How to Grow Onions
Allium cepa
Day-length-driven bulbs — choosing the right type for your latitude is everything.
By the Plants by Zone Editorial Team · Reviewed June 1, 2026
Quick answer · Updated July 2026
Plant onion sets or transplants 2–4 weeks before your last spring frost, as soon as soil is workable and around 50°F. Starting from seed? Sow indoors 8–10 weeks before that. In Zones 7 and warmer, plant again in fall (October–December) to overwinter for the earliest, biggest bulbs. One rule decides everything else: match the day-length type to your latitude — long-day onions in the North, short-day in the South, intermediate in between.
About onions
Onions are a long-season cool-weather crop with one crucial quirk: bulbing is triggered by day length, so you must match the variety to your latitude. Long-day types suit northern zones, short-day types the South, and day-neutral types grow well almost anywhere. Start from seed, sets, or transplants for a pantry staple that stores for months.
When to plant and harvest onions
Timing is relative to your frost dates. Find your USDA zone for exact dates, or browse the month-by-month calendars.
Start seeds indoors
8–10 weeks before last frost
Transplant outdoors
As soon as soil is workable
Direct sow
Sets or seed in early spring
Harvest
Mid-to-late summer
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When to plant onions in every zone
Onion timing confuses more gardeners than any other crop, because two clocks run at once. The calendar clock says plant early — onions are cold-hardy, and more leaves grown before bulbing begins means bigger bulbs, since each leaf becomes a ring. The day-length clock decides when bulbing starts: long-day varieties trigger at ~14–16 hours of daylight (right for Zones 3–6 latitudes), short-day at ~10–12 hours (Zones 8–13 South), and intermediate/day-neutral types in between. Plant the wrong type for your latitude and you get lush tops with marble-sized bulbs, no matter how well you grow them.
That’s why the planting window differs so sharply by zone. Northern gardeners start long-day seed indoors in late winter and plant out as soil becomes workable in spring. Southern gardeners flip the calendar entirely: short-day onions go in during fall and grow through the mild winter, bulbing in early spring. Zone 7–8 sits in the overlap and gets both options — spring-planted intermediates or fall-planted overwintering onions that finish weeks earlier.
Whatever the zone, the sequence at planting is the same: loose, rich, well-drained soil; sets pushed in about an inch deep and 4 inches apart (or transplants at the same spacing); steady water while the tops grow; and a stop to watering once half the tops flop over — the signal that bulbs are curing for harvest.
Onion planting dates by USDA zone
| Zone | When to plant | Best onion type |
|---|---|---|
| Zones 3–4 | Seed indoors Feb–Mar; sets/transplants out mid-Apr–May | Long-day |
| Zones 5–6 | Seed indoors Jan–Feb; sets/transplants out Mar–Apr | Long-day |
| Zone 7 | Spring: out Feb–Mar · or fall-plant Oct–Nov to overwinter | Intermediate |
| Zone 8 | Fall-plant Oct–Nov (best) · or spring out by Feb | Intermediate / short-day |
| Zones 9–10 | Fall-plant Oct–Dec; grow through winter | Short-day |
| Zones 11–13 | Plant Nov–Jan in the cool dry season | Short-day |
Fall-planted onions in Zones 7+ overwinter as small plants and bulb in spring — they finish earlier and larger than spring-planted ones.
Bigger bulbs, every zone
- ✓Buy the right day-length type before anything else — it matters more than variety, soil, or feeding.
- ✓Plant as early as your window allows: every leaf grown before bulbing adds a ring to the bulb.
- ✓Use transplants or sets for reliability; seed is cheapest but adds 8–10 indoor weeks.
- ✓Feed nitrogen every 2–3 weeks until bulbing starts, then stop — late nitrogen delays maturity.
- ✓Stop watering when half the tops have flopped, then cure bulbs in a dry, airy spot for 2–3 weeks before storing.
How to grow onions step by step
- 1
Choose long-day (North), short-day (South), or day-neutral types to match your latitude.
- 2
Start seed indoors 8–10 weeks early, or plant sets/transplants in early spring.
- 3
Set out pencil-thick transplants 4 in apart as soon as soil can be worked.
- 4
Keep weed-free and evenly watered while bulbs size up.
- 5
Stop watering and harvest when the tops yellow and fall over.
Common problems growing onions
⚠ Small bulbs
Wrong day-length type for your latitude, or crowding/weed competition.
⚠ Bolting (flower stalks)
From cold stress on large transplants/sets — use smaller sets and avoid early cold snaps.
⚠ Rot in storage
Cure thoroughly and store thick-necked bulbs first; they don’t keep.
✓ Good companions for onions
✗ Keep away from
🧺 Harvesting onions
When most tops have flopped over and started to brown, lift the bulbs and cure them in a dry, airy place for 2–3 weeks until the necks are papery. Then trim and store.
Onions: frequently asked questions
When should you plant onions?＋
In most regions you start seeds indoors 8–10 weeks before last frost, then transplant as soon as soil is workable — or direct sow sets or seed in early spring. Timing is relative to your last frost, so find your USDA hardiness zone for the exact planting dates where you live.
What onion type do I need for my zone?＋
It follows latitude: long-day onions for Zones 3–6 (northern US), short-day for Zones 8–13 (southern US), and intermediate/day-neutral types for the Zone 6–8 middle band. The wrong type produces leafy tops but tiny bulbs.
Can you plant onions in the fall?＋
Yes — in Zones 7 and warmer, fall planting (October–December) is actually the better window. The onions overwinter as small plants, resume growth in late winter, and produce earlier, larger bulbs than spring planting. In Zones 6 and colder, stick to spring.
What are long-day and short-day onions?＋
They bulb at different day lengths. Long-day onions suit northern gardens, short-day the South, and day-neutral types work in between.
Should I grow onions from seed or sets?＋
Sets are easiest and fastest; seed offers far more variety and better storage but needs an early indoor start.
Sources & review
Written and maintained by the Plants by Zone Editorial Team. Planting times are based on USDA hardiness zones and NOAA frost-date normals, with care guidance drawn from Cooperative Extension sources. Last reviewed June 1, 2026.
USDA Plant Hardiness Zone MapNOAA U.S. climate normalsCooperative Extension
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