When to plant onions in Minnesota
Minnesota’s average last spring frost is May 6, and the first fall frost typically arrives around September 28 — about 145 frost-free days (USDA zones 3a, 3b, 4a, 4b, 5a). Onion needs 90–120 days to mature, which fits comfortably within Minnesota's growing season.
When to plant onions in Minnesota: the short answer
For a spring planting, plant out Apr 8 – Apr 25, with seeds started indoors Feb 18 – Mar 4. Seeds germinate in soil from 40 to 90°F.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow onions in Minnesota, step by step
- Start seeds indoors (Feb 18 – Mar 4). Sow seeds ½ inch deep in seed-starting mix under bright light. They take 7-14 days to sprout, fastest in soil around 70°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 12 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Apr 8 – Apr 25). Space plants about 4 inches apart in rows 12 inches apart. It handles frost well, so it can go out before the last frost date. As a cool-season crop it does best before summer heat arrives in Minnesota.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: onion maggot, onion thrips. Diseases and disorders: downy mildew, white rot, neck rot, purple blotch.
- Harvest (from about Jul 14 – Aug 13). Onion takes 90-120 days to reach harvest. Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.
Figures come from our crop data; dates use Minnesota’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in Minnesota
Minnesota spans USDA zones 3a, 3b, 4a, 4b, 5a. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the onion dates shift with them.
When to plant onions in nearby states
Questions gardeners in Minnesota ask
When is the best time to plant onions in Minnesota?
Plant onion transplants outdoors in Minnesota between April 8 and April 25. That timing is set by Minnesota's average last spring frost of May 6.
When should I start onion seeds indoors in Minnesota?
Start seeds indoors between February 18 and March 4, about 10 weeks before the last frost, so the seedlings are ready to plant out from April 8.
When will onions planted in Minnesota be ready to harvest?
Planted on time, onions are usually ready from about July 14 to August 13, 90-120 days after transplanting.
When is the last frost in Minnesota?
On average around May 6, with the first fall frost around September 28 - about 145 frost-free days. Your town may differ by two weeks or more.