Seed Starting Calendar for Wisconsin

Count back from the median last frost where you garden. At Madison (Madison Dane Regional Airport) that date is May 1, so for spring 2027 onions start indoors Jan 23 to Feb 6, peppers Mar 6 to Mar 20 and tomatoes Mar 13 to Mar 27. Across Wisconsin's 200 NOAA stations the median last frost runs from Apr 24 to May 24 for the middle 90 percent, so a garden near North Pelican starts later than one near Kenosha.

It depends on the station. The earliest median last spring freeze in Wisconsin is April 19 at Kenosha; the latest is June 3 at North Pelican. A median is a coin flip: at Madison the last 32°F freeze comes after May 1 in half of years, and has passed by May 16 in 9 of 10 years (NOAA 1991 to 2020 normals).

UNH Extension sets tomatoes out 1 to 2 weeks after the last frost, which at Madison is May 8 to May 15 in 2027. Tomatoes are frost tender, so the date to watch is May 16, when 9 of 10 springs at Madison have had their last 32°F night. Hardening off starts two weeks before setting out (Apr 24 to May 8).

University of Wisconsin-Madison Division of Extension publishes planting months by region (Madison (southern Wisconsin); about 1 week later on the lower lake shore and central Wisconsin, about 2 weeks later in northern counties). For Madison (southern Wisconsin); about 1 week later on the lower lake shore and central Wisconsin, about 2 weeks later in northern counties, tomatoes go in during April. Seeds for transplants go in 4 to 10 weeks ahead of those months, depending on the crop.

University of Wisconsin-Madison Division of Extension's The Vegetable Garden puts it this way: "Plant about 1 week later along the lower lake shore and in the central part of state and about 2 weeks later in northern counties." The guide is linked below with the date it was read (September 21, 2026).

UNH Extension gives 8 to 10 weeks from seed to transplant for both, and sets peppers out 2 weeks after the last frost and eggplant 2 to 3 weeks after, so the sowing date is 6 to 8 weeks before the median last frost. At Madison that is Mar 6 to Mar 20 for peppers.

About two weeks. University of Missouri Extension: "Hardening should be started about two weeks before planting in the garden." University of Minnesota Extension adds that seedlings should go out on a cloudy day or in the late afternoon.

Milwaukee on Lake Michigan reaches its median last frost around Apr 19, La Crosse around Apr 23, Madison around May 1, and Wausau and Grantsburg in the north around May 6 to May 11. Each station carries its own dates: Milwaukee Apr 19 and Grantsburg May 11 at the median.