Montana · zones 4a–5b
When to plant onions in Montana.
Sow or transplant between Mar 7 to Apr 20 in a typical year, keyed to Montana's median last frost around May 15. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Montana season.
1 · Start indoors
Jan 24 to Mar 9
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Mar 7 to Apr 20
Keyed to the ~May 15 last frost. Sow 0.5" deep, 4" apart.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Montana spans more than one climate: Zone 4 is the most common zone, but the state runs 4a to 5b. That's why the planner works from your ZIP code's frost dates, not your zone — mountain and coastal gardens in the same state can be three weeks apart.