When to Plant Onions
Planting Window - Sets, transplants, or seed
- Start seed indoors (optional)
- 10-12 weeks before last frost
- Plant sets or transplants
- A few weeks before last frost
| USDA Zone | Typical Window |
|---|---|
| Zone 5 and colder | April, long-day varieties |
| Zone 6-7 | March, often intermediate-day varieties |
| Zone 8 and up | Fall (Oct-Nov), short-day varieties, overwinter |
Onion timing depends on a factor most other vegetables don't involve at all: day length. Onions form bulbs in response to how many hours of daylight they're getting, not primarily in response to temperature or plant age, which makes variety selection as important as planting date.
What Day Length Means for Your Planting Date
Practically, this splits the calendar in two: long-day varieties (roughly north of the 35th parallel) go in during early spring for a summer harvest, the same general window most other spring crops follow, while short-day varieties in the Deep South are planted in fall instead and overwinter, an entirely different schedule the zone table above reflects. The full mechanism behind why day length rather than temperature drives bulbing is covered on the main Onions growing guide.
Sets, Transplants, or Seed
Onions can be started from small dormant bulbs called sets, from nursery-grown transplants, or from seed started indoors 10-12 weeks before the last frost date - sets are the fastest and most forgiving option for beginners, seed offers the widest variety selection but the longest lead time, and transplants split the difference.
Reading the Harvest Signal
Onions tell you when they're close to done in a way most root vegetables don't: the green tops naturally yellow and fall over on their own as the bulb finishes sizing up, usually in mid-to-late summer depending on planting date. Bending tops over by hand to force this early is an old practice some gardeners still use, though it's more of a minor speed-up than something that changes the underlying planting-to-harvest timeline.
Onions Tolerate Real Cold as Young Plants
Unlike most of the vegetables planted around the same time in colder zones, young onion sets and transplants handle a fairly hard frost without much damage, which is part of why they're planted so early relative to the last frost date compared to tender crops like tomatoes or peppers started weeks later.
Sets Are the Most Bolt-Prone Start Method
Onion sets exposed to an unusually cold snap shortly after planting can be triggered into bolting - sending up a flower stalk instead of continuing to bulb - which is one of the few real risks of planting sets too early in a spring with unpredictable late cold; seed-grown and transplant-grown onions are somewhat less prone to this than sets.
Zone boundaries follow the USDA Plant Hardiness Zone Map (2023 revision); frost windows reflect NOAA/NCEI climate normals. Both are typical historical ranges, reviewed annually - not a forecast or a guarantee for any single year. See Methodology & Sources.
Frequently Asked Questions
If I planted a bit later than usual this spring, does that change which day-length variety I should have chosen?
No - the day-length trigger is a fixed trait of the variety itself, not something that shifts based on exactly when within the season you planted. Planting a long-day variety two weeks later than usual doesn't turn it into a better match for a southern, short-day location; the mismatch happens regardless of planting date, so which type of onion you buy matters more than fine-tuning the calendar.
What happens if I plant the wrong day-length variety for my zone?
The plant will still grow foliage normally, but bulb formation triggers too early or too late relative to the plant's size, producing small, underdeveloped bulbs even with otherwise ideal soil, water, and spacing - it's a genetic mismatch with day length, not a fixable growing-condition problem.
How do I know when my onions are ready to harvest?
The tops falling over on their own is the clearest natural signal - once a good portion of the foliage has yellowed and flopped, the bulbs are close to mature, and most gardeners wait for this rather than guessing based on a fixed calendar date alone.