Seed Starting Calendar for Iowa

Count back from the median last frost where you garden. At Cedar Rapids (Cedar Rapids Municipal Airport) that date is April 27, so for spring 2027 onions start indoors Jan 19 to Feb 2, peppers Mar 2 to Mar 16 and tomatoes Mar 9 to Mar 23. Across Iowa's 144 NOAA stations the median last frost runs from Apr 17 to May 4 for the middle 90 percent, so a garden near Cresco 1 NE starts later than one near Keokuk Lock Dam 19.

It depends on the station. The earliest median last spring freeze in Iowa is April 10 at Keokuk Lock Dam 19; the latest is May 8 at Cresco 1 NE. A median is a coin flip: at Cedar Rapids the last 32°F freeze comes after April 27 in half of years, and has passed by May 11 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 Cedar Rapids is May 4 to May 11 in 2027. Tomatoes are frost tender, so the date to watch is May 11, when 9 of 10 springs at Cedar Rapids have had their last 32°F night. Hardening off starts two weeks before setting out (Apr 20 to May 4).

Iowa State University Extension and Outreach publishes planting months by region (Central Iowa (southern Iowa about one week earlier, northern Iowa about one week later); months are outdoor transplant or direct sow). For Central Iowa (southern Iowa about one week earlier, northern Iowa about one week later); months are outdoor transplant or direct sow, tomatoes go in during May and June. Seeds for transplants go in 4 to 10 weeks ahead of those months, depending on the crop.

Iowa State University Extension and Outreach's Planting and Harvesting Times for Garden Vegetables puts it this way: "The dates listed are for Central Iowa. (Southern Iowa can shift the sowing, planting, and harvesting dates approximately one week earlier. Northern Iowa can shift the dates approximately one week later)." 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 Cedar Rapids that is Mar 2 to Mar 16 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.

Des Moines reaches its median last frost around Apr 18, Cedar Rapids around Apr 27, and Decorah in the Driftless northeast around May 3, so an Iowa seed calendar shifts about two weeks north to south. Each station carries its own dates: Des Moines Apr 18 and Decorah May 3 at the median.