Minnesota · zones 3b–5a
When to plant onions in Minnesota.
Sow or transplant between Feb 25 to Apr 10 in a typical year, keyed to Minnesota's median last frost around May 5. A second, fall sowing fits Aug 6 to Sep 17. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Minnesota season.
1 · Start indoors
Jan 14 to Feb 27
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 25 to Apr 10
Keyed to the ~May 5 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 6 to Sep 17
Counted back from the ~Oct 1 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Minnesota spans more than one climate: Zone 4 is the most common zone, but the state runs 3b to 5a. 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.
Minnesota growing notes
Minnesota onions start indoors before winter even lets go.
The math that makes an early indoor start non-negotiable
An onion transplant needs roughly 10 to 12 weeks of growth indoors before it is sturdy enough to set outside, and the bulb itself needs another 95 days or so after that before it is ready to pull. Add those two numbers together and the crop needs close to 6 months from seed to harvest, in a state where the frost-free window between the last spring freeze and the first fall one runs only about 149 days.
The only way that math works is to start seed well before the ground even thinks about thawing, so the transplant is already 10 or 12 weeks old and hardened off by the time it is safe to set outside. Growers who wait for reliably mild weather before starting seed indoors are, in effect, beginning the crop with a deficit the rest of the season cannot make up.
Grow for the root cellar, not the salad bowl
A short growing season rewards onions bred to store rather than ones bred for eating fresh, and Minnesota's winters make that choice easy: a variety that keeps only 3 months is not much use against 7 months of cold ahead. Patterson, a long-day hybrid, is the modern benchmark keeper and holds firm in a cool, dry space for 8 to 10 months, long enough to bridge one harvest to the next; it took over that role from Copra, which growers here favored for years before its breeder discontinued the seed line. Redwing, a red-skinned long-day type, keeps nearly as long and adds a second color to the cellar shelf.
All three varieties need the long summer daylight of Minnesota's 45 to 47 degree latitude to trigger bulbing properly; a short-day onion bred for a southern garden starts forming a bulb far too early up here and never reaches real size. Set transplants 4 inches apart in the row, since crowding is the fastest way to end up with a cellar full of small bulbs instead of large ones.
Curing indoors when the calendar runs out
Minnesota's autumn closes fast once it turns, and a crop still curing in the garden when the first hard freeze arrives will not store well. Pull bulbs once at least half the tops in the row have fallen over on their own, then move curing indoors to a garage or shed with a small fan running, rather than counting on several more weeks of open-air drying the way a longer-season state can afford.
Spread the bulbs in a single layer with air moving across them and check the necks after 2 weeks; they should feel dry and papery all the way around before the tops get trimmed and the bulbs go into storage. Onions cured too fast against an early freeze often trap moisture at the neck, and that trapped moisture is where storage rot starts within a month.
In Minnesota: watch for
The pests that find onions here.
The short season here, about 149 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: Minnesota gives onions about 149 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 Minnesota: questions
When should I plant onions in Minnesota?+
In a typical Minnesota year, sow onions or set out transplants between Feb 25 to Apr 10, keyed to the state's median last frost around May 5. There's also a fall window: Aug 6 to Sep 17. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Minnesota?+
Start seeds inside about 6 weeks before set-out: roughly Jan 14 to Feb 27 for a typical Minnesota season.
What USDA zone is Minnesota?+
Minnesota spans zones 3b–5a, 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.