When to Plant Onions in Montana
The backbone of the kitchen garden. Choose short-day, intermediate, or long-day varieties based on your latitude.
The Short Answer
Montana Frost Dates
Your planting dates depend on which part of Montana you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Western Montana | 4b, 5a, 5b | May 10 - May 30 | Sep 10 - Sep 30 |
| Central Montana | 3b, 4a, 4b | May 15 - Jun 5 | Sep 5 - Sep 20 |
| Eastern Montana | 3a, 3b, 4a | May 10 - Jun 1 | Sep 10 - Sep 25 |
Onions Planting Schedule for Montana
Western Montana (Zones 4b, 5a, 5b)
Average last frost: May 10 - May 30 · Average first frost: Sep 10 - Sep 30
Central Montana (Zones 3b, 4a, 4b)
Average last frost: May 15 - Jun 5 · Average first frost: Sep 5 - Sep 20
Eastern Montana (Zones 3a, 3b, 4a)
Average last frost: May 10 - Jun 1 · Average first frost: Sep 10 - Sep 25
Growing Onions in Montana
Onions in Montana's Climate
You're in long-day onion territory — plant varieties that bulb when days exceed 14-16 hours. This means Copra, Walla Walla, and Yellow Globe Danvers. Plant sets or transplants from mid-April. The critical distinction: short-day onions from southern catalogs will NOT form bulbs in your zone because your long summer days trigger bulbing too early, before the plant is large enough.
Soil Considerations for Montana
Alkaline soils common. Thin, dry soils in east. Better soils in valleys. Wind erosion challenges. Loose, well-drained soil is especially important for onions since the edible portion grows underground. If your soil is heavy clay, consider raised beds.
Montana Climate & Growing Season
Short, intense growing season. Low humidity. Wide day-night temperature swings. Wind protection valuable. Onions can handle frost well, which is an advantage in Montana's climate. You can push planting dates earlier in spring and extend into fall.
Growing season length varies across Montana: Western Montana (4b, 5a, 5b) has a last frost around May 10 - May 30, while Eastern Montana (3a, 3b, 4a) sees frost end around May 10 - Jun 1. This difference matters for onions — but onions handle frost well, so the timing difference is less critical.
Growing Tips
Day length triggers bulbing. Northern gardeners need long-day varieties. Southern gardeners need short-day varieties.
Companion Planting
Plant onions alongside these companions for better growth:
Keep onions away from:
The Bottom Line
Last reviewed: March 29, 2026