When to plant · 16 cities · zones 4b–6a
When to plant onions in Wisconsin
Start Spanish Onion seeds indoors around Feb 9 and transplant around Apr 6 — the Wisconsin statewide median, counted from a last spring frost of May 4 across 779 ZIP codes and 202 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Wisconsin onions, by step
Statewide median last frost May 4, first fall frost Oct 7, 2027 season. Each cultivar's own indoor lead time and days to maturity, same engine as the Plan Builder.
| Cultivar | Start indoors | Plant out | First harvest | Days |
|---|---|---|---|---|
| Spanish OnionYellow Sweet Spanish (long-day) | Feb 9 | Apr 6 | Jul 25 | 110 from transplant |
Tight means first harvest lands within two weeks of the median first fall frost. Statewide dates are the ZIP-weighted median of every NOAA 1991–2020 station Wisconsin's ZIP codes resolve to; 4 of 783 ZIPs have no datable freeze and are left out.
Onions in Wisconsin, city by city
Spanish Onion, scheduled from each city's nearest NOAA station — the same dates as its frost page.
| City | Last frost | Start indoors | Plant out | First harvest |
|---|---|---|---|---|
| Milwaukeezone 6a | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Madisonzone 5a | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Green Bayzone 5b | May 7 | Feb 12 | Apr 9 | Jul 28 |
| Kenoshazone 5b | Apr 28 | Feb 3 | Mar 31 | Jul 19 |
| Racinezone 6a | Apr 24 | Jan 30 | Mar 27 | Jul 15 |
| Appletonzone 5b | May 4 | Feb 9 | Apr 6 | Jul 25 |
| Eau Clairezone 4b | May 8 | Feb 13 | Apr 10 | Jul 29 |
| Waukeshazone 5b | Apr 30 | Feb 5 | Apr 2 | Jul 21 |
| Oshkoshzone 5b | Apr 24 | Jan 30 | Mar 27 | Jul 15 |
| Janesvillezone 5b | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| West Alliszone 5b | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| La Crossezone 5a | Apr 23 | Jan 29 | Mar 26 | Jul 14 |
| Sheboyganzone 6a | May 2 | Feb 7 | Apr 4 | Jul 23 |
| Wauwatosazone 5b | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Fond du Laczone 5b | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Brookfieldzone 5b | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
What these onions become
What Wisconsin onions need
Yellow Sweet Spanish Onion Seed
A long-day storage onion with enough sugar to sweat down without scorching. The aromatic base under every tomato sauce on this site.
Organic Seed Starter Potting MixMust have
Seed-starting mix is not potting soil with a nicer label — it is finer, lighter, and low in nutrients on purpose, because a seedling's first roots want air and moisture, not fertiliser.
72-Cell Trays with 1020 Bottoms, 5-PackMust have
The cheap black nursery trays crack after one season and flex enough to break roots when you lift them. These do not. Over five seasons the price per use is lower than the throwaways.
T8 Full-Spectrum Grow Lights, 1 ft 4-packMust have
A south window in March delivers a fraction of the light a pepper seedling needs, and the plant tells you by stretching. Two inches of LED above the canopy for 16 hours a day fixes it permanently.
Rapitest Soil Test Kit
In-ground growers are guessing until they test. pH outside 6.0–6.8 locks nutrients out no matter how much fertiliser you add, and no amount of feeding fixes that.
Pure Organic Worm Castings, 15 lb
Not a fertiliser so much as a soil-biology inoculant with a mild nutrient charge. A cup in each planting hole is a cheap, unglamorous improvement you can measure by August.
Garden Harvest Basket with Tool Caddy
A harvest basket that doubles as a colander means the hose rinse happens in the garden, and the dirt stays outside. On a 20-pound tomato day that is a genuine time saving.
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Onions in Wisconsin: questions
When should I plant onions in Wisconsin?
Transplant around Apr 6 at the statewide median; across the state that date runs from Mar 29 in the warmest tenth of Wisconsin's ZIP codes to Apr 21 in the coldest. To cut the freeze risk to one year in ten, count from May 20 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Wisconsin?
Around Feb 9 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Wisconsin?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 4. In the warmest tenth of Wisconsin's ZIP codes the median last frost falls on or before Apr 26; in the coldest tenth, on or after May 19.
When will Wisconsin onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Jul 25: 110 days from transplant.
Which onions grow in Wisconsin?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Wisconsin city listed here is north of that line.
More for Wisconsin
- When to plant tomatoes
- When to plant peppers
- When to plant tomatillos
- When to plant basil
- When to plant cilantro
- When to plant carrots
- When to plant mustard greens
Onions in every state → · Wisconsin frost dates → · How frost-date math works →