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The short answer: plant onions in early spring in the North (March to April, 4 to 6 weeks before your last frost) and in fall in the South (October to December). The deciding factor is not just the calendar but matching the day-length type to your latitude — short-day for the South, long-day for the North, day-neutral for the middle — because daylight, not date, is what triggers an onion to form a bulb. Onions are cold-hardy, so early planting is an asset: the more leaves a plant builds before bulbing starts, the bigger the bulb.
Onion timing trips up more gardeners than it should because the day-length rule is invisible. This guide follows extension-service planting data and the results growers consistently report, described in how we rank. For the full growing process from soil to storage, see how to grow onions, and for garlic's opposite fall-only schedule, when to plant garlic.
Day length decides your planting season
Before you pick a month, pick a type, because the type dictates the season:
| Type | Bulbs at | When to plant | Best latitude |
|---|---|---|---|
| Short-day | 10 to 12 hrs daylight | Fall (South) | Below ~35°N |
| Day-neutral | 12 to 14 hrs daylight | Early spring, most regions | ~32 to 42°N |
| Long-day | 14 to 16 hrs daylight | Early spring (North) | Above ~37°N |
A short-day onion planted in a northern spring bulbs while still tiny; a long-day onion in the South never gets enough daylight and stays a scallion. Day-neutral types are the forgiving middle-ground choice.
Spring planting in the North and central US
Across most of the country, spring is onion season:
- Timing: as soon as the soil can be worked, typically 4 to 6 weeks before the last frost, when soil reaches about 50°F.
- Why early: onions build leaves in cool weather and bulb as days lengthen. An early start banks maximum leaf growth before the daylight trigger arrives, and each leaf becomes a bulb ring.
- Cold tolerance: established onions shrug off light freezes to around 20°F, so a late cold snap rarely hurts them.
Fall planting in the South
In zones 8 to 10, the schedule flips:
- Timing: October to December, so plants establish over a mild winter.
- Type: short-day onions, which bulb as spring days reach 10 to 12 hours and finish in late spring.
- Overwintering: the plants grow slowly through winter, then surge and bulb as days lengthen, giving large early-season bulbs.
Planting windows by USDA zone
| Zone | Onion type | Planting window |
|---|---|---|
| Zones 3 to 5 | Long-day | April to early May |
| Zone 6 | Long-day or day-neutral | March to April |
| Zone 7 | Day-neutral | February to March |
| Zone 8 | Short-day or day-neutral | Oct to Nov (fall) or Feb (spring) |
| Zones 9 to 10 | Short-day | October to December |
Use these as a starting point and adjust to your own microclimate and average frost dates, which your local extension office can confirm.
Timing for sets, seed and transplants
Your starting method shifts the schedule:
- Seed indoors: start 10 to 12 weeks before you want to transplant, so late winter for a spring set-out.
- Transplants: set out 4 to 6 weeks before the last frost, the same window as sets.
- Sets: plant 2 to 4 weeks before the last frost; choose sets smaller than a dime to avoid bolting.
For green onions rather than bulbs, you can sow or plant sets almost any time in the growing season, since day length does not matter when you are not waiting for a bulb.
Reading soil temperature, not just the calendar
The calendar and your average frost date get you close, but soil temperature is the more reliable signal for when onions are ready to go in. Onions germinate and root best once the soil reaches about 50°F, and they will sit and sulk in colder ground:
| Soil temp | What onions do |
|---|---|
| Below 45°F | Roots barely grow; sets can rot in cold, wet soil |
| 50 to 60°F | Ideal for planting sets and transplants; strong rooting |
| 60 to 75°F | Fast leaf growth; the leaf-building phase you want before bulbing |
| Above 75°F with long days | Bulbing accelerates; leaf growth slows |
A cheap soil thermometer takes the guesswork out. If your target planting week arrives but the soil is still cold and soggy, wait a few days rather than planting into conditions that rot sets and stall transplants. A short delay into workable soil beats an early start into cold mud.
Timing for green onions versus storage bulbs
Everything above assumes you want mature bulbs. If you are after green onions (scallions), the timing rules relax completely, because day length only matters when you are waiting for a bulb to form:
- Green onions: sow seed or plant sets almost any time from early spring through late summer, in succession every few weeks, and harvest young at pencil thickness. Day-length type barely matters.
- Storage bulbs: plant in the correct season for your zone with the matched day-length type, giving maximum leaf-building time before bulbing.
- Overwintered scallions: in mild zones, a late-summer or fall sowing overwinters for very early spring green onions.
This flexibility is why onions are worth planting in more than one window: a spring batch for storage bulbs, plus staggered small sowings for a steady supply of scallions all season.
Frost, cold snaps and protecting young onions
One of the advantages of onions is how much cold they shrug off, which is exactly why they can go in so early. Established plants tolerate light freezes down to about 20°F, and a spring cold snap after planting rarely does lasting harm. That said, a few precautions protect an early crop:
- Harden off transplants and seed-started plants for several days before setting them out, so tender indoor growth is not shocked by cold nights.
- Mulch lightly with straw after planting to buffer soil-temperature swings and protect shallow roots.
- Cover with row cover if a hard freeze below 20°F threatens newly set transplants, then remove it once the danger passes.
- Avoid a prolonged cold spell right after planting sets, which can trigger bolting later; this is why timing to workable, warming soil matters more than the earliest possible date.
The balance to strike is starting early enough to bank leaf growth before bulbing, without exposing freshly planted, un-hardened seedlings to a damaging freeze. In most zones, planting into soil that has warmed to about 50°F hits that balance naturally.
Timing mistakes that shrink your bulbs
- Wrong day-length type for your latitude — the number-one reason onions stay small.
- Planting too late — fewer weeks of leaf growth before bulbing means a smaller bulb, no matter how well you feed it.
- Large sets — they bolt to flower instead of bulbing; go small.
- Waiting for warm weather — onions want a cool start; planting late in warm soil costs you leaf-building time.
Get the season and the type right and the rest of onion growing is straightforward. Walk through soil prep, spacing, feeding and curing in how to grow onions, feed the bed with a nitrogen-forward blend from our vegetable garden fertilizer picks, and browse the full seeds and vegetable growing hub for the rest of your planting calendar.