Planting calendar
When to plant onions in Illinois
Illinois spans USDA hardiness zones 5a-7a. Average last spring frost: 05-05. This calendar gives you the indoor start, transplant, and direct-sow dates for Allium cepa.
Illinois planting dates for onions
| Phase | Date in Illinois | Notes |
|---|---|---|
| Start seeds indoors | February 10 | 12 weeks before last frost |
| Transplant outdoors | April 21 | 2 weeks before last frost |
Why these dates
Onions are day-length sensitive. Long-day varieties for the North (above latitude 35), short-day varieties for the South (below 35), day-neutral varieties work anywhere.
The dates above are anchored to Illinois's average last spring frost date of 05-05, with offsets standard across university Extension publications. For zone-specific local timing, use the frost date lookup tool with your ZIP code.
More on growing onions
The full growing guide for onions - varieties, soil, fertilization, pests, harvest, storage - is here: Onions growing guide.
More Illinois planting information
- Full Illinois planting calendar (all crops + ornamentals)
- Find your exact USDA zone by ZIP
- Look up your average frost dates
- Seed starting timeline calculator