Grow Onions at an Industrial Level: Complete Guide

An onion field is the least forgiving block on most vegetable farms. The crop has a shallow root system, almost no canopy, a bulb-triggering switch tied to day length, and a harvest window measured in days rather than weeks. Miss any one of those and you don’t lose 5 percent. You lose the block.

Here’s the number that gets people’s attention. Roughly 134,700 acres of onions were harvested in the United States in 2020, averaging 503 hundredweight per acre, with a farm value near $877.8 million, according to the Agricultural Marketing Resource Center’s summary of USDA data. Now look at what sits underneath that average: strong growers in the Pacific Northwest routinely push past 900 cwt per acre, while a poorly managed block in the same county might scrape 350. Same seed. Same rainfall. Same market. The gap is management.

This guide is about closing that gap at scale. Not backyard onions. Not a market-garden half acre. We’re talking 200 to 3,000 acres, mechanized harvest, forced-air curing, bulk storage, and a packing line that has to move eight to fifteen tons an hour without bruising the product.

I’ve organized it the way the season actually runs, because that’s how decisions actually get made. Soil comes before seed. Seed comes before water. Water comes before fertility. And every single one of those choices shows up again in November when you open a storage door and find out whether the crop held.

Who This Guide Is For

• Growers scaling up from 40 acres of fresh-market onions to several hundred acres of storage production.
• Farm managers and agronomists who need a single reference that connects agronomy to storage engineering and cost accounting.
• Ag investors and operations leads evaluating onions as a rotation crop or a standalone enterprise.
• Processors and packers who want to understand what their growers are up against in the field.

HOW TO USE THIS GUIDE Every section ends with a short ‘What to do next’ block. If you’re mid-season and firefighting, skip to Troubleshooting: Fifteen Ways Onion Crops Fail and Your Twelve-Month Operating Calendar. If you’re planning a new block from bare ground, start at Site Selection and Land Suitability and work forward in order.

A Note on Sources and Local Adaptation

Onion production is intensely regional. A fertility program that works on Treasure Valley silt loam will wreck a crop on Georgia sandy loam. Throughout this guide I point to land-grant university extension services and federal agencies, because those are the people running replicated trials in your soil type. Use their numbers to calibrate mine.

Two ground rules before we start. First, the pesticide label is the law. Nothing in this guide overrides it, and product registrations change every season. Verify current registrations through your state lead agency and the

U.S. Environmental Protection Agency: epa.gov/pesticide-registration. Second, run your own soil and tissue tests. Any number in this guide is a starting point for a conversation with your agronomist, not a prescription.

Know the Plant Before You Farm It

Allium cepa is a biennial that we grow as an annual. That single sentence explains most of what goes wrong in commercial onion fields. The plant wants to spend year one building a bulb and year two making seed. We interrupt it halfway, and our job is to make year one as productive as possible before the plant catches on.

The Six Stages, and Why Two of Them Matter Most

An onion crop moves through six recognizable stages. Stages one through three build the factory. Stages four through six convert it into sellable tonnage.

Figure 1. The six growth stages of a commercial onion crop, with approximate timing for a long-day storage variety.

Here’s the part that catches new growers off guard. Every leaf the plant makes before bulb initiation becomes a storage scale inside the bulb. Once day length trips the bulbing switch, leaf production stops cold. The plant redirects everything into filling the scales it already has.

So bulb size isn’t decided at harvest. It’s decided somewhere around day 90, by how many leaves you managed to grow. A plant with thirteen healthy leaves at bulb initiation has a shot at a colossal. A plant with seven leaves, stunted by thrips or a nitrogen gap or crusted soil, is going to give you a medium no matter what you do afterward.

THE ONE THING TO REMEMBER Everything you do in the first 90 days sets the yield ceiling. Everything you do after that determines how much of that ceiling you actually collect. You cannot fertilize your way out of a bad stand in August.

Photoperiod: The Decision You Cannot Undo

Onions bulb in response to day length, not calendar date and not temperature. Breeders sort varieties into three broad photoperiod classes, and matching that class to your latitude is the single highest-stakes call in the whole operation.

Figure 2. Photoperiod classes and the latitudes where each one performs.

Class	Bulbing Trigger	Typical Latitude	Where It’s Grown	Storage Life
Short-day	10 to 12 hours	25° to 35° N	South Texas, Vidalia region of Georgia, Imperial Valley, Arizona	30 to 90 days
Intermediate-day	12 to 14 hours	32° to 38° N	Central California, New Mexico, Kansas, Missouri	90 to 150 days
Long-day	14 to 16 hours	36° to 50° N	Idaho, Oregon, Washington, Colorado, New York, Michigan, Wisconsin	150 to 270 days

Table 1. Photoperiod classes, triggers, and production regions. Storage life assumes proper curing and controlled holding conditions.

Get this wrong in either direction and the block is a write-off. Plant short-day seed in Idaho and the plants bulb far too early, at four or five leaves, giving you golf balls. Plant long-day seed in south Texas and the days never get long enough to trigger bulbing at all. The crop just makes leaves, then bolts.

There’s a fourth consideration that trips people up: day-neutral varieties exist, but they’re a marketing term more than a physiological class. Read the seed company’s actual latitude recommendation, not the category name.

Bolting, Vernalization, and Why Planting Date Is Not Flexible

Bolting is the plant deciding it’s already been through a winter and jumping to flower. A seed stalk means an unsaleable bulb, a hollow center, and a rot pathway straight into your storage.

The trigger is cold. Once a plant reaches a certain size, usually around pencil-thickness at the neck, exposure to sustained temperatures below about 45°F for several weeks tells it winter has happened. When warm weather returns, it flowers.

This is exactly why fall-planted short-day onions in the South have such narrow planting windows. Plant too early and the plants get big enough to vernalize before winter. Plant too late and they’re too small to size up before spring day lengths trigger bulbing. Regional extension services publish planting date windows down to the county. Use them.

• Bolting risk rises with early planting, large transplants, sets larger than about 3/4 inch, and cold springs.
• Bolting risk falls with variety selection (ask your seed rep for bolt-tolerance ratings), on-time planting, and avoiding early nitrogen that pushes size too fast in fall.
• Scout for it at the 8-to-10 leaf stage. Rogue seed stalks out if the incidence is low and the block is otherwise clean.

Pungency, Dry Matter, and Why Sweet Onions Don’t Store

Onion pungency comes from sulfur compounds. Sweet onions are sweet mostly because they’re grown in low-sulfur soils with abundant water, which dilutes those compounds and raises water content.

That same chemistry is why sweet onions rot. High water, low dry matter, and thin skins make a bulb that a customer loves and a warehouse hates. Storage yellows run 12 to 16 percent dry matter. Dehydration varieties run 18 to 22 percent. Sweets often sit below 8 percent.

PRACTICAL IMPLICATION You cannot grow a sweet onion and store it for eight months. If your business model depends on long holding, you need a high-solids storage variety and a sulfur program that supports it. Pick one lane.

What to Do Next

• Confirm your farm’s latitude and pull the recommended photoperiod class from your state extension vegetable guide.
• Ask your seed supplier for bolt-tolerance and dry-matter ratings in writing, not just a catalog description.
• Decide now whether you’re a fresh-market operation or a storage operation. The next section, Pick Your Market Before You Pick Your Seed, walks through that call.

Pick Your Market Before You Pick Your Seed

Most failed onion enterprises I’ve seen didn’t fail agronomically. They failed because someone ordered seed before they had a buyer, then discovered in September that the only people who wanted 4,000 tons of medium yellows were paying less than the cost of the bag.

Onions are a volume commodity with brutal price swings. USDA Economic Research Service reporting showed onion prices falling by 43 percent year over year through October 2025, from 46 cents per pound down to 26 cents. A grower who penciled a budget at 46 cents and locked in nothing lost money on a good crop.

Figure 3. Four commercial onion production models and what each one demands from the operation.

The Four Models, Compared Honestly

Factor	Fresh Sweet	Storage Yellow	Processing / Dehy	Specialty & Organic
Typical scale	100 to 1,500 ac	300 to 3,000 ac	500 to 5,000 ac	20 to 300 ac
Dry matter target	6 to 9%	12 to 16%	18 to 22%	Varies by type
Capital: storage	Low (ship fast)	Very high	Moderate (contract flow)	Low to moderate
Labor intensity	High	Moderate	Low	Very high
Price volatility	Extreme	High	Low (contracted)	Moderate
Marketing window	4 to 8 weeks	8 to 10 months	Scheduled delivery	Year-round niche
Main risk	Glut at harvest	Storage loss	Contract terms	Labor availability
Who wins here	Growers with brand + regional identity	Growers with capital and storage discipline	Growers who want predictable cash flow	Growers with direct buyer relationships

Table 2. Comparing the four commercial onion production models across the factors that actually drive profitability.

Contracts Versus Open Market

Here’s the honest tradeoff. Contract acres cap your upside and protect your downside. Open-market acres do the opposite. Most well-run operations I know split the difference, roughly 50 to 70 percent contracted and the balance speculative.

What to check before signing anything:

1. Grade specification. Who defines a defect, and who does the counting? Third-party inspection language matters more than the price line.
2. Delivery schedule. A contract that requires January delivery means you’re carrying storage cost and shrink risk for five months. Price that in.
3. Shrink allowance. Storage onions lose weight. If the contract prices on delivered weight with no allowance, you’re eating 5 to 12 percent.
4. Force majeure and crop failure clauses. What happens if hail takes the block in August?
5. Payment terms. Net 30 versus net 60 on a 2,000-ton delivery is real working capital.

Yield Benchmarks You Can Budget Against

Production System	Realistic Yield (cwt/ac)	Top-Decile Yield	Notes
Short-day fresh, transplanted	400 to 600	700+	Sweet types, high hand-labor, short window
Short-day fresh, direct-seeded	350 to 550	650+	Lower cost, more size variability
Intermediate-day, drip-irrigated	550 to 750	850+	Good balance of size and tonnage
Long-day storage, furrow-irrigated	500 to 700	800+	Legacy systems, higher water use
Long-day storage, drip-irrigated	700 to 900	1,000+	Best-in-class Pacific Northwest blocks
Dehydration contract	600 to 850	950+	Paid on solids, not just tonnage
Certified organic	300 to 500	600+	Weed pressure is the binding constraint

Table 3. Yield benchmark ranges by production system. National average across all U.S. onion types has run near 500 cwt per acre. Local extension variety trials are the best calibration for your county.

BUDGET DISCIPLINE Build your break-even at the low end of the realistic range, not the top-decile number. If the enterprise only works at 900 cwt per acre, it doesn’t work.

What to Do Next

• Write down your target market and buyer before ordering seed. One page. Buyer name, volume, grade spec, delivery window, price mechanism.
• Pull three years of price history for your onion type from USDA Agricultural Marketing Service market news.
• Decide your contracted-versus-open split and stick to it.

Site Selection and Land Suitability

You can fix a lot of things on an onion farm. You cannot fix the field. Land selection is where the ceiling gets set, and it’s the cheapest decision to get right because it costs nothing but discipline to walk away from a bad block.

The Non-Negotiables

Requirement	Target	Why It Matters	Deal-Breaker If…
Soil texture	Sandy loam, silt loam, or well-structured muck	Shallow roots need low resistance and fast drainage	Heavy clay that crusts or waterlogs
Internal drainage	Water moves through the top 24 in freely	Saturated roots invite Pythium and pink root within days	Perched water table above 30 in
Field slope	0 to 2 percent, uniform	Even water distribution and clean mechanical harvest	Broken topography that pockets water
Stone content	Minimal above 1 in	Stones bruise bulbs on the harvester and jam the line	Heavy stone that needs annual picking
Soil pH	6.0 to 6.8	Nutrient availability plus disease suppression	Below 5.5 without a lime program
Salinity (ECe)	Below 1.2 dS/m	Onions are among the most salt-sensitive crops grown	Above 2.5 dS/m with no leaching capacity
Water supply	Reliable 24 to 36 in of applied water per season	Peak demand tolerates no interruption	Allocation that can be curtailed in July
Rotation history	3 to 5 years since last allium	White rot and Fusarium persist in soil for decades	Known white rot history, at any level
Herbicide carryover	Clean plant-back interval verified	Onions are sensitive to several common residuals	Recent ALS or triazine applications

Table 4. Site suitability screen. Score a candidate field against all nine before you commit a season to it.

The White Rot Question

Sclerotium cepivorum deserves its own paragraph because it’s the one soil problem that ends onion production on a piece of ground. The sclerotia survive in soil for twenty years or longer without a host. There’s no rescue treatment once a field is infested at commercial levels.

Practical rules:

• Never move soil, equipment, or cull piles from a suspect field to a clean one. Wash down harvesters between fields.
• Ask for the full allium cropping history of any rented ground, going back at least fifteen years. Get it in writing.
• If you find isolated infection centers, flag them, harvest them last, and keep that soil out of your storage.
• Talk to your regional extension plant pathologist before you take on ground with any history at all.

Water: Quantity, Quality, and Legal Security

An onion crop uses roughly 24 to 36 acre-inches of applied water per season depending on climate, soil, and irrigation method. That demand is not evenly spread. Peak use lands squarely in the bulb-sizing window, when a three-day interruption costs you size you never get back.

Test the water, not just the well yield. The numbers that matter:

Parameter	Preferred	Caution Zone	Consequence If Exceeded
Electrical conductivity	Below 0.7 dS/m	0.7 to 1.5 dS/m	Yield loss begins; leaching fraction required
Sodium adsorption ratio	Below 3	3 to 9	Soil structure collapse, infiltration problems
Chloride	Below 70 ppm	70 to 140 ppm	Leaf tip burn, reduced sizing
Bicarbonate	Below 90 ppm	90 to 500 ppm	Emitter plugging, calcium precipitation in drip
Iron	Below 0.2 ppm	0.2 to 1.5 ppm	Iron bacteria slime, drip system failure
Total suspended solids	Below 50 ppm	50 to 100 ppm	Filter load, emitter clogging
Boron	Below 0.7 ppm	0.7 to 2.0 ppm	Toxicity symptoms, especially on light soils

Table 5. Irrigation water quality screen for onion production, with emphasis on drip system longevity.

FIELD-SCOUTING TIP Dig four soil pits, not one, and put them in the worst-looking parts of the field, not the best. A 30-inch hole tells you more about a block’s onion potential than any lab report. Look for compaction pans, mottling from a fluctuating water table, and root restriction layers.

Climate Fit

Onions want a long, mild growing season with dry conditions at the end. Two climate factors deserve a hard look before you commit:

• Rainfall at maturity. Wet weather during dry-down is the single biggest driver of bacterial rot in storage. Regions with reliable September rain need either earlier varieties or covered curing.
• Heat during bulb fill. Sustained temperatures above 90°F during bulb sizing cause translucent scale, sunscald, and split centers. Shade never fixes this at scale; variety and planting date do.
• Frost dates. Emerging onions tolerate light frost. Bulbs exposed at harvest do not. Freezing a windrow ends the crop.

Pull thirty years of local climate normals from NOAA’s National Centers for Environmental Information before you plan a planting date. Historical data beats intuition every time.

Reference: NOAA National Centers for Environmental Information and the USDA Plant Hardiness Zone Map.

What to Do Next

• Score every candidate field against Table 4. Anything failing two or more lines goes into a different crop.
• Send a water sample to a certified lab and screen against Table 5 before designing an irrigation system.
• Document the allium cropping history of all rented ground.

Soil Preparation and Bed Engineering

Onion roots are lazy. They’re fine, unbranched, and mostly confined to the top 8 to 12 inches of soil. They don’t push through compaction, they don’t chase moisture sideways, and they don’t recover from a crusted seedbed. Which means your tillage program isn’t housekeeping. It’s the foundation of yield.

Fix the pH and Salinity First

These two numbers set a ceiling you cannot raise mid-season.

Figure 4. The two soil numbers that cap your yield potential: pH and salinity.

Lime moves pH slowly, over six to twelve months. If a soil test comes back at 5.3, that’s a correction you make the season before onions go in, not the week before. Sulfur to lower a high pH is even slower and rarely economical at field scale.

Salinity is the crueler one. Onions start losing yield at an ECe around 1.2 dS/m, which is low enough that many growers never think to check. By 4 dS/m you’ve lost more than 40 percent of your potential. If you’re farming in an arid region with pressurized irrigation, budget a leaching fraction into your water plan every season.

Building the Seedbed

The target is straightforward to describe and hard to achieve: a firm, level, fine-textured bed with no clods larger than a marble in the top three inches, and no compaction pan above 14 inches.

1. Deep tillage in fall. Subsoil or rip to below the pan, at 16 to 20 inches, when soil moisture is low enough to shatter rather than smear. Do this in fall so winter can settle the profile.
2. Incorporate residue early. Onions do not tolerate heavy surface trash. It interferes with seed placement, harbors seedcorn maggot, and fouls the harvester. Chop and incorporate the prior crop at least 60 days ahead.
3. Primary tillage. Moldboard, chisel, or heavy disk depending on your system and residue load. Aim to get the profile uniform, not pulverized.
4. Secondary passes. Two to three passes with a field cultivator or rotary harrow. Each pass costs money and destroys structure, so use the fewest that gets you to the target seedbed.
5. Bed forming. Build beds, then firm them. A rolling basket or bed press behind the bed shaper is what separates good stands from patchy ones.
6. Pre-plant fertility and fumigation. Apply and incorporate before the final bed pass, not after.
7. Let the bed settle. Two to three weeks between bed formation and seeding, with a light irrigation if you can, gives you a bed that won’t slump and bury your seed.

THE MISTAKE THAT COSTS THE MOST Working ground when it’s too wet. A smeared pan at 8 inches is invisible from the cab and will cost you 20 percent of your yield. If a shovel of soil ribbons instead of crumbling, park the tractor.

Bed Geometry

Bed width is largely dictated by your existing equipment fleet, and there’s no single right answer. What matters is that the bed top is wide enough to carry your seed lines without crowding the shoulders, and that your tractor, sprayer, cultivator, and harvester all share the same wheel spacing.

Figure 5. Raised bed cross-section with two subsurface drip tapes and five seed lines.

Bed System	Bed Centers	Bed Top	Seed Lines	Typical Region	Notes
Narrow twin-row	40 in	20 in	2	Georgia, Texas fresh market	Easy cultivation, lower population
Standard four-line	80 in	22 in	4	Pacific Northwest storage	Jumbo-biased size profile
Standard five-line	80 in	24 in	5	Idaho, Oregon, Washington	The industry workhorse
Wide six-line	80 to 84 in	28 in	6	Processing, dehydration	Max tonnage, smaller bulbs
Flat ground, no bed	22 in rows	n/a	1 per row	Muck soils, New York, Michigan	Requires excellent natural drainage

Table 6. Common commercial onion bed systems in U.S. production.

Organic Matter and Cover Crops

Onions respond well to organic matter, but timing is everything. Fresh, high-carbon residue immediately before planting ties up nitrogen and creates a seedcorn maggot magnet. Decomposed organic matter improves water-holding capacity and structure without either problem.

• Good rotation partners: small grains, corn, sudangrass, and non-host cover crops that build residue and break disease cycles.
• Risky partners: brassicas can host some shared pests; legumes can leave excess residual nitrogen that delays maturity and softens bulbs.
• Avoid entirely: any allium in the preceding three to five years.
• Compost: apply the fall before, at rates based on a compost nutrient analysis, and account for the nitrogen it releases in year one and year two.

What to Do Next

• Pull soil samples by management zone, not by whole field. Test to 12 inches, plus a 12-to-24-inch sample for nitrate and salinity.
• Probe for compaction with a penetrometer at field capacity, on a grid.
• Schedule deep tillage for fall, not spring.

Variety Selection and Seed Sourcing

Seed is one of the smallest lines in an onion budget and one of the largest drivers of outcome. Growers who shop on price here almost always pay for it twice.

The Selection Matrix

Rank these criteria in the order your market demands, then screen varieties against them. Don’t let a seed rep reorder your priorities.

Criterion	Why It Matters	How to Verify
Photoperiod class	Non-negotiable. Wrong class means no crop.	Seed company latitude chart plus local trial data
Days to maturity	Determines harvest sequencing and storage loading order	Regional variety trials, not catalog claims
Dry matter / solids	Predicts storage life and processing acceptance	Ask for refractometer and dry-matter trial data
Skin retention	Buyers pay for skin. Poor retention = downgrades	Handling trials, packing line observation
Bolting tolerance	Critical for fall-planted and early-planted crops	Multi-year trial data from your region
Disease package	Pink root, Fusarium, downy mildew resistance ratings	Published resistance ratings with pathogen races
Bulb shape and uniformity	Drives packout percentage and line throughput	Field observation, packout records
Single-centeredness	Essential for onion ring and processing contracts	Cut 100 bulbs from a trial and count
Storage rating	Weeks of holding under commercial conditions	Commercial storage trials, not lab data
Neck size and closure	Thick necks cure poorly and rot in storage	Trial observation at curing

Table 7. Variety selection criteria matrix. Screen every candidate against all ten before committing acres.

Buy Seed Like a Professional

A few practices separate operations that get consistent stands from those that gamble every spring.

• Order early. Good storage-onion seed sells out. Commit twelve months ahead for large volumes.
• Buy by seed count, not weight. Onion seed size varies. Specify seeds per pound and target plants per acre, then let the seed company calculate the bag count.
• Demand a current germination test. Onion seed loses viability faster than most vegetable seed. Ask for a test date within six months, and a vigor test if you’re seeding into cold soil.
• Specify pelleted or raw based on your planter. Precision seeders need consistent pellet size; check that the pellet spec matches your seed plates or vacuum disks.
• Ask about seed treatment. Fungicide and insecticide treatments for damping-off and seedcorn maggot are standard and worth every cent.
• Never plant a single variety on more than 30 percent of your acres. Spread maturity dates and genetic risk.

TRIAL DISCIPLINE Put 2 to 5 percent of your acres into a replicated variety trial every year, with your current standard as the check. Keep the data. Three years of your own trial results beats any catalog.

Where to Find Independent Variety Data

Land-grant universities publish annual onion variety trials with yield, size distribution, and storage results. Start with Oregon State University Extension, Cornell Cooperative Extension Vegetable Program, University of Georgia Extension, and Texas A&M AgriLife Extension.

What to Do Next

• Build your own variety scorecard from Table 7 and score every candidate.
• Request three years of regional trial data from each seed supplier.
• Lock in seed orders twelve months ahead of planting for anything above 200 acres.

Establishment: Seed, Transplants, or Sets

How you get the crop in the ground shapes cost, timing, uniformity, and risk. At industrial scale, direct seeding wins on economics almost everywhere. But the exceptions matter.

Figure 6. Three establishment methods compared on season length and cost profile.

Method	Establishment Cost/Acre	Days to Harvest	Uniformity	Best Use Case	Main Risk
Direct seeding	$180 to $450	150 to 190	Excellent when stand is achieved	Storage and processing at scale	Stand failure from crusting or cold soil
Transplants	$900 to $2,200	110 to 140	Very good, but transplant shock varies	Early fresh market, short-day sweets	Labor supply, bolting from oversized plants
Sets (bulblets)	$600 to $1,400	90 to 110	Moderate	Green onions, earliest slot, niche markets	High bolting rate, disease carried on sets

Table 8. Establishment method comparison. Cost ranges are planning estimates; verify against current regional custom rates.

Direct Seeding Done Right

The whole game is achieving a uniform stand. Everything else follows.

Parameter	Target	Notes
Seeding depth	0.5 to 0.75 in	Deeper in dry sand, shallower in heavy soil. Consistency beats absolute depth.
Soil temperature at seeding	50°F and rising	Germination below 45°F is slow and uneven; Pythium takes advantage.
Seeding rate	280,000 to 450,000 seeds/ac	Set by target population and expected field emergence.
Expected field emergence	70 to 85%	Adjust rate accordingly. Never assume lab germination equals field emergence.
Final stand target	110,000 to 180,000 plants/ac	Lower for jumbo-biased fresh market, higher for processing.
In-row spacing	2 to 4 in	Wider spacing gives bigger bulbs and fewer of them.
Planter speed	2.5 to 3.5 mph	Faster speeds destroy spacing accuracy on precision vacuum planters.
Press wheel setting	Firm, not compacted	Seed-to-soil contact without a crust-forming seal.

Table 9. Direct seeding parameters for commercial onion production.

Population Drives Size Distribution

This is where growers leave money on the table. Population isn’t a yield lever so much as a size-distribution lever. More plants per acre means more total tonnage but a smaller average bulb. If your buyer pays a premium for jumbo and colossal, chasing tonnage costs you money.

Figure 7. Seed line configuration sets both bulb size and total tonnage.

Target Population	Approx. Size Profile	Best For
100,000 to 120,000/ac	45% jumbo, 30% colossal, 25% medium and smaller	Premium fresh market, onion ring contracts
130,000 to 155,000/ac	50% jumbo, 15% colossal, 35% medium and smaller	General storage market, retail bulk
160,000 to 185,000/ac	35% jumbo, 5% colossal, 60% medium and pre-pack	Consumer bags, processing, dehydration
190,000+/ac	Mostly medium, small, and boiler grades	Boilers, pickling, specialty packs

Table 10. How plant population translates into size distribution. Actual results shift with variety, water, and fertility.

Crusting: The Silent Stand Killer

Onion seedlings emerge with a loop-shaped cotyledon that has to push through the surface. A hard crust after a rain or a heavy irrigation stops them dead. You lose the stand in patches, and by the time you can see it from the road it’s too late to replant.

• Keep the surface moist with light, frequent irrigations from seeding through emergence. Small and often beats heavy and occasional.
• Run a rotary hoe or crust-breaker if a crust forms before emergence. Timing is measured in hours, not days.
• Consider a light straw or mulch application on the most crust-prone fields.
• Choose bed-forming equipment that leaves a slightly rough surface instead of a sealed, polished one.

What to Do Next

• Calibrate your planter with the actual seed lot you’ll plant, on the actual bed, before the first field.
• Set your population target from your buyer’s grade preferences, working backward from Table 10.
• Do stand counts at 21 days after seeding, on a grid, and record them. This is your earliest yield forecast.

Irrigation Engineering for Onions

Onions are shallow-rooted, salt-sensitive, and completely intolerant of water stress during bulb fill. That combination makes irrigation the highest-leverage system on the farm. Get it right and you convert the yield ceiling you built in June into tons in September.

How Much Water, and When

Seasonal applied water typically runs 24 to 36 acre-inches. But the season total matters far less than the daily match between supply and demand. Use the reference-ET method: measure or obtain local reference evapotranspiration, then multiply by the onion crop coefficient for the current growth stage.

Figure 8. Onion crop coefficient curve across the season, following the FAO-56 single-coefficient method.

The Kc values above follow the approach set out in the FAO Irrigation and Drainage Paper 56, the standard reference for crop water requirements: Crop Evapotranspiration (FAO-56). Many states also run free ET networks; check your extension service for a local station.

Growth Stage	Kc	Allowable Depletion	Root Depth	Typical Frequency (drip, sandy loam)
Emergence to 2-leaf	0.35 to 0.45	20%	3 to 5 in	Daily, short sets, keep surface moist
3 to 5-leaf	0.50 to 0.70	25%	5 to 7 in	Every 1 to 2 days
6 to 9-leaf	0.75 to 0.95	25%	7 to 10 in	Daily to every other day
Bulb initiation	0.95 to 1.05	20%	10 to 12 in	Daily; do not let it stress here
Bulb fill (peak)	1.05	20%	10 to 14 in	Daily, sometimes twice daily in heat
Maturation	0.75 to 0.90	35%	10 to 14 in	Tapering, every 2 to 3 days
Dry-down	0.40 to 0.55	n/a	n/a	Terminate 7 to 14 days before undercutting

Table 11. Irrigation scheduling by growth stage. Allowable depletion is the fraction of plant-available water you let the soil lose before refilling.

THE RULE THAT SAVES THE MOST YIELD Keep allowable depletion at or below 25 percent from bulb initiation through bulb fill. Onions show almost no visible wilt before yield is already lost. By the time the crop looks thirsty, the size is gone.

Choosing an Irrigation Method

System	Application Efficiency	Capital Cost/Acre	Best Fit	Watch Out For
Furrow	50 to 65%	$150 to $500	Level ground, abundant low-cost water	Uneven distribution, high salinity risk at bed center
Center pivot / linear	75 to 88%	$800 to $1,600	Large uniform blocks, PNW and Plains	Wheel track damage, foliar disease from wet leaves
Solid-set sprinkler	70 to 85%	$900 to $1,800	Germination phase, frost protection	Leaf wetness drives downy mildew and botrytis
Surface drip	88 to 94%	$700 to $1,400	High-value fresh market	Tape damage, rodents, retrieval labor
Subsurface drip (SDI)	90 to 95%	$1,100 to $2,400	Storage and processing at scale	Emitter plugging, root intrusion, higher filtration need

Table 12. Irrigation system comparison for commercial onion production. Costs are planning estimates and vary widely by region and design.

Designing a Drip System That Lasts

Figure 9. Drip headworks and zone layout for a commercial onion block.

Three design decisions determine whether your drip system delivers 95 percent uniformity in year one and still delivers it in year five.

1. Filtration sized for your worst water day, not your average. Sand media plus a screen or disc backup. Automatic backflush on a differential pressure trigger, not a timer.
2. Zone sizing for pressure uniformity. Design each zone so pressure varies less than 10 percent from the submain to the far end of the lateral. Use pressure-compensating tape on any run over 500 feet or any slope above 1 percent.
3. A maintenance chemistry program from day one. Acid injection to hold line pH near 6.5 and prevent carbonate scaling; chlorine or another approved biocide to control biofilm. Flush laterals on a written schedule, not when you notice a problem.

Drip Design Parameter	Typical Onion Spec	Why
Tape per bed	2 lines	Wets the full bed top across all seed lines
Tape spacing	10 to 14 in apart	Overlapping wetting fronts, no dry stripe at bed center
Burial depth (SDI)	2 to 5 in	Below tillage damage, above the shallow root zone
Emitter spacing	8 to 12 in	Continuous wetted strip in sandy soils
Emitter flow	0.20 to 0.34 gpm/100 ft	Matches peak ET without runoff
Operating pressure	8 to 12 psi at the tape	Below this, uniformity collapses
Wall thickness	6 to 15 mil	Thinner for single-season, thicker for multi-year SDI
Filtration	120 to 200 mesh	Emitter passage protection
Flush velocity	1.0 ft/sec minimum	Actually removes sediment instead of moving it

Table 13. Drip system specifications for onion production.

Terminating Irrigation

When you shut the water off is as important as how you ran it. Cut too early and you sacrifice size in the final bulking weeks. Cut too late and you get thick necks, delayed maturity, poor skin set, and a crop that will not cure.

The general rule: stop irrigation when roughly 10 to 30 percent of tops have lodged, which usually means 7 to 14 days before you plan to undercut. On sandy soils in hot climates that gap narrows; on heavy soils it widens. Watch your own fields for two seasons and you’ll dial it in.

What to Do Next

• Install soil moisture sensors at two depths in at least three representative zones. Sensors pay for themselves in one season.
• Run a distribution uniformity test on every zone before the season starts. Target 90 percent or better.
• Write a written flush-and-chemigation schedule and assign it to a named person.

Nutrient Management and Fertigation

Onions are heavy feeders with a shallow root system, which is an awkward combination. They need a lot, and they can only reach a small volume of soil to get it. That’s why fertigation, feeding through the irrigation system in small frequent doses, outperforms broadcast fertility almost everywhere onions are grown with drip.

When the Crop Actually Needs It

Figure 10. Cumulative nutrient uptake across the onion season.

Note the shape of those curves. The crop takes up very little in the first six weeks, then demands most of its season total in a sixty-day window centered on bulb initiation and early bulb fill. A single heavy pre-plant application is out of sync with that curve, and a good share of it leaches past the root zone before the plant wants it.

Nitrogen: The Nutrient Most Growers Get Wrong

Nitrogen drives leaf number, and leaf number sets bulb size. So more nitrogen means bigger onions, right? Only up to a point, and past that point it actively damages the crop.

• Too little N early limits leaf production, and you never recover the lost yield ceiling.
• Too much N late produces thick necks, delayed maturity, soft bulbs, poor skin set, and dramatically higher storage rot. It also feeds thrips populations.
• The fix is a split program that front-loads a modest starter, peaks through leaf building and bulb initiation, then cuts off hard around 3 to 4 weeks before expected maturity.

HARD STOP ON NITROGEN Stop all nitrogen applications at least 3 to 4 weeks before expected bulb maturity. A late nitrogen application is the most reliable way to turn a good field crop into a storage disaster.

Sulfur, and the Pungency Tradeoff

Sulfur is essential for onions, and it directly influences pungency. Storage and processing varieties benefit from adequate sulfur, which builds the compounds that give firmness and shelf life. Sweet onion production deliberately restricts sulfur to keep pungency down.

Decide which you’re growing before you build the fertility plan, because the sulfur program is where the two diverge most sharply.

A Planning Framework for the Nutrient Budget

Nutrient	Typical Removal (per 100 cwt)	Season Budget Range	Form and Timing Notes
Nitrogen (N)	12 to 18 lb	120 to 220 lb/ac	Split 6 to 10 ways. Nitrate forms are taken up fastest. Cut off 3 to 4 weeks pre-maturity.
Phosphorus (P₂O₅)	5 to 8 lb	60 to 150 lb/ac	Band at planting. Onions respond strongly to starter P even at high soil test levels.
Potassium (K₂O)	14 to 22 lb	120 to 250 lb/ac	Peak demand during bulb fill. Sulfate of potash preferred on saline or chloride-sensitive sites.
Sulfur (S)	6 to 10 lb	30 to 80 lb/ac	Raise for storage and processing types. Restrict for sweets.
Calcium (Ca)	8 to 14 lb	Soil-dependent	Supports scale firmness and skin quality. Check base saturation.
Magnesium (Mg)	2 to 4 lb	Soil-dependent	Watch Ca:Mg balance, especially after heavy liming.
Zinc (Zn)	0.03 to 0.06 lb	2 to 8 lb/ac	Commonly deficient above pH 7.2. Foliar correction is fast.
Boron (B)	0.02 to 0.05 lb	0.5 to 2 lb/ac	Narrow window between deficiency and toxicity. Test first.
Manganese (Mn)	0.05 to 0.10 lb	1 to 4 lb/ac	Deficiency common on high-pH and muck soils.

Table 14. Nutrient budget planning framework. These are planning ranges only. Build the actual program from your soil test, yield goal, and in-season tissue results.

Splitting the Program

Figure 11. A split fertigation program across seven crop stages.

A reasonable starting structure looks like this. Adjust it every year with tissue test results.

1. Pre-plant band: 10 to 20 percent of N, most of the P, a share of K and S, placed 2 to 3 inches below and to the side of the seed line.
2. Emergence to 3-leaf: minimal. The plant is tiny and the roots are shallow. Overfeeding here just leaches.
3. 4 to 6 leaf: ramp up. This is where leaf building starts to accelerate.
4. 7 to 9 leaf: peak leaf production. Highest sustained N and K rates of the season.
5. Bulb initiation: maintain N, push K. Calcium supplementation supports scale firmness.
6. Early bulb fill: K demand peaks here. N tapers.
7. Late fill and dry-down: N off completely. Small K and Ca applications only if tissue tests justify them.

Tissue Testing: The Feedback Loop

Soil tests tell you what’s in the ground. Tissue tests tell you what the plant actually got. Sample the youngest fully mature leaf, every two to three weeks from the 5-leaf stage through early bulb fill, and adjust fertigation from the results.

Nutrient	Sufficiency Range (whole leaf, % or ppm)	Deficiency Symptom
Nitrogen	3.0 to 4.5%	Pale, upright, stunted leaves; slow growth
Phosphorus	0.25 to 0.50%	Dark green then purple leaf tips, poor rooting
Potassium	2.5 to 4.5%	Leaf tip and margin necrosis, weak bulbs
Sulfur	0.50 to 1.00%	General yellowing of youngest leaves
Calcium	0.80 to 1.60%	Soft scales, poor skin retention
Magnesium	0.25 to 0.50%	Interveinal chlorosis on older leaves
Zinc	20 to 60 ppm	Stunted twisted leaves, delayed maturity
Boron	25 to 60 ppm	Brittle leaves, cross-banding, growing point damage
Manganese	40 to 120 ppm	Yellow striping on young leaves
Iron	50 to 150 ppm	Interveinal chlorosis on youngest leaves

Table 15. Tissue sufficiency ranges for onions. Ranges vary by lab and growth stage. Always interpret against your lab’s own reference values.

What to Do Next

• Build the season’s nutrient budget from a yield goal and a current soil test, then divide it across the seven stages in Figure 11.
• Book tissue testing for the whole season now, on the calendar, so it actually happens.
• Set a hard calendar date for nitrogen shutoff and write it on the wall of the shop.

Weed Management: The Biggest Single Cost Center

Onions are the worst competitors in the vegetable aisle. Narrow, upright, waxy leaves cast almost no shade, so the canopy never closes and weeds get full sun all season. Uncontrolled weeds can cut marketable yield by more than half, and hand weeding is often the single largest labor line in the budget.

Figure 12. The critical weed-free period and the tactics that cover it.

Build a Layered Program, Not a Spray Plan

No single tactic holds a ten-week weed-free window in a crop with no canopy. Stack them.

Tactic	Timing	What It Handles	Practical Notes
Rotation and field selection	Years ahead	Perennials, nutsedge, resistant biotypes	The cheapest weed control is not planting onions in a dirty field
Stale seedbed	2 to 4 weeks pre-plant	First flush of annuals	Form beds, irrigate, let weeds emerge, then kill with tillage or a burndown
Pre-emergent residual	At or just after seeding	Annual grasses and many broadleaves	Requires incorporation by irrigation. Check plant-back and rate by soil texture
Post-emergent selective	2-leaf onion onward	Escapes and later flushes	Onion tolerance is stage-dependent. Never apply before the labeled leaf stage
Mechanical cultivation	3-leaf through canopy	Between-row weeds	Camera-guided or GPS-steered rigs work within 1 to 2 in of the row
Precision and laser weeders	3-leaf onward	In-row escapes	High capital cost, but replacing hand crews changes the math fast
Hand weeding	As needed	Everything that got through	The most expensive option. Treat it as a rescue, not a plan

Table 16. Layered weed management program for commercial onions.

HERBICIDE SELECTION AND THE LABEL Registered onion herbicides differ by state, by soil type, and by whether the crop is direct-seeded or transplanted. Product registrations also change every season. Verify current options with your state extension weed specialist and read the full label before every application. The label is a legal document, and it overrides anything in this guide.

Resistance Management

Herbicide-resistant weeds are now the norm rather than the exception in most onion regions. The mechanics are simple: use the same site of action repeatedly, and you select for the biotypes that survive it.

• Rotate sites of action across the season and across years. Track them by the HRAC/WSSA group number, not by trade name. Two different brands can be the same chemistry.
• Tank-mix effective partners with different sites of action rather than sequencing single actives.
• Never let escapes go to seed. One pigweed that sets seed replaces years of clean control.
• Map your problem areas and change tactics on those blocks specifically.

Where Mechanical Weeding Pays

Camera-guided cultivators and laser weeders carry serious capital cost, often $80,000 to $400,000 depending on width and technology. Run the math against your actual hand-weeding spend, not a national average. Operations spending $600 to $1,400 per acre on hand crews frequently pay back a precision cultivator inside two seasons.

Two practical requirements before you invest: beds have to be straight and consistently spaced, and the crop has to be planted with GPS guidance so the machine knows where the rows are. Sloppy bed forming makes precision weeding impossible.

What to Do Next

• Map weed species and pressure by field, and update the map every year.
• Write your herbicide program by site-of-action group number, then check that no group appears twice in a season.
• Calculate your true hand-weeding cost per acre from payroll, then compare it against a precision cultivator lease quote.

Insect Pest Management

Three pests do most of the damage in commercial onions, and one of them, onion thrips, does more damage than everything else combined, because it feeds on the crop and vectors a virus at the same time.

Figure 13. Scouting route design and the thrips action threshold.

Onion Thrips: Public Enemy Number One

Thrips feed by rasping leaf tissue and sucking the contents, leaving silvery streaking. At high populations, photosynthetic capacity collapses and bulb size drops sharply. Worse, thrips transmit Iris yellow spot virus (IYSV), which has no cure and no resistant commercial varieties in most markets.

The scouting protocol that works:

1. Walk a zig-zag route with at least 10 stops per 20 to 40 acre block. Avoid headlands and field edges as your primary sample; check them separately since populations build there first.
2. Pull 10 plants per stop. Count thrips on all green leaves, including down inside the neck where they hide.
3. Calculate thrips per green leaf, not thrips per plant. Leaf number changes through the season, and per-plant counts will mislead you.
4. Compare to your regional action threshold, commonly around 1 to 3 thrips per green leaf. Confirm the local figure with your extension entomologist.
5. Scout weekly from the 4-leaf stage, then twice weekly once populations start climbing.

RESISTANCE IS REAL AND IT’S ALREADY HERE Thrips populations have developed resistance to several insecticide classes across U.S. production regions. Rotate by IRAC mode-of-action group, not by product name, and never make more than two consecutive applications from the same group.

The Pest Table

Pest	Damage	Peak Risk Window	Management Approach
Onion thrips	Leaf scarring, reduced bulb size, IYSV transmission	Bulb initiation through bulb fill, hottest and driest weeks	Weekly scouting, threshold-based sprays, MOA rotation, avoid excess N, manage weedy borders
Onion maggot	Larvae tunnel into bulbs and seedlings; stand loss and rot entry	Emergence through early growth; multiple generations	Seed treatments, sanitation of cull piles, avoid fresh manure and green residue, rotation
Seedcorn maggot	Attacks germinating seed and seedlings	First 3 weeks after seeding, cool wet soil	Seed treatment, delay planting after incorporating green residue
Bulb mites	Feeding wounds that open the door to bacterial and fungal rots	Establishment, especially after heavy residue	Rotation away from grain residue, seed treatment, good drainage
Cutworms and armyworms	Stand loss, clipped seedlings	Early season and after weedy field edges are mowed	Scouting for clipped plants, targeted treatment, border management
Leafminers	Mines in leaves reducing photosynthesis	Mid-season, especially in warmer regions	Conserve natural enemies, treat only above threshold
Wireworms	Tunneling into bulbs and roots	Fields recently out of sod or grain	Pre-plant soil sampling with bait stations, field selection
Nematodes (stem and bulb)	Bloated, distorted plants; soft bulbs	Establishment through mid-season	Soil assay before planting, rotation, avoid moving infested soil

Table 17. Key insect and nematode pests of commercial onions. Confirm regional thresholds and registered products with your state extension service.

Beneficials and Biological Control

Minute pirate bugs, predatory mites, lacewings, and several parasitoid wasps all suppress thrips populations. Broad-spectrum insecticides wipe them out and frequently cause thrips flare-ups two weeks later.

Practical steps: use selective chemistries where they’re effective, treat only above threshold, and leave insectary strips or flowering borders where field layout allows it. This isn’t idealism. On operations that have measured it, conserving natural enemies reduces total insecticide applications per season.

For detailed, regularly updated IPM guidance on onion pests, the UC Statewide IPM Program onion and garlic pages are among the most thorough public resources available.

What to Do Next

• Hire or contract a dedicated scout. On 500+ acres this is a full-time role during the season, and it pays for itself.
• Build a written spray program organized by IRAC group with no group repeated more than twice.
• Clean up cull piles and volunteer onions. They’re overwintering sites for both thrips and IYSV.

Disease Management

Onion diseases fall into three buckets: soilborne problems you prevent with rotation and field selection, foliar problems you manage with fungicides and canopy conditions, and storage problems that are actually field problems you didn’t notice until January.

Figure 14. When each major onion disease hits hardest.

The Disease Reference Table

Disease	Pathogen Type	What You See	Favors	Core Management
Downy mildew	Oomycete	Pale oval lesions, fuzzy gray-purple growth in the morning	Cool nights, heavy dew, 6+ hours leaf wetness	Forecast-driven protectant fungicides, wider beds, drip instead of overhead
Purple blotch	Fungus (Alternaria)	Small white centers expanding to purple-brown zonate lesions	Warm, humid, extended leaf wetness	Protectant program from bulb initiation, avoid late-day irrigation
Stemphylium leaf blight	Fungus	Tan to brown lesions that coalesce and kill leaf tips	Warm humid conditions, often follows thrips injury	Thrips control plus fungicide rotation across FRAC groups
Botrytis leaf blight	Fungus	Small white spots with light green halos	Cool wet weather, dense canopy	Canopy management, protectant sprays, avoid excess nitrogen
Botrytis neck rot	Fungus	Softening at the neck after weeks in storage, gray mold	Immature necks, wet harvest, poor curing	Cure the neck fully, harvest at correct maturity, cool the pile promptly
Fusarium basal rot	Soilborne fungus	Yellowing, wilting, brown decay at the basal plate	Warm soil, root injury, tight rotations	Resistant varieties, 4+ year rotation, avoid mechanical root damage
Pink root	Soilborne fungus	Roots turn pink then purple and shrivel; plant stunts	Warm soil, continuous allium ground	Resistant varieties, long rotation, cool soil management
White rot	Soilborne fungus	Fluffy white mycelium and tiny black sclerotia at the base	Cool moist soil; sclerotia survive 20+ years	Prevention only. Strict sanitation, never move infested soil
Bacterial soft rots	Bacteria	Water-soaked scales, foul odor, mushy bulbs	Warm wet weather at maturity, hail or thrips wounds	Avoid rain at harvest, cure quickly, minimize mechanical injury
Iris yellow spot (IYSV)	Virus (thrips-vectored)	Straw-colored diamond lesions on scapes and leaves	High thrips pressure, drought stress	Thrips management, weed host removal, avoid stress
Damping-off	Pythium, Rhizoctonia	Seedlings collapse at the soil line, patchy stands	Cold wet soil, poor drainage, crusting	Seed treatment, warm soil at seeding, drainage
Black mold	Fungus (Aspergillus)	Black powdery spores under the outer scale	High temperature during curing and storage	Cure below 95°F, cool the pile, control humidity

Table 18. Major onion diseases, conditions that favor them, and core management strategies.

Building a Fungicide Program That Doesn’t Break

The goal is protection ahead of infection, not curing after the fact. By the time you can see downy mildew in the field, you’re two weeks behind.

1. Start protectant coverage at bulb initiation, or earlier in high-pressure regions and wet seasons.
2. Use a disease forecasting model where one exists for your region. Several states run onion downy mildew and botrytis models that convert weather data into spray timing.
3. Rotate FRAC groups rigorously. Single-site chemistries are the most effective and the fastest to lose. Always tank-mix or alternate with a multi-site protectant.
4. Get coverage right. Onion leaves are waxy and vertical, which is the worst possible spray target. Use appropriate nozzles, adequate carrier volume, and a labeled adjuvant.
5. Adjust for canopy and weather. Tighten intervals in wet weather, widen them in hot dry conditions, and always respect the maximum applications per season on the label.

STORAGE ROT STARTS IN THE FIELD Nearly every storage disease traces back to a field decision: nitrogen applied too late, harvest during wet weather, mechanical injury on the harvester, or an incomplete cure. Fix it in the field and storage gets easy.

What to Do Next

• Get last year’s problem fields diagnosed properly by a plant diagnostic lab. Guessing at a pathogen leads to spraying the wrong thing.
• Build a season-long fungicide program mapped by FRAC group before the season starts.
• Subscribe to your regional disease forecasting alerts if they exist.

Precision Agriculture, Data, and Farm Records

At 2,000 acres you cannot walk every field. What you can do is instrument the operation so the fields tell you when they need attention.

What’s Actually Worth the Money

Technology	What It Delivers	Typical Payback	Verdict
Soil moisture sensors	Real irrigation scheduling instead of guesswork	1 season	Buy first. Highest return of anything on this list
RTK GPS guidance	Straight beds, no overlap, enables precision cultivation	1 to 2 seasons	Essential if you’re mechanizing weed control
Variable-rate fertility	Matches inputs to soil zones	2 to 4 seasons	Worth it on variable ground, marginal on uniform ground
Drone or satellite NDVI	Early detection of stand gaps, stress, irrigation failures	1 to 2 seasons	Very high value for catching drip failures fast
Weather stations on farm	Local ET, leaf wetness, and disease model inputs	1 to 2 seasons	Cheap and high leverage for spray timing
Storage environment monitoring	Continuous temp, RH, and CO₂ logging inside the pile	1 season	Non-negotiable for long-term holding
Yield and packout mapping	Connects field decisions to actual returns	3+ seasons	Slow payback, but it’s how you get better every year
Farm management software	Traceability, spray records, food safety compliance	Immediate	Required for most buyer audits anyway

Table 19. Precision agriculture technologies ranked by practical return in onion production.

The Records You’re Legally and Commercially Required to Keep

• Pesticide application records: product, EPA registration number, rate, date, time, applicator, weather, field, and acres. Required by law in every state.
• Irrigation water testing under the FSMA Produce Safety Rule, on the schedule your water source requires.
• Worker training records on food safety and hygiene.
• Traceability records connecting each lot from field to pallet.
• Harvest and packing sanitation logs.
• Storage environment logs with continuous data, not spot readings.

The federal framework for produce safety records is the FDA Food Safety Modernization Act. Start with the FDA FSMA resource center and your state’s Produce Safety Program for the version that applies to your operation.

Harvest Timing and Operations

Harvest is where a whole season gets confirmed or destroyed. The window is short, the equipment is unforgiving, and every bruise you put in a bulb in September becomes a rot in January.

Reading Maturity

Figure 15. Reading lodging to time the harvest.

The industry standard trigger is 80 percent of tops down. But lodging percentage on its own isn’t enough. Check three things together:

1. Lodging. Roughly 80 percent of tops fallen over naturally, not knocked down by wind or equipment.
2. Neck tightness. Squeeze the neck just above the bulb. It should feel firm and closed, not soft and juicy.
3. Skin set. Rub a bulb. The outer scales should be dry, papery, and firmly attached. Two to three intact skins is the commercial target.

DO NOT FORCE LODGING Rolling or crimping tops to force early maturity produces immature necks, poor skin set, and storage failure. If the crop isn’t ready, it isn’t ready. Fix the scheduling problem next year with variety maturity spread, not with a roller.

The Harvest Sequence

Step	Operation	Key Settings and Cautions
1	Terminate irrigation	7 to 14 days before undercutting. Longer on heavy soils.
2	Undercut	Sever roots 1 to 2 in below the basal plate. Sharp blades, correct depth, minimal soil disturbance.
3	Field cure in windrow	5 to 10 days in dry climates. Tops shade the bulbs from sunscald. Skip or shorten if rain is forecast.
4	Top and lift	Leave a 1 to 1.5 in neck stub. Topping too close opens a direct rot pathway.
5	Load into bins or trucks	Keep every drop height under 6 in. Padding at every transfer point.
6	Haul to storage	Same day. Do not leave loaded bins in the sun.
7	Load and start airflow	Fans on immediately. Curing begins the hour the pile is built.

Table 20. The commercial onion harvest sequence, step by step.

Bruising: The Damage You Cannot See

Onion bruises don’t show up as marks. They show up eight weeks later as a soft spot with bacterial rot spreading out from it. A single bruised bulb in a bin can start a rot pocket that costs you a ton of product.

• Drop height is the number one variable. Every drop over 6 inches damages tissue. Audit your entire chain with a pressure-sensing test sphere.
• Pad every transfer point on the harvester, the conveyor, and the bin filler.
• Slow the line down during loading. Faster is not cheaper if it costs you 3 percent of the crop.
• Harvest at the right pulp temperature. Bulbs harvested in extreme heat bruise more easily and enter storage with a heat load you have to remove.
• Train the crew and re-train them mid-season when the pace picks up.

Weather Contingencies

Two situations require immediate decisions:

• Rain forecast during field curing. Get the crop up and into forced-air curing rather than letting it sit wet in the windrow. Wet bulbs plus warm weather equals bacterial soft rot within 48 hours.
• Frost forecast with bulbs exposed. Bulbs that freeze in the windrow are unmarketable. Cover or lift before the event.
• Extreme heat during undercut. Sunscald on exposed bulbs shows up as bleached, sunken areas that rot in storage. Orient windrows so tops shade bulbs, and shorten field curing.

What to Do Next

• Do a drop-height audit on your entire harvest and handling chain before the season, using an instrumented test sphere.
• Sequence your harvest so the longest-storing varieties come out of the field in the best weather.
• Assign one person the authority to stop harvest for weather. Committee decisions are always too slow.

Curing: The Most Underrated Step in the Whole Operation

Curing does two things. It seals the neck so pathogens can’t walk into the bulb, and it dries the outer scales into the papery skin that buyers pay for. Skip it, rush it, or overheat it, and no amount of refrigeration will save the crop.

An onion is properly cured when the neck is tight and completely dry and the outer scales rustle when you handle them. That’s the standard. Everything else is a means to that end.

Figure 16. The three-phase cure, dry, and cool schedule.

Field Curing Versus Forced-Air Curing

Method	How It Works	Best For	Limitations
Field curing in windrow	Bulbs cure in the field under their own tops for 5 to 10 days	Arid regions with reliable dry harvest weather	Rain ruins it. Sunscald risk. No control over the process.
Forced-air in storage	Heated air pushed through the pile on a controlled schedule	Any region with harvest rain risk; all long-term storage	Capital cost, energy cost, requires good building design
Hybrid	Short field cure then finish with forced air	Most commercial storage operations	Requires the discipline to pull the crop when weather turns
Artificial dryers	Dedicated batch drying before storage loading	Processing and dehydration contracts	Extra handling step, more bruise exposure

Table 21. Curing method comparison.

The Three-Phase Schedule

A widely used commercial approach runs curing in three distinct phases, each with a different job.

Phase	Duration	Air Temp	Relative Humidity	Objective and Endpoint
1. Cure (heat)	3 to 5 days	85 to 95°F	65 to 75%	Seal the neck. Done when exhaust air temp matches inlet air temp and necks are dry.
2. Dry	5 to 10 days	Ambient, 75 to 85°F	60 to 75%	Finish the outer scales. Done when necks are paper-dry and skins rustle.
3. Cool	2 to 6 weeks	Step down about 1°F per day	65 to 70%	Reach holding temperature without condensation or chill injury.

Table 22. Three-phase curing schedule for commercial storage onions. Adapt setpoints to your variety, region, and building.

THE TEMPERATURE CEILING Hold curing air below about 95°F. Above that you’re not curing, you’re cooking. Excess heat drives black mold (Aspergillus) and translucent scale, and both show up as storage losses.

Airflow Is the Whole Game

Heat without airflow is useless. You need enough air moving through the pile to carry moisture out, and you need it distributed evenly so there are no dead spots.

• Curing airflow: commercial guidance commonly lands in the range of 50 to 70 cubic feet per minute for every 35 cubic feet of onions, tapering after the cure is complete.
• Even distribution beats raw horsepower. A big fan pushing air through a channel does nothing for the corner of the pile that isn’t getting any.
• Watch the exhaust air. When exhaust temperature equals inlet temperature and exhaust humidity stops dropping, the cure phase is finished.
• Don’t over-dry. Excessive drying causes shrink and scale cracking. The target is dry outer scales and a firm neck, not a dehydrated bulb.

Common Curing Mistakes

• Loading wet onions and delaying fan startup. Fans go on the same hour the pile is built.
• Curing too hot to save time. You’ll pay for it in black mold.
• Cooling too fast. Rapid cooling causes condensation on the bulbs, which is exactly the moisture you spent two weeks removing.
• Piling too deep for the fan capacity. Deeper piles need proportionally more static pressure and airflow, not the same fan running longer.
• Mixing lots with different maturities in one pile. The immature portion sets the rot risk for the whole building.

Storage Engineering

A storage building is a machine, not a shed. Its job is to hold hundreds or thousands of tons of living, respiring product in dormancy for up to nine months. Every part of the design either supports that or works against it.

Figure 17. Bulk storage building with underfloor ventilation.

The Setpoints

Parameter	Target	Why	Consequence of Getting It Wrong
Temperature	32 to 34°F for long-term holding	Suppresses respiration, sprouting, and rot organisms	Above 40°F sprouting accelerates; freezing damages tissue
Relative humidity	65 to 70%	Low enough to suppress rot and root growth, high enough to limit shrink	Above 75% invites rot and rooting; below 60% causes excessive shrink
Airflow (holding)	Reduced from curing rate, periodic cycling	Removes respiration heat and CO₂, prevents stratification	Dead spots become rot pockets
Carbon dioxide	Below about 0.5% in most operations	Elevated CO₂ causes physiological disorders and off-odors	Internal breakdown, off-flavors, rejected loads
Light	Dark	Light triggers greening and sprouting	Unsaleable product
Pile depth	Typically 12 to 16 ft in bulk storage	Deeper piles need more static pressure and better duct design	Compression damage and poor air distribution at depth
Space allowance	Roughly 90 to 110 cu ft per ton	Basic capacity planning figure for bulk piles	Undersized buildings force premature marketing

Table 23. Storage setpoints and design parameters for commercial onions.

Building Design Essentials

1. Underfloor plenum with lateral ducts sized so air reaches every part of the pile at similar velocity. This is the single most important design element.
2. Fresh air intake, recirculation, and exhaust with automatic mixing dampers, so the control system can use free outside air whenever conditions allow it.
3. Refrigeration sized for the warmest expected loading conditions, not average conditions. You need to pull field heat out of a fresh pile.
4. Humidity control that can both add and remove moisture. In many climates a desiccant system is the practical answer for holding 65 to 70 percent RH at 32°F.
5. Insulation and a continuous vapor barrier. Condensation on a cold wall drips into the pile and starts a rot column.
6. Continuous monitoring inside the pile at several depths, not just a thermostat on the wall. Pile temperature and wall temperature are different numbers.
7. Sanitation access. Design so the building can actually be swept and disinfected between seasons.

Warming Onions Out of Storage

This step gets skipped constantly and causes enormous losses. When you pull 33°F onions into a 75°F humid loading dock, condensation forms on every bulb instantly. That surface moisture is a perfect rot medium, and the load rots in transit.

The fix is straightforward: warm the product gradually until it’s above the dew point of the destination environment, then move it. A common guideline limits warming to about 2°F per day, which means planning a week or more into your shipping schedule.

Storage Loss Budgeting

Loss Category	Typical Range	Primary Driver
Moisture shrink	3 to 8%	Time in storage, RH control, initial dry matter
Rot and decay	2 to 10%	Field maturity, curing quality, mechanical injury
Sprouting and rooting	0 to 6%	Temperature control, variety dormancy, late nitrogen
Grade-out on packing	5 to 15%	Size distribution, skin quality, defects
Total from bin to sale	12 to 30%	Cumulative

Table 24. Storage loss categories. Budget conservatively; a well-run operation lands at the low end, and a bad crop year can exceed the high end.

MONITOR BY WALKING THE PILE Instruments catch trends. Your nose catches problems. Walk every storage weekly, smell the exhaust air, and check for warm spots by hand. A rot pocket announces itself by odor days before it shows on a sensor.

Grading, Packing, and Food Safety

Everything up to this point produced a pile of onions. This section turns it into a product with a price.

Figure 18. Packing line flow with quality checkpoints.

Size Grades and Their Markets

Figure 19. Commercial onion size grades and the markets that buy them.

Federal grade standards for onions are published by the USDA Agricultural Marketing Service. Get the current standard for your onion type from USDA AMS grades and standards. Note that buyer specifications are frequently tighter than the federal minimum, so contract language is what you actually pack to.

Defects That Cost You Money

Defect	Cause	Prevention
Doubles and splits	Excess water or nitrogen at bulb initiation, temperature swings	Even irrigation, variety selection, avoid stress cycles
Bottlenecks / thick necks	Late nitrogen, immature harvest, excess water at maturity	Nitrogen cutoff, correct harvest timing, irrigation termination
Sunscald	Bulbs exposed to direct sun during field curing	Orient windrows so tops cover bulbs; shorten field cure in heat
Greening	Bulb shoulders exposed to light in field or storage	Adequate bed cover, dark storage
Skin slip / bald onions	Rough handling, over-drying, immature skin set	Reduce drop heights, correct curing, harvest at proper skin set
Translucent scale	Heat stress during bulb fill or overheating during cure	Cure below 95°F, manage heat during bulb sizing
Black mold	Aspergillus in warm, humid curing or storage	Temperature control, prompt cooling
Sprouting	Storage temperature too high, dormancy break	Hold at 32 to 34°F, variety with strong dormancy
Root growth	Storage humidity too high	Hold RH at 65 to 70%
Bruising and internal decay	Drop heights, rough handling, harvest at high pulp temp	Padding, slower line speeds, drop-height audits

Table 25. Onion quality defects, their causes, and prevention.

Food Safety Compliance

Onions are a covered commodity under the FDA’s Produce Safety Rule for most commercial operations. Beyond the legal requirement, virtually every retail and foodservice buyer now requires a third-party audit.

Requirement Area	What You Need	Who Requires It
Agricultural water	Water source assessment, testing on the required schedule, corrective actions	FDA Produce Safety Rule
Biological soil amendments	Records of source, treatment, and application intervals	FDA Produce Safety Rule
Worker health and hygiene	Documented training, facilities, illness reporting policy	FDA Produce Safety Rule; all audit schemes
Domesticated and wild animals	Pre-harvest assessment, exclusion measures, records	FDA Produce Safety Rule
Equipment, tools, buildings	Cleaning and sanitation SOPs and logs	FDA Produce Safety Rule; all audit schemes
Traceability	Lot coding from field through pallet, one-up one-back records	FSMA; buyer requirements
Third-party audit	GAP/GHP, PrimusGFS, GlobalG.A.P., or SQF certification	Retail and foodservice buyers
Recall program	Written mock recall performed at least annually	Audit schemes; good practice regardless

Table 26. Food safety compliance requirements for commercial onion operations.

START THE AUDIT PROCESS EARLY First-time GAP certification typically takes three to six months of documentation work before the auditor ever arrives. Do not wait until a buyer asks. Contact your state Department of Agriculture Produce Safety Program for free assistance, which most states offer.

Packing Line Design Principles

• Minimize transfers. Every transfer point is a bruise point. Fewer, gentler handoffs beat more sorting stages.
• Control drop heights everywhere. Under 6 inches, with padding at every drop.
• Size before you grade optically. Pre-sizing rollers reduce optical grader load and improve accuracy.
• Keep a human in the loop. Optical graders miss internal defects and neck problems that trained hands catch.
• Design for washdown and sanitation from the start, not as a retrofit.
• Log a quality checkpoint at every stage. Those logs double as your traceability record.

Labor, Equipment, and Mechanization

Labor is usually the largest single cost line in onion production, and it’s the one that’s hardest to plan around. Availability keeps tightening, and wage rates keep climbing. That pressure is the main reason mechanization keeps advancing in this crop.

The Core Equipment Fleet

Equipment	Purpose	Scale Threshold	Own or Custom Hire?
Subsoiler / ripper	Break compaction pans below the root zone	Any scale	Own
Bed shaper with press	Form and firm uniform beds	Any scale	Own
Precision vacuum planter	Accurate seed spacing and depth	Any scale	Own
Drip tape layer / retriever	Install and remove tape efficiently	Drip systems	Own
Fertigation injection skid	Deliver split nutrition through irrigation	Drip systems	Own
High-clearance sprayer	Timely fungicide and insecticide coverage	200+ acres	Own; timeliness is everything
Camera-guided cultivator	Mechanical weed control near the row	300+ acres	Own if hand-weeding spend is high
Undercutter / lifter	Sever roots ahead of harvest	Any scale	Own
Topper / windrower	Remove tops, form windrows	Any scale	Own
Self-propelled harvester	Lift, clean, and load bulbs	500+ acres	Own above 500 ac; custom hire below
Bin filler with padding	Gentle bin loading	Any scale with bins	Own
Storage fans and controls	Curing and holding environment	Storage operations	Own
Optical grader	Size and defect sorting at throughput	1,000+ acres or shared packing	Own or co-op
Bagging and palletizing line	Consumer and bulk pack	Packing operations	Own or co-op

Table 27. Core equipment fleet for commercial onion production, with scale thresholds for ownership.

Planning Labor Requirements

Labor demand in onions is spiky. Two peaks dominate: weeding in early summer and harvest in late summer. Between them, the crew requirement drops by 70 percent or more.

• Weeding peak: the largest hand-labor demand in most operations, and the one mechanization targets first.
• Harvest peak: shorter but more intense, and the one where a labor shortage costs you the crop rather than just the margin.
• H-2A and seasonal programs: if you use them, applications go in months ahead. Build that into your calendar.
• Cross-training pays. A crew that can move between the packing line and the field absorbs weather disruptions without idle time.
• Housing and transport are often the real constraints on crew size, not wages.

The Mechanization Decision

Run this calculation before any major equipment purchase:

1. Total your actual annual spend on the task you’d be replacing, including wages, payroll taxes, housing, transport, and supervision.
2. Get a real quote on the machine, including installation, training, and expected annual maintenance.
3. Estimate the labor you’ll still need after mechanization. It’s rarely zero.
4. Divide the net machine cost by the net annual savings. If payback is under three years and the labor supply is uncertain, buy it.
5. Sanity-check the operational fit: does it match your bed width, your row spacing, and your soil conditions?

The Economics: What It Costs and What It Returns

This is the section that decides whether everything before it matters. Onion production carries a high cost per acre relative to most field crops, and the returns swing hard from year to year.

Figure 20. Cost structure and break-even economics.

A Representative Cost of Production

The table below is a planning framework, not a quote. Costs vary enormously by region, irrigation method, land tenure, and whether you own storage. Build your own version with your actual numbers.

Cost Category	Typical Range ($/acre)	Notes
Land rent or ownership cost	$300 to $1,200	Highest in irrigated valleys with limited ground
Seed	$180 to $700	Higher for pelleted hybrid storage varieties
Transplants (if used)	$800 to $2,000	Replaces the seed line, not additional to it
Tillage and bed forming	$120 to $300	Fuel, wear parts, labor
Fertility program	$400 to $900	Split fertigation, micronutrients, soil amendments
Irrigation: system amortization	$150 to $500	Drip tape, filters, pumps, annualized
Irrigation: water and energy	$180 to $600	Highly regional; pumping lift matters
Herbicide program	$150 to $400	Multiple applications across the season
Insecticide program	$150 to $450	Thrips pressure is the main driver
Fungicide program	$200 to $550	Wet regions at the high end
Hand weeding	$200 to $1,400	The widest range on this table, and the biggest lever
Scouting and crop consulting	$25 to $80	Cheapest insurance you can buy
Harvest: undercut, top, lift	$300 to $700	Custom rates or owned equipment
Hauling to storage	$80 to $220	Distance-dependent
Curing and storage	$250 to $800	Energy, depreciation, monitoring, shrink
Grading and packing	$400 to $1,100	Labor, materials, line operation
Packaging materials	$200 to $600	Bags, cartons, pallets, labels
Crop insurance	$40 to $180	Coverage level dependent
Overhead, management, compliance	$150 to $450	Food safety, records, admin
Total (indicative)	$3,500 to $12,000	Direct-seeded storage crops typically land in the middle

Table 28. Indicative cost of production framework for commercial onions. Use your own regional data and current custom rates; these ranges are for structuring a budget, not for quoting one.

Break-Even Math

The break-even equation is simple, and worth writing on a whiteboard:

Break-even price per cwt  =  Total cost per acre  ÷  Marketable yield in cwt per acre

Two things follow from that, and both are worth internalizing.

• Yield is the cheapest cost-reduction tool you have. Going from 600 to 800 cwt per acre at a fixed $8,000 cost drops break-even from $13.33 to $10.00 per cwt. You can’t cut $2,600 per acre out of the budget without wrecking the crop, but you might be able to add 200 cwt with better irrigation scheduling.
• Marketable yield is what counts, not gross yield. A 900 cwt field with a 25 percent grade-out is a 675 cwt field. Quality and yield are the same conversation.

Sensitivity: The Numbers That Move Profit Most

Variable	Change	Approximate Profit Impact per Acre	Controllability
Market price	± $2.00/cwt on 700 cwt	± $1,400	Low. Manage with contracts.
Marketable yield	± 100 cwt/ac	± $1,000 to $1,600	High. This is where management pays.
Storage loss	± 5 percentage points	± $350 to $700	High. Curing and storage discipline.
Hand-weeding cost	± $400/ac	± $400	High. Program design and mechanization.
Packout percentage	± 5 percentage points	± $350 to $700	Moderate to high.
Water and energy	± 25%	± $45 to $150	Moderate. Efficiency and scheduling.
Crop protection spend	± 25%	± $125 to $350	Moderate. Threshold-based decisions.

Table 29. Profit sensitivity analysis. Notice that the variables you control most are also among the largest.

WHERE TO FOCUS If you only have bandwidth to improve two things this season, make them irrigation scheduling and storage management. Together they influence roughly half the profit variance in a typical commercial onion enterprise.

Risk Management and Marketing

Onion prices are volatile enough that a grower can do everything right agronomically and still lose money. Risk management isn’t optional at this scale.

The Risk Register

Risk	Likelihood	Impact	Mitigation
Price collapse at harvest	High	Severe	Forward contracts, storage capacity, staggered marketing
Storage loss above budget	Moderate	Severe	Curing discipline, monitoring, inspect and market problem lots first
Hail or wind at bulb fill	Moderate	Severe	Crop insurance, geographic spread of acreage
Rain during harvest	Moderate to high	Severe	Forced-air curing capacity, flexible harvest scheduling
Labor shortage at harvest	High	Severe	Mechanization, H-2A planning, cross-trained crews
Thrips or disease outbreak	High	Moderate	Scouting program, MOA rotation, forecasting models
Water curtailment	Regional	Severe	Water rights security, storage ponds, efficient systems
Buyer default or rejection	Low to moderate	Moderate	Credit checks, clear grade language, diversified buyers
Food safety incident	Low	Catastrophic	Certified program, traceability, mock recalls, insurance
Herbicide carryover injury	Low	Moderate	Documented field history, bioassay before planting

Table 30. Risk register for commercial onion production.

Federal crop insurance options for onions vary by county and program year. Start with the USDA Risk Management Agency and talk to a crop insurance agent who has actually written onion policies in your region. Coverage details differ meaningfully from row-crop programs.

Marketing Strategy

• Stagger your marketing. Selling everything in October puts you at the mercy of the harvest-window price. Storage exists precisely so you don’t have to.
• Know your carrying cost. Holding onions costs money every month: energy, shrink, capital tied up, and rot risk. If the forward price doesn’t beat your carrying cost, sell.
• Market your problem lots first. The lot that looks marginal in November will be worthless in February. Inspect, sort, and move it.
• Diversify buyers. One buyer taking 80 percent of your production is a single point of failure.
• Track market news weekly. USDA Agricultural Marketing Service publishes daily and weekly onion market reports covering shipping-point prices by region.

Rotation, Soil Health, and Sustainability

Onions are hard on soil and hard on rotations. They come out late, they leave little residue, and they demand a long interval before the next allium crop. Building a rotation that works over a decade is what keeps the enterprise viable.

Designing the Rotation

Rotation Slot	Good Choices	Why	Avoid
Year before onions	Small grains, sweet corn, sudangrass	Residue builds structure and breaks disease cycles	Any allium; heavy legume residue that releases late N
Year after onions	Cover crop then a cereal	Rebuilds organic matter after a low-residue crop	Another high-value shallow-rooted crop
Minimum allium interval	3 to 5 years, longer with disease history	Fusarium, pink root, and white rot persist in soil	Tight rotations of 2 years or less
Cover crop windows	Fall-seeded cereal rye or a mixed cover	Erosion control, organic matter, nematode suppression	Brassica covers where shared pests are a concern

Table 31. Rotation design for sustainable onion production.

Practices That Pay for Themselves

• Controlled traffic. Confining wheel traffic to permanent lanes prevents the compaction that limits onion rooting.
• Reduced tillage where it fits. Full conventional tillage is standard for onions, but strip-till and reduced secondary passes work on some soils and save real money.
• Cover crops in every gap. Bare ground between onion crops is lost organic matter and lost erosion protection.
• Irrigation efficiency. Drip conversion cuts water use 25 to 40 percent versus furrow while raising yield. That’s the rare change that improves both the balance sheet and the watershed.
• Nutrient budgeting to actual crop removal. Applying more nitrogen than the crop takes up costs money and moves into groundwater.
• Cull and waste management. Onion culls are a substantial waste stream and a pest reservoir. Compost them properly, well away from production fields.

Technical and cost-share assistance for conservation practices is available through the USDA Natural Resources Conservation Service. Many onion-relevant practices, including irrigation efficiency upgrades and cover cropping, qualify for cost-share programs.

Troubleshooting: Fifteen Ways Onion Crops Fail

Find the symptom, work back to the cause. Most of these are preventable next season even when they can’t be fixed this one.

Symptom	Most Likely Causes	What To Do Now	Prevent Next Season
Patchy, thin stand	Soil crusting, cold soil at seeding, seed depth variation, damping-off	Stand count and map it; decide replant vs. accept early	Light frequent irrigation to emergence, seed treatment, planter calibration
Plants stunted and pale	Nitrogen shortfall, compaction, pink root, salinity	Tissue test plus dig plants to inspect roots	Compaction management, split fertigation, resistant varieties
Bulbs too small at harvest	Too few leaves at bulb initiation, high population, water stress, thrips	Nothing this season. Document and diagnose	Protect early growth, cut population, tighten irrigation scheduling
Widespread bolting	Wrong planting date, oversized plants before cold, susceptible variety	Rogue seed stalks; harvest and market bolted bulbs immediately	Correct planting window, bolt-tolerant varieties, avoid early N push
Doubles and splits	Moisture or temperature swings at bulb initiation, excess N	Grade them out at packing	Steady irrigation, controlled N, variety selection
Thick necks at harvest	Late nitrogen, irrigation continued too long, immature harvest	Cure aggressively; market these lots first	Hard N cutoff, irrigation termination discipline
Silvery streaking on leaves	Onion thrips feeding	Scout, confirm counts, treat above threshold with a rotated MOA	Weekly scouting program from 4-leaf stage
Straw-colored diamond lesions	Iris yellow spot virus	No cure. Manage thrips to slow spread; harvest affected blocks first	Thrips control, weed host removal, avoid drought stress
Leaves dying from tip down	Purple blotch, Stemphylium, or botrytis leaf blight	Get a lab diagnosis, then apply the correct FRAC-rotated program	Protectant program from bulb initiation, forecasting models
Rot at the basal plate	Fusarium basal rot	Harvest and market affected blocks first; do not store	4+ year rotation, resistant varieties, avoid root injury
White fluffy growth with black specks	White rot	Isolate. Harvest last. Sanitize all equipment	Prevention only. Field selection and strict sanitation
Bulbs soft and foul-smelling	Bacterial soft rot	Sort aggressively; do not put these in storage	Avoid wet harvest, minimize wounds, cure promptly
Black powder under outer scale	Black mold (Aspergillus)	Cool the pile; grade out affected bulbs	Keep curing below 95°F, cool promptly
Sprouting in storage	Storage temperature too high, dormancy break, late N	Move the lot to market immediately	Hold 32 to 34°F, choose dormant varieties, N cutoff
Condensation and rot on shipped loads	Cold onions moved into warm humid air	Stop shipping; warm the product above dew point first	Plan a gradual warm-up of about 2°F per day into the shipping schedule

Table 32. Onion crop troubleshooting matrix.

Your Twelve-Month Operating Calendar

Onion farming is a scheduling problem as much as an agronomic one. The calendar below is built for a northern long-day storage crop. For a southern short-day fall-planted crop, shift the whole block roughly six months earlier.

Figure 21. Twelve-month operating calendar for a northern long-day storage crop.

Period	Priority Actions
Off-season (Nov to Jan)	Soil sampling by zone; rotation planning; variety selection and seed orders; equipment overhaul; storage monitoring; budget and contract negotiation; food safety documentation review
Pre-plant (Feb to Mar)	Deep tillage if not done in fall; lime or amendments; bed forming and pressing; drip system installation and pressure testing; pre-plant fertility; planter calibration
Planting (Mar to Apr)	Seeding or transplanting on the calibrated planter; pre-emergent herbicide with incorporation irrigation; light frequent irrigation through emergence; crust monitoring
Establishment (Apr to May)	Stand counts at 21 days; first post-emergent herbicide at the labeled leaf stage; begin cultivation; start weekly scouting; first tissue test at 5-leaf
Leaf building (May to Jun)	Peak fertigation; cultivation and hand weeding; thrips scouting twice weekly; begin protectant fungicide program; irrigation scheduling from soil moisture data
Bulb initiation (Jun to Jul)	Maximum irrigation discipline; K and Ca emphasis; sustained IPM pressure; monitor for IYSV; final tissue tests
Bulb fill (Jul to Aug)	Peak water demand; nitrogen cutoff at 3 to 4 weeks before expected maturity; continue fungicide rotation; storage building preparation and sanitation
Maturation (Aug to Sep)	Monitor lodging, neck tightness, and skin set; terminate irrigation 7 to 14 days before undercut; final equipment checks; crew scheduling
Harvest (Sep to Oct)	Undercut at 80% lodging; field cure; top and lift; haul same day; load storage and start fans immediately; drop-height discipline
Curing (Sep to Nov)	Three-phase cure, dry, and cool schedule; continuous monitoring; identify and market any problem lots early
Packing and shipping (Oct to Jun)	Grading and packing; gradual warm-up before shipping; staggered marketing against carrying cost; inventory and quality checks
Year-round	Records: spray logs, irrigation, food safety, traceability, storage environment, labor

Table 33. Twelve-month priority actions for a commercial onion operation.

Scaling From 50 Acres to 2,000

The agronomy doesn’t change much as you scale. Everything else does.

What Changes	At 50 Acres	At 500 Acres	At 2,000 Acres
Irrigation	One zone, manual scheduling	Multiple zones, sensor-driven	Automated control, full-time water manager
Scouting	The owner walks the fields	Dedicated seasonal scout	Scouting team plus consulting agronomist
Weeding	Hand crew	Hand crew plus cultivation	Precision cultivation, hand crew as rescue only
Harvest	Custom hire	Owned harvester, one crew	Multiple crews, staggered maturity varieties
Curing	Field cure, maybe a shed	One forced-air building	Multiple buildings, staged loading
Packing	Co-op or third party	Basic owned line	Full optical grading and packing operation
Marketing	One or two buyers	Contract plus open market split	Sales team, multi-buyer program
Records	Spreadsheets	Farm management software	Integrated ERP and traceability system
Biggest risk	Weather	Labor and storage loss	Price exposure and food safety

Table 34. What changes as an onion operation scales.

THE SCALING TRAP The most common failure in scaling onion production is adding acres without adding curing and storage capacity to match. You end up forced to sell everything at harvest, in the worst price window of the year, which is exactly the outcome storage exists to prevent. Add the building before you add the acres.

Frequently Asked Questions

How many acres do you need to grow onions commercially?

There’s no single threshold, but the economics change at recognizable points. Below about 50 acres, custom hiring your harvest and packing through a co-op usually makes more sense than owning equipment. Between 200 and 500 acres, owning a harvester and a curing building starts to pencil out. Above 1,000 acres you’re running a full operation with dedicated scouting, storage, and packing staff. What matters more than acreage is whether you have committed buyers and enough curing capacity for the volume you plant.

What is a good onion yield per acre?

U.S. onion yields have averaged around 500 hundredweight per acre across all types. Drip-irrigated long-day storage onions in the Pacific Northwest commonly run 700 to 900 cwt per acre, with top blocks exceeding 1,000. Short-day fresh-market crops typically run 350 to 600. Budget at the low end of your system’s realistic range, not the top.

How long does it take to grow onions from seed to harvest?

Direct-seeded onions generally need 150 to 190 days from seeding to harvest, depending on variety and climate. Transplanted onions reach harvest in 110 to 140 days after setting, though you add six to ten weeks of greenhouse time before that. Sets are fastest at 90 to 110 days but carry the highest bolting risk.

Why are my onions small?

Almost always because the plants had too few leaves when bulb initiation started. Each leaf becomes a bulb scale, and no new leaves form after bulbing begins. Common causes are a poor early stand, nitrogen shortfall during leaf building, water stress, thrips damage, compaction limiting rooting, or a plant population that’s simply too high for the size you want.

What is the best soil for growing onions at scale?

Well-drained sandy loam, silt loam, or well-managed muck soil with a pH between 6.0 and 6.8, salinity below 1.2 dS/m, and no compaction pan above 14 inches. Onions have shallow, unbranched roots that don’t push through resistance, so soil physical condition matters as much as fertility.

How much water do onions need per acre?

Roughly 24 to 36 acre-inches of applied water per season, depending on climate, soil type, and irrigation method. More important than the total is the timing: keep soil moisture depletion at or below 25 percent from bulb initiation through bulb fill, because onions show almost no visible wilt before yield is already lost.

When should you stop watering onions before harvest?

Generally when 10 to 30 percent of tops have lodged, which usually means 7 to 14 days before undercutting. Stopping too early sacrifices bulb size in the final weeks. Stopping too late produces thick necks, delayed maturity, poor skin set, and onions that won’t cure or store.

How do you know when onions are ready to harvest?

Check three signs together: about 80 percent of tops have fallen over naturally, the necks feel firm and closed when squeezed, and the outer scales are dry, papery, and firmly attached. Lodging alone isn’t enough, because wind and equipment can knock tops down before the bulb is actually mature.

What is curing and why does it matter so much?

Curing seals the neck against pathogens and dries the outer scales into the papery skin buyers pay for. An onion is cured when the neck is tight and completely dry and the scales rustle. Without a proper cure, bulbs develop neck rot in storage no matter how good the refrigeration is.

What temperature and humidity should onions be stored at?

For long-term storage, hold onions at 32 to 34°F with relative humidity between 65 and 70 percent, in the dark, with enough airflow to remove respiration heat and carbon dioxide. Higher humidity invites rot and rooting; lower humidity causes excessive shrink.

How long can onions be stored?

It depends entirely on the type. Sweet short-day onions hold 30 to 90 days. Intermediate-day varieties manage 90 to 150 days. High-solids long-day storage varieties, properly cured and held at 32°F, can go 150 to 270 days. Dry matter content is the best single predictor of storage life.

Why do onions rot in storage?

Nearly always because of a field decision, not a storage failure. The usual culprits are nitrogen applied too late in the season, harvest during wet weather, mechanical bruising during handling, incomplete curing, or cooling the pile too quickly and causing condensation.

What is the most damaging onion pest?

Onion thrips, by a wide margin. They feed on leaves and reduce photosynthesis, and they transmit Iris yellow spot virus, for which there’s no cure. Weekly scouting from the 4-leaf stage and threshold-based treatment with rotated modes of action is the standard commercial approach.

Can you grow onions in the same field every year?

No. A minimum three-to-five-year rotation away from all alliums is standard practice, and longer if you have any history of Fusarium basal rot or pink root. White rot is the extreme case, with sclerotia surviving in soil for twenty years or more, which is why field selection and sanitation matter so much.

Are sweet onions and storage onions the same crop?

They’re the same species but very different products. Sweet onions have low sulfur compounds and low dry matter, often below 8 percent, which is why they taste mild and why they don’t store. Storage onions run 12 to 16 percent dry matter, and dehydration varieties reach 18 to 22 percent. You cannot grow a sweet onion and hold it for eight months.

What does it cost to grow an acre of onions?

Depending on region, irrigation method, land tenure, and whether you own storage, total costs commonly land somewhere between $3,500 and $12,000 per acre. Labor and harvest are typically the largest category, followed by packing and storage. Build your own budget from local custom rates rather than relying on national figures.

Is drip irrigation worth it for onions?

In most commercial situations, yes. Drip typically reaches 88 to 95 percent application efficiency versus 50 to 65 percent for furrow, cuts water use 25 to 40 percent, keeps foliage dry to reduce disease pressure, and enables the split fertigation that onions respond to strongly. The capital cost is real, but so is the yield response.

How do I stop onions from bolting?

Bolting happens when a plant large enough to respond is exposed to sustained cold, then warms up. Prevention means planting within your region’s recommended window so plants don’t get too big before winter, choosing bolt-tolerant varieties, avoiding early nitrogen that pushes fall growth, and using sets no larger than about 3/4 inch if you plant sets.

What certifications do commercial onion growers need?

Most operations are covered by the FDA Produce Safety Rule under FSMA. Beyond that legal baseline, nearly all retail and foodservice buyers require a third-party audit such as USDA GAP/GHP, PrimusGFS, GlobalG.A.P., or SQF. First-time certification usually takes three to six months of documentation work, so start well ahead of when a buyer asks.

What’s the single highest-return improvement for an onion operation?

For most growers, irrigation scheduling based on real soil moisture data rather than a calendar. It usually pays back within one season, and it directly protects the bulb-sizing window where yield is won or lost. A close second is storage management discipline, which protects the crop you already paid to grow.

Bringing It Together

If you take one idea from this guide, make it this: to grow onions at an industrial level, you’re running an integrated system, not a sequence of independent tasks. The bed you form in March determines the stand you get in April. The stand determines the leaf count in June. The leaf count sets your yield ceiling in July. The nitrogen cutoff in August determines whether the crop cures in September. And the cure determines whether you still have a crop in February.

Every one of those links is a place where a shortcut costs you more than it saves. That’s the hard part of this crop, and it’s also why well-run onion operations are so consistently profitable while poorly run ones aren’t. The gap between a 400 cwt block and a 900 cwt block, on the same ground with the same seed, is entirely management.

Start with the three that move the needle most: match your variety to your latitude, schedule irrigation from actual soil moisture data, and cure the crop properly before you cool it. Get those three right and the rest of the system has a chance to work.

YOUR NEXT STEP Pick the single section that maps to your biggest current weakness, work through its ‘What to do next’ list this week, and put the actions on the calendar with names attached. Systems improve when specific people own specific tasks on specific dates. If you’d like help building a site-specific production plan, soil and water testing schedule, or storage design review, get in touch with our agronomy team.