Vegetable · Amaryllidaceae
When to plant onions
Bulbing is triggered by day length, so your latitude decides which cultivars will work.
Start seed indoors
10 weeks before
Counted from your last spring frost.
Plant out
4 weeks before
Counted from your last spring frost.
Days to maturity
100-120
From planting out to a first picking.
Those weeks become real dates once you name a place.
Timing, in frost-relative terms
Seed packets rarely give calendar dates, because a date that suits one latitude is wrong at another. They give offsets from the last spring frost instead, and those offsets are what make onions plantable anywhere.
- Start seed indoors 10 weeks before the last frost.
- Plant out 4 weeks before the last frost.
- Expect 100–120 days to maturity, counted from planting out.
- Can also be sown in late summer for an autumn crop or to overwinter.
Getting the timing right
Start seed ten weeks before last frost. Choose long-day cultivars above roughly 38 degrees latitude and short-day types below it - the wrong choice produces no bulb regardless of timing.
Frost tolerance
Young plants take -8C. Autumn-planted sets overwinter reliably in zone 6 and warmer.
Classified as hardy, with damage expected below -8°C (17.6°F)17.6°F (-8°C).