Seed to Harvest Growing Guide
How to grow onions from seed
Build strong seedlings, match day length to your latitude, grow a full canopy before bulbing, and cure dry onions for dependable storage.
Crop family: Amaryllidaceae
Before you begin
Day length determines whether a bulb onion succeeds.
Bulb onions begin enlarging after daylight reaches a variety-specific threshold. Short-day onions fit southern latitudes, intermediate-day types fit the middle of the country, and long-day onions fit northern gardens. Bunching onions are grown for stems and are less dependent on bulbing.
Build your onion growing plan
Time seed starting, plan close spacing, and estimate one seed for every bulb.
Onions reward an early start, steady growth, shallow cultivation, and a dry finish.
Onions at a glance
The path from seed to stored bulb
Select any milestone to jump directly to its full instructions.
Stage 1–3
Start onion seed early and keep seedlings growing steadily
Onions grow slowly at first. Every healthy leaf supports a ring in the eventual bulb, so the goal is a sturdy transplant with a strong leaf canopy before day length triggers bulbing.
Choose the correct day-length class
Match the variety to latitude first, then compare color, sweetness, pungency, maturity, disease resistance, and storage.
- Short-day onions bulb at roughly 10–12 hours of daylight and suit southern regions.
- Intermediate-day onions bulb at roughly 12–14 hours and suit many central regions.
- Long-day onions need about 14–16 hours and suit northern regions.
Sow indoors or direct sow very early
For transplants, sow about 8–12 weeks before planting out. Cover seed 1/4–1/2 inch, keep evenly moist, and provide strong light after emergence. Direct sow into a firm, finely prepared bed as soon as local soil conditions allow.
Transplant shallowly and space for bulb size
Harden seedlings mainly by gradually reducing water and exposure—not by chilling them enough to trigger bolting. Set roots into moist soil with the growing point above the soil line.
- Space bulb onions about 3–4 inches apart; closer spacing produces smaller bulbs.
- Leave bunching onions closer and harvest them progressively.
- Water immediately after planting.
- Keep cultivation shallow because onion roots stay near the surface.
Stage 4
Build leaves first, then let bulbs mature dry
Onions compete poorly with weeds and depend on steady moisture and available fertility while leaves are growing.
Water
Keep the shallow root zone evenly moist. Water stress reduces leaf growth and bulb size; stop regular irrigation as tops fall and bulbs mature.
Feed
Use a soil test. Supply nitrogen during active leaf growth, then stop late feeding so necks mature and cure properly.
Weed
Remove weeds early and often by hand or very shallow cultivation. Deep hoeing damages roots and reduces growth.
Scout
Inspect leaf folds and bases for thrips, streaking, mildew, rot, and plants that wilt without explanation.
Growth checkpoints
Know what to watch for.
Use plant development—not only the calendar—to time thinning, feeding, irrigation changes, and harvest.
Trim only if needed
Shorten floppy tops to keep indoor seedlings upright and improve airflow; retain healthy green tissue.
Thin for final size
Leave 3–4 inches for full bulbs or harvest crowded plants early as green onions.
Stop heavy nitrogen
Maintain moisture, but avoid late lush growth that produces thick necks and weak storage.
Prepare to cure
Reduce water, lift mature bulbs in dry weather, and protect them from rain and sunscald.
Stage 5
Diagnose onion problems early
Open a symptom to compare what you see, likely causes, and the next practical check.
PestOnion thripsSilvery streaks, stippling, distorted leaves, and tiny slender insects
Look deep between leaves where thrips hide. Reduce dusty, weedy conditions, avoid water stress, preserve beneficial insects, and use a labeled control only when monitoring shows it is needed.
PestOnion maggotSeedlings wilt or disappear and bulb bases contain cream-colored larvae
Remove affected plants, rotate away from alliums, clean up cull onions, and avoid leaving fresh undecomposed organic matter in the seed row. Once larvae are inside the plant, rescue options are limited.
DiseaseDowny mildewLong pale lesions develop gray-violet growth in cool, humid weather
Improve spacing and airflow, irrigate early enough for leaves to dry, rotate alliums, remove volunteer onions, and begin any labeled protection before disease is widespread.
Growing conditionPremature boltingA stiff central flower stalk forms before the bulb matures
Cold exposure after seedlings reach a responsive size can trigger flowering in this biennial crop. Harvest bolted onions promptly; the thick center will not cure or store like a normal bulb.
Stage 6
Harvest when tops fall, then cure completely
Storage onions are ready when most tops yellow, weaken at the neck, and fall naturally. Lift in dry weather rather than forcing still-green tops down.
- Lift: Loosen soil and handle bulbs gently; bruises invite decay.
- Cure: Dry in a warm, shaded, well-ventilated place for roughly 2–3 weeks until necks are tight, roots dry, and skins rustle.
- Store: Trim tops to 1–2 inches or braid suitable varieties. Keep only sound bulbs in cold, dry, ventilated storage.
Choose with purpose
Find the right onion seed for your latitude
These verified Shopify products represent short-, intermediate-, and long-day choices.
Questions gardeners ask
Growing onions from seed FAQ
Quick answers for day length, sowing, trimming, bulbing, curing, and storage.
When should onion seed be started indoors?
Most gardeners start seed about 8–12 weeks before outdoor planting. Earlier sowing gives slow-growing seedlings time to build leaves before transplanting.
What is the difference between short-, intermediate-, and long-day onions?
They begin bulbing at different daylight thresholds. Short-day types suit southern latitudes, intermediate-day types suit central regions, and long-day types suit northern regions.
How deep should onion seed be planted?
Cover seed about 1/4–1/2 inch. Use a fine, firm mix or seedbed and keep it evenly moist through emergence.
Should onion seedlings be trimmed?
Floppy indoor tops may be shortened to about 4 inches to improve airflow and handling. Use clean scissors and never cut into the newest central growth.
Can onion seedlings tolerate frost?
Established seedlings tolerate cool weather, but prolonged cold can damage plants or trigger bolting after they reach a responsive size. Harden mainly through gradual exposure and careful watering.
How far apart should bulb onions be planted?
Space about 3–4 inches apart for full bulbs. Closer spacing produces smaller onions; thin crowded rows early and eat the extras as green onions.
Why did my onions make small bulbs?
Common causes include the wrong day-length class, late planting, crowding, weeds, water stress, low fertility during leaf growth, or disease damage.
Why are my onions flowering?
Onions are biennials. Cold exposure can make a sufficiently developed plant behave as though winter has passed, causing a seed stalk. Bolted bulbs should be used promptly.
How much water do onions need?
Keep the shallow root zone consistently moist while leaves and bulbs are growing. Reduce and stop irrigation as tops fall so bulbs can mature and cure.
When should nitrogen feeding stop?
Stop heavy nitrogen once bulbs begin enlarging. Late lush growth can create thick necks, delayed maturity, and poor storage.
When are bulb onions ready to harvest?
Harvest when most tops yellow and fall naturally and the neck tissue softens. Lift before prolonged rain or a hard freeze.
How are onions cured for storage?
Spread them in a warm, shaded, airy place for about 2–3 weeks. They are cured when necks are completely dry, roots are dry, and outer skins feel papery.
How should cured onions be stored?
Keep sound bulbs in a cold, dry, well-ventilated place without freezing. Check regularly and use sweet, damaged, softening, or sprouting bulbs first.
Ready to plant?
Choose onion seed for your region.
Compare day length, color, flavor, maturity, storage, disease resistance, and bunching versus bulbing habits.