Three methods to grow onions, three different economics, and one selection mistake that causes more failure than anything else. The mistake: buying long-day onion sets in a short-day climate, or short-day transplants in a long-day climate. The onion either never bulbs properly or bolts before maturing. Getting the day-length type right is the prerequisite before any financial analysis matters.
Long-Day vs. Short-Day: The Non-Negotiable Starting Point
Onions (Allium cepa) form bulbs in response to day length. The trigger is how many hours of light the plant receives per day.
Long-day varieties: bulb when days exceed 14-16 hours. This happens from late spring through summer at latitudes above 35-36°N. Long-day onions are the correct choice for most of the northern US - everything north of roughly the Tennessee/Kentucky border and southward through Kansas. Varieties: ‘Stuttgarter’, ‘Yellow Sweet Spanish’, ‘Walla Walla’, ‘Patterson’.
Short-day varieties: bulb when days exceed 10-12 hours. This happens from late winter through early spring in the southern US, where short-day onions can be planted in fall and harvested the following spring. Short-day onions are the correct choice for zones 7-9 in the Deep South. Varieties: ‘Granex’ (the Vidalia type), ‘Texas Super Sweet’, ‘Southern Belle’.
Day-neutral (intermediate) varieties: bulb across a wider day-length range (12-14 hours). Work across a broader geographic range and are useful in the transition zone between long and short-day regions (roughly zones 6-7). Varieties: ‘Candy’, ‘Red Amposta’, ‘Sierra Blanca’.
The failure mode: a short-day onion planted in a northern long-day climate will form a small bulb too early (when days reach 10-12 hours in late spring), then bolt to seed. A long-day onion planted in a southern short-day climate never receives the 14-hour trigger and stays vegetative without forming a proper bulb. Neither produces a usable harvest.
Three Starting Methods Compared
| Method | Cost per 100 plants | Variety selection | Bolting risk | Yield quality | Best for |
|---|---|---|---|---|---|
| Sets (small dry bulbs) | $4-5 | Limited (typically yellow and red) | Higher | Smaller bulbs on average | Beginners; minimum lead time |
| Transplants (nursery starts) | $6-12 | Moderate | Low | Good | Zone 7-9 fall planting; time-constrained |
| Seeds (direct or indoor start) | $1-2 | Full - 100+ varieties | Low when timed correctly | Best potential | Experienced growers; variety selection priority |
Sets are small dormant onion bulbs, typically 3/4 to 1 inch in diameter, sold in bags at garden centers in spring. They’re the fastest path to a planted row because there’s no germination step and no 10-week indoor start period. The trade-offs: variety selection is almost universally limited to ‘Stuttgarter’ yellow, a red variety, and occasionally white. Sets also have a higher bolting rate than transplants or seeds because the small bulb has already completed some of its growth cycle and can be triggered to flower prematurely under certain conditions.
Transplants are 8-12-week-old seedlings, typically 6-8 inches tall with pencil-thin stalks, sold in bunches of 60-100. They’re established plants that have already been through germination and early growth. For Zone 7-9 gardeners doing a fall onion planting, transplants timed to the local fall planting window are the standard choice. Quality transplants from a reliable nursery perform well and eliminate the indoor start work.
Seeds provide access to the full catalog of onion varieties, including long-day storage types (Patterson, Copra, Cortland) that hold 6-12 months in storage and are not available as sets or transplants at most garden centers. The cost per plant is the lowest of the three methods, but requires 10-12 weeks of indoor start time and slightly more germination management than some crops.
Seed Economics in Detail
A packet of onion seed runs $2-4 and contains 250-400 seeds. At 70-75% germination rate, one packet produces 175-300 plants.
| Metric | Seeds | Sets | Transplants |
|---|---|---|---|
| Packet/bunch cost | $2-4 | $3-5/lb (~80-100 sets) | $4-8/bunch (60-80 plants) |
| Plants per dollar | 60-100 | 16-25 | 8-15 |
| Weeks of lead time | 10-12 weeks before outdoor planting | Plant directly | 2-4 weeks acclimation |
| Bolting susceptibility | Low (with correct timing) | Higher | Low |
| Storage variety access | Full catalog | Limited | Moderate |
The seed-to-plant cost advantage is substantial: $0.01-0.02 per plant with seed versus $0.04-0.06 per plant with sets. For 100 plants, that’s $1-2 versus $4-5 - not a large absolute difference, but seed also gives you access to the long-storage varieties that set-sourced onions typically don’t.
Yield and Gross Value Math
A typical home garden onion produces 0.5-1.0 lb per plant under reasonable conditions. A well-grown spring-planted long-day onion in good soil reaches 0.75 lb average without difficulty.
100-plant scenario:
- Yield: 75 lbs (100 plants × 0.75 lb average)
- Retail value: $1.00-1.75/lb for yellow storage onions (USDA ERS, Vegetables and Pulses Yearbook, 2024)
- Gross at $1.25/lb: $93.75
| Method | Input cost (100 plants) | Gross value (75 lbs at $1.25/lb) | Net |
|---|---|---|---|
| Seeds | $2-4 seed + $5-8 amendments = $7-12 | $93.75 | $82-87 |
| Sets | $4-5 sets + $5-8 amendments = $9-13 | $93.75 | $81-85 |
| Transplants | $5-8 transplants + $5-8 amendments = $10-16 | $93.75 | $78-84 |
The net difference between methods is small in absolute dollars when growing 100 plants. The real advantage of seeds at scale - 300+ plants - where the cost-per-plant gap compounds, and the variety access advantage (long-storage cultivars worth $1.50-2.00/lb versus commodity yellows at $1.00/lb) adds to the revenue side.
Curing: The Step That Creates the Storage Window
Freshly harvested onions are high in moisture and won’t store. Curing removes excess moisture, hardens the outer skin layers, and seals the neck - the entry point for storage rots.
The curing process: after pulling or digging, spread onions in a single layer in a warm (75-80°F), dry, well-ventilated location out of direct sun. A slatted shelf, a wire rack, or newspaper on a table works. Allow 2-4 weeks of curing - longer in humid conditions. The neck should feel completely dry and papery to the touch; the outer skin should be dry and papery throughout.
Curing signs you’re done: neck feels completely dry and shows no moisture when squeezed; outer skins are papery and crinkle when handled; roots are completely shriveled and dry.
Storage after curing: at 35-50°F with low humidity, cured long-day storage onions keep 6-12 months. ‘Copra’, ‘Patterson’, and ‘Cortland’ consistently hold 8-10 months under good conditions (Cornell Cooperative Extension, Onion Storage, 2019). At room temperature (65-70°F), even well-cured storage onions last only 1-3 months before sprouting or softening.
The storage duration is the value multiplier. Onions harvested in July and still contributing to meals in February have captured 7 months of retail price differential. At $1.25/lb for grocery onions, 75 lbs stored from July through February represents $93.75 in grocery substitution that accrues over 8 months from one garden investment.
The Storage Variety Advantage
The biggest financial gap between growing your own onions and buying commercial onions is not the per-pound cost - it’s variety access.
Commercial grocery store onions are almost universally short-storage types bred for high yield, uniform size, and the ability to look acceptable after weeks in cold storage and transit. The dominant commercial varieties (‘XZ500’, ‘Vaquero’, and their relatives) taste fine but lack the concentrated sugar-sulfur balance of old storage varieties grown to full maturity in home garden conditions.
Long-storage varieties grown and cured correctly tell a different story. ‘Copra’ is the standard home garden storage recommendation - medium-sized, golden brown, intensely flavored when cooked, holds 8-10 months without sprouting. ‘Patterson’ is slightly milder with better yield, similar storage. ‘Red Zeppelin’ is a red storage type (most red onions don’t store well; Red Zeppelin is the exception) with 6-8 month storage life.
These varieties are not available in grocery stores and are rarely available as sets. They come primarily from seed, which is one more argument for starting onions from seed rather than sets: you access a catalog that the set-buying gardener never sees.
At farmers markets, locally grown storage onions from named heritage varieties regularly command $1.75-2.50/lb versus commodity grocery pricing at $0.75-1.25/lb. If you’re growing for home use, the quality argument is real. If you’re growing for market, the premium pricing makes the seed-starting work financially worthwhile.
Pest and Disease Management
Onion thrips (Thrips tabaci): the most common onion pest. Tiny (1/25 inch), yellowish insects that rasp leaf tissue and cause silver streaking on foliage. High populations stunt bulb development. Spinosad or insecticidal soap at early infestation; reflective silver mulch as a deterrent (thrips use visual cues to find hosts).
Botrytis leaf blight: gray-green lesions on leaves in wet, humid weather. The same Botrytis that causes storage rots if bulbs enter storage with damaged neck tissue. Avoid overhead irrigation; provide good airflow between plants; do not harvest when foliage is wet.
White rot (Stromatinia cepivora): a soil-borne fungus that causes yellowing, collapse, and a white cottony growth at the base. Survives in soil for 15-20 years. No cure once established; rotate alliums out of affected beds for as long as possible. This is the pest that requires long crop rotations (5-7 years minimum without alliums) once it establishes.
Downy mildew (Peronospora destructor): gray-purple sporulation on leaves in cool, humid weather. Serious in wet springs. Copper-based fungicide preventively when conditions favor the disease.
Most home garden onion plantings in well-drained soil with good airflow avoid serious pest and disease pressure. The risks above are worth knowing, but onions are genuinely not a high-maintenance crop in terms of pest management compared to tomatoes or cucumbers.
Growing Requirements and Spacing
Onions are shallow-rooted and need frequent, consistent moisture. The top 6 inches of soil should stay consistently moist from the time bulbing begins (triggered by the appropriate day length) through harvest. Irregular watering during bulbing causes double bulbs (two bulbs fused together), thick necks, and uneven sizing.
Spacing: 4 inches between plants, 12 inches between rows for large bulb production. You can plant as close as 2 inches for green onion harvest, then thin to 4 inches for bulb development.
Weed management: the critical window is the first 6 weeks after planting. Onion seedlings and transplants are easily outcompeted by weeds during early establishment. After canopy closes (around week 8-10), competition decreases. Hand weeding near onions is important because their shallow roots are easily disturbed by hoes and cultivators.
Fertilization: nitrogen in the first half of the growing season (vegetative phase) drives leaf development, which directly correlates with bulb size - more leaves equal larger bulbs. Apply nitrogen fertilizer every 3-4 weeks from planting until the onions begin to bulb. Stop nitrogen application once bulbing starts; nitrogen at that stage produces soft bulbs that don’t store well. Switch to a potassium-dominant fertilizer at bulbing to support storage quality.
When Onions Pencil Out Best
Onions make the most financial sense in specific situations: when you have the indoor starting infrastructure to start from seed (reducing per-plant cost to $0.01-0.02 and opening the full variety catalog), when you have cool dry storage for the 6-12 month holding period, and when you’re growing 100+ plants rather than a small patch.
At small scale - 25 plants from a set bag - the grocery comparison math is thin. A 25-plant set planting at $0.05/plant and 0.75 lb/plant yields 18 lbs at $1.25/lb = $22.50 gross minus $5-8 in inputs. The returns are real but not dramatic.
At 200 plants started from seed, the calculation changes: $3 in seed produces 140-180 plants, yielding 105-135 lbs at $1.25-1.50/lb = $131-203 gross against $3-12 in total inputs. The per-unit efficiency of onion production at scale is genuine. Plan your bed space accordingly.
Related reading: Root Vegetable ROI - onion compared against garlic, leeks, and other alliums; Succession Planting Calendar - planning fall onion planting for zones 6-9
Related crops: Onion - full growing guide with day-length zone map and storage variety recommendations