When to Plant Onions in Wisconsin
The backbone of the kitchen garden. Choose short-day, intermediate, or long-day varieties based on your latitude.
The Short Answer
Wisconsin Frost Dates
Your planting dates depend on which part of Wisconsin you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Northern Wisconsin | 3b, 4a | May 15 - Jun 1 | Sep 10 - Sep 25 |
| Central Wisconsin | 4a, 4b | May 5 - May 20 | Sep 20 - Oct 5 |
| Southern Wisconsin | 4b, 5a, 5b | Apr 25 - May 10 | Oct 1 - Oct 15 |
Onions Planting Schedule for Wisconsin
Northern Wisconsin (Zones 3b, 4a)
Average last frost: May 15 - Jun 1 · Average first frost: Sep 10 - Sep 25
Central Wisconsin (Zones 4a, 4b)
Average last frost: May 5 - May 20 · Average first frost: Sep 20 - Oct 5
Southern Wisconsin (Zones 4b, 5a, 5b)
Average last frost: Apr 25 - May 10 · Average first frost: Oct 1 - Oct 15
Growing Onions in Wisconsin
Onions in Wisconsin's Climate
You're in long-day onion territory — plant varieties that bulb when days exceed 14-16 hours. This means Copra, Walla Walla, and Yellow Globe Danvers. Plant sets or transplants from mid-April. The critical distinction: short-day onions from southern catalogs will NOT form bulbs in your zone because your long summer days trigger bulbing too early, before the plant is large enough.
Soil Considerations for Wisconsin
Rich agricultural soils in southern half. Sandy soils in central plain. Heavy clay in some lake regions. Generally fertile. Loose, well-drained soil is especially important for onions since the edible portion grows underground. If your soil is heavy clay, consider raised beds.
Wisconsin Climate & Growing Season
Cold winters. Warm summers. Lake Michigan moderates eastern climate. Good growing season in south. Short season in north. Onions can handle frost well, which is an advantage in Wisconsin's climate. You can push planting dates earlier in spring and extend into fall.
Growing season length varies across Wisconsin: Northern Wisconsin (3b, 4a) has a last frost around May 15 - Jun 1, while Southern Wisconsin (4b, 5a, 5b) sees frost end around Apr 25 - May 10. This difference matters for onions — but onions handle frost well, so the timing difference is less critical.
Growing Tips
Day length triggers bulbing. Northern gardeners need long-day varieties. Southern gardeners need short-day varieties.
Companion Planting
Plant onions alongside these companions for better growth:
Keep onions away from:
The Bottom Line
Last reviewed: March 29, 2026