Planting calendar
When to plant onions in Montana
Montana spans USDA hardiness zones 3-6. Average last spring frost: 05-25. This calendar gives you the indoor start, transplant, and direct-sow dates for Allium cepa.
Montana planting dates for onions
| Phase | Date in Montana | Notes |
|---|---|---|
| Start seeds indoors | March 2 | 12 weeks before last frost |
| Transplant outdoors | May 11 | 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 Montana's average last spring frost date of 05-25, 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 Montana planting information
- Full Montana planting calendar (all crops + ornamentals)
- Find your exact USDA zone by ZIP
- Look up your average frost dates
- Seed starting timeline calculator