Cool-season crop
When to Start Onions Before the Last Frost
Plan long-season onions from seed, sets, or transplants using local spring freeze context, with spacing, depth, maturity, and day-length limits.
Reviewed by Garden By ZIP Editorial Review ·
- Spacing
- 3–4 in
- Seed depth
- 0.25–0.5 in
- Typical maturity
- 90–120 days
Quick answer
Start onion seed indoors 10–12 weeks before the last spring freeze reference. Direct-sow about 6–4 weeks before it, or transplant about 4–2 weeks before it. The planting calendar places those ranges locally.
Planting methods
Onions can be grown from seed, transplants, or sets. The best choice depends on season length and the correct day-length class for the latitude.
Spacing, depth, and maturity
Sow seed 1/4–1/2 inch deep and space bulb onions about 3–4 inches. Seed-grown bulbs often need 90–120 days.
Worked local-calendar example
With an April 19 last spring freeze reference, an indoor seed window begins in late January or early February, while transplanting falls in late March. This example assumes an appropriate onion type for the location.
Common mistakes
- Choosing the wrong short-, intermediate-, or long-day class.
- Starting seed too late for bulb development.
- Planting sets too deeply.
Limitations
Day length, cultivar, bulb goal, storage goal, and latitude are essential. A frost date alone cannot determine the correct onion class.