When to plant onions in Wisconsin
Wisconsin’s average last spring frost is May 10, and the first fall frost typically arrives around September 30 — about 143 frost-free days (USDA zones 3b, 4a, 4b, 5a, 5b). Onion needs 90–120 days to mature, which fits comfortably within Wisconsin's growing season.
When to plant onions in Wisconsin: the short answer
For a spring planting, plant out Apr 12 – Apr 29, with seeds started indoors Feb 22 – Mar 8. 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 Wisconsin, step by step
- Start seeds indoors (Feb 22 – Mar 8). 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 12 – Apr 29). 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 Wisconsin.
- 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 18 – Aug 17). 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 Wisconsin’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in Wisconsin
Wisconsin spans USDA zones 3b, 4a, 4b, 5a, 5b. 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 Wisconsin ask
When is the best time to plant onions in Wisconsin?
Plant onion transplants outdoors in Wisconsin between April 12 and April 29. That timing is set by Wisconsin's average last spring frost of May 10.
When should I start onion seeds indoors in Wisconsin?
Start seeds indoors between February 22 and March 8, about 10 weeks before the last frost, so the seedlings are ready to plant out from April 12.
When will onions planted in Wisconsin be ready to harvest?
Planted on time, onions are usually ready from about July 18 to August 17, 90-120 days after transplanting.
When is the last frost in Wisconsin?
On average around May 10, with the first fall frost around September 30 - about 143 frost-free days. Your town may differ by two weeks or more.