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. A second, fall sowing fits Jul 29 to Sep 9. 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.
Fall round
Jul 29 to Sep 9
Counted back from the ~Sep 23 first frost.
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.
In Montana: watch for
The pests that find onions here.
The short season here, about 131 frost-free days, keeps onion, bulb in front of onion thrips only briefly. One early row cover usually carries a planting through the worst of it.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: Montana gives onions about 131 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Montana: questions
When should I plant onions in Montana?+
In a typical Montana year, sow onions or set out transplants between Mar 7 to Apr 20, keyed to the state's median last frost around May 15. There's also a fall window: Jul 29 to Sep 9. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Montana?+
Start seeds inside about 6 weeks before set-out: roughly Jan 24 to Mar 9 for a typical Montana season.
What USDA zone is Montana?+
Montana spans zones 4a–5b, mostly zone 4. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.