When to plant onions
Onions tolerate light frost and go out early: plant sets or transplants two to four weeks before the last frost. What makes a bulb is day length, so pick the right type: long-day onions in the North (above about 37°N), short-day in the South, intermediate in between.
Seed-grown onions need a long start: sow indoors ten to twelve weeks before the last frost and trim tops to about 3 inches to keep them stocky.
- Start seeds indoors
- 10–12 weeks before last frost
- Plant sets or transplants
- 2–4 weeks before last frost
- Frost
- Hardy: tolerates frost
- Soil temp
- 50°F (10°C) or warmer
- Depth
- 1 in (sets)
- Spacing
- 4 in apart, rows 12 in · 9 per square foot
- Harvest
- 90–120 days from sets or transplants
Your dates for onions
Tips for growing onions
- Long-day onions planted in the South never bulb properly; short-day onions planted in the North bulb small and early.
- Keep beds weeded: onions have thin leaves and lose to weeds quickly.
- Stop watering when tops start to flop over, then lift and cure for two weeks in a dry, airy place.
Choosing neighbours? See companion plants for onions. Working out how many fit? Use the spacing calculator.
Onions: common timing questions
Which onions should I grow?
North of roughly 37° latitude (a line through Virginia to central California) choose long-day types; south of about 35° choose short-day types; intermediate types suit the band in between.
Can onions take frost?
Yes, young onions handle frosts down to the mid-20s °F. Hard freezes after a long warm spell can trigger bolting.