Iowa City, Iowa: planting zone & frost dates
Iowa City is in USDA plant hardiness zone 5b, where the average coldest winter night reaches -15 to -10°F (-26.1 to -23.3°C). The typical last spring frost is around Apr 28 and the first fall frost around Oct 13.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 6 | Oct 4 |
| Typical planning date (30% risk) | Apr 28 | Oct 13 |
| Median (50%) | Apr 21 | Oct 18 |
How to read this: after the spring "10% risk" date, a 32°F frost happens in only about 1 year in 10. In fall, the "10% risk" date is the early end — a frost before it happens about 1 year in 10.
Frost data comes from NOAA station IOWA CITY MUNI AP, IA (USW00014937), 1.7 mi (2.7 km) from the city center and 3 ft lower than the city center (-1 m). Record used for the 1991–2020 normals: 23 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Iowa City
All 4 ZIP codes covering Iowa City are in zone 5b.
Vegetable planting calendar for Iowa City
Dates are counted from the 30%-risk frost dates above (last frost Apr 28, first frost Oct 13). Shift a week later in cold spots or low-lying ground.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato | Mar 3 – Mar 17 | May 5 – May 19 | — | — |
| Pepper | Feb 17 – Mar 3 | May 12 – May 26 | — | — |
| Lettuce | Mar 17 – Mar 31 | Apr 14 – May 5 | Mar 31 – May 12 | Aug 4 – Sep 1 |
| Green Beans | — | — | May 5 – Jun 9 | Jul 21 – Aug 4 |
| Sweet Corn | — | — | May 5 – Jun 2 | — |
| Zucchini & Summer Squash | Mar 31 – Apr 7 | May 5 – May 19 | May 5 – May 26 | Jul 21 – Aug 4 |
| Cucumber | Mar 31 – Apr 7 | May 12 – May 19 | May 5 – May 26 | Jul 21 – Aug 4 |
| Carrot | — | — | Apr 7 – May 12 | Jul 21 – Aug 4 |
| Onion | Feb 3 – Feb 17 | Mar 31 – Apr 14 | — | — |
| Potato | — | Mar 31 – Apr 21 | — | — |
| Basil | Mar 17 – Mar 31 | May 12 – May 26 | May 12 – Jun 2 | — |
| Spinach | — | — | Mar 17 – Apr 14 | Aug 18 – Sep 15 |
| Kale | Mar 3 – Mar 17 | Mar 31 – Apr 21 | Mar 31 – Apr 21 | Aug 4 – Sep 1 |
| Broccoli | Mar 3 – Mar 17 | Apr 14 – May 5 | — | Jul 21 – Aug 4 |
| Peas | — | — | Mar 17 – Apr 7 | Aug 4 – Aug 18 |
| Eggplant | Feb 17 – Mar 3 | May 12 – May 26 | — | — |
| Cabbage | Mar 3 – Mar 17 | Mar 31 – Apr 14 | — | Jul 21 – Aug 4 |
| Radish | — | — | Mar 24 – May 12 | Sep 1 – Sep 29 |
| Strawberry | — | Mar 31 – Apr 21 | — | — |
| Beets | — | — | Apr 7 – May 19 | Aug 4 – Aug 18 |
Nearby cities in Iowa
- Cedar Rapids — zone 5a, last frost May 3, first frost Oct 5
- Davenport — zone 5b, last frost Apr 24, first frost Oct 17
- Ankeny — zone 5b, last frost Apr 29, first frost Oct 9
- Des Moines — zone 5b, last frost Apr 24, first frost Oct 14
- Sioux City — zone 5a, last frost May 4, first frost Oct 1
More: all Iowa cities · other zone 5b cities: Madison, WI, Des Moines, IA, Aurora, IL, Fort Collins, CO, Naperville, IL, Rockford, IL, Elgin, IL, Greeley, CO
Iowa City gardening questions
When is the last frost in Iowa City, IA?
Plan around Apr 28. At NOAA's Iowa City Municipal Airport, IA station, 1.7 mi from the city center, there is a 30% chance of a 32°F frost after that date. Half of springs have their last frost by Apr 21, and after May 6 the risk drops to about 1 in 10. A hard freeze (28°F) usually ends earlier, around Apr 9. The 90% and 10% dates are 31 days apart, close to the 36-day median for cities on this site.
When should I plant tomatoes in Iowa City, IA?
Start seeds indoors Mar 3 – Mar 17 and set transplants outside May 5 – May 19, once nights stay above 50°F and the soil is near 60°F. That leaves 168 days between the 30% frost dates, enough for most varieties including 80–85-day slicers. More: tomato planting dates by zone.
How long is the growing season in Iowa City?
About 180 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 159 days, and in 1 year in 10 it runs 201 days or more. In fall there is a 10% chance of frost by Oct 4 and a 30% chance by Oct 13. That ranks 3rd longest of 6 Iowa cities on this site.
When should I plant a fall garden in Iowa City?
Count back from the first fall frost (Oct 13): transplant broccoli Jul 21 – Aug 4; sow kale Aug 4 – Sep 1; sow lettuce Aug 4 – Sep 1; sow radishes Sep 1 – Sep 29. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 28. How the math works: succession and fall planting.
Did Iowa City's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Iowa City is zone 5b on both the 2012 and the 2023 USDA maps (PRISM/OSU ZIP listings). About 42% of U.S. ZIP codes kept their half-zone in 2023. Why the 2023 map is warmer.
How does Iowa City compare with Cedar Rapids, IA?
Iowa City is zone 5b and Cedar Rapids is zone 5a. The typical last spring frost in Iowa City (Apr 28) comes about 5 days earlier than in Cedar Rapids, IA (May 3), 23 miles away, and the first fall frost is Oct 13 versus Oct 5. The two cities use different NOAA stations.