When to Plant Onions in Montana
In Montana, the time to plant onions outside runs from Apr 6 – Apr 20 in Billings to May 16 – May 30 in Butte-Silver Bow. In Billings, it's Apr 6 – Apr 20. The average last spring frost differs by about 6 weeks across the state (May 4 in Billings, Jun 13 in Butte-Silver Bow), so use the dates for the town nearest you.
Onions are cold-hardy. Plant sets or transplants early so they grow leaves before long summer days trigger bulbing.
- Start indoors: 10–12 weeks before the last frost.
- Plant outside: 2–4 weeks before the last frost.
Onions planting dates in Montana by city
| City | Zone | Start indoors | Plant outside |
|---|---|---|---|
| Anaconda-Deer Lodge County | 5a | Mar 9 – Mar 23 | May 4 – May 18 |
| Belgrade | 5a | Mar 3 – Mar 17 | Apr 28 – May 12 |
| Billings | 5a | Feb 9 – Feb 23 | Apr 6 – Apr 20 |
| Bozeman | 5a | Mar 3 – Mar 17 | Apr 28 – May 12 |
| Butte-Silver Bow | 5a | Mar 21 – Apr 4 | May 16 – May 30 |
| Columbia Falls | 5a | Mar 8 – Mar 22 | May 3 – May 17 |
| Great Falls | 4b | Feb 11 – Feb 25 | Apr 8 – Apr 22 |
| Hamilton | 5b | Feb 26 – Mar 12 | Apr 23 – May 7 |
| Havre | 3b | Feb 20 – Mar 6 | Apr 17 – May 1 |
| Helena | 5a | Feb 14 – Feb 28 | Apr 11 – Apr 25 |
| Kalispell | 5b | Mar 8 – Mar 22 | May 3 – May 17 |
| Laurel | 5a | Feb 10 – Feb 24 | Apr 7 – Apr 21 |
| Lewistown | 4b | Feb 28 – Mar 14 | Apr 25 – May 9 |
| Livingston | 5a | Mar 4 – Mar 18 | Apr 29 – May 13 |
| Miles City | 4b | Feb 13 – Feb 27 | Apr 10 – Apr 24 |
| Missoula | 5b | Feb 15 – Mar 1 | Apr 12 – Apr 26 |
| Polson | 6a | Feb 15 – Mar 1 | Apr 12 – Apr 26 |
| Sidney | 4a | Feb 16 – Mar 2 | Apr 13 – Apr 27 |
| Whitefish | 5a | Feb 21 – Mar 7 | Apr 18 – May 2 |
Calculated from each town's average (50%) frost dates, from the nearest NOAA station (1991–2020). Typical dates for the 2027 planting season, based on NOAA's 1991–2020 averages. They are long-term odds, not a forecast: always check your local forecast before planting. Zones: 2023 USDA Plant Hardiness Zone Map (USDA-ARS and Oregon State University). Not the official map; see planthardiness.ars.usda.gov.
See all frost dates in Montana, or the general guide to when to plant onions.
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