When to Plant Summer Squash and Zucchini in Illinois: Spring and Fall Dates by Region
One Illinois calendar, one crop: summer squash, with the source’s dates left as printed and NOAA frost dates for Chicago O'Hare International Airport, Peoria Greater Peoria Airport, and Springfield Capital Airport.
Updated
Figures checked against their sources on ; next check .
Illinois is covered by University of Illinois Extension’s own table, and summer squash sit under the heading “Squash (summer) from Seed”, in it as a separate row for 3 regions. The earliest date in the table is April 28 in Southern Illinois, and the latest is May 24 in Northern Illinois, a gap of 26 days.
Summer Squash and Zucchini: the Illinois table
The page adds this line for the row: “Sow 2 seeds 24-36" apart, 1" deep. Thin weaker seedlings.”
| Region | First planting | Second planting |
|---|---|---|
| Northern Illinois | May 24 to June 30 | — |
| Central Illinois | May 10 to June 15 | — |
| Southern Illinois | April 28 to June 1 | — |
How the Illinois calendar is built
Illinois Extension splits the state into northern, central and southern Illinois and prints one planting range for each. It says planting dates are set by the first and last frost dates and by how long a crop takes to mature.
Starting summer squash indoors
A calendar of outdoor dates says nothing about when the seed tray goes on the windowsill. For that, UNH offers 3 to 4 weeks of indoor growth before summer squash go out.
Expect less guidance here than for tomatoes: summer squash get a single timed source and a note on pots.
In the garden itself, UNH sows summer squash once the soil reaches 70 °F, which a soil thermometer answers better than any calendar.
| Source | What it says |
|---|---|
| University of New Hampshire | 3 to 4 weeks from seed to transplant; 2 weeks after the last frost is safe to set out |
| University of Minnesota | Cucumbers and squash transplant poorly, so raise them in fiber or paper pots that break down in the soil |
Typical frost dates at Illinois stations
A 50% frost date is a coin flip, not a promise. In half of the years the last spring freeze comes later than the date shown, and in half of them the first fall freeze comes earlier.
A station is tied to a region of the calendar only where the source’s own wording makes that call; the rest are left blank on purpose.
| Station | Last spring 32°F | First fall 32°F | Region in Illinois’s calendar |
|---|---|---|---|
| Chicago O'Hare International Airport, Illinois | April 16 | October 28 | Northern Illinois |
| Peoria Greater Peoria Airport, Illinois | April 16 | October 23 | Not stated by the source |
| Springfield Capital Airport, Illinois | April 14 | October 20 | Not stated by the source |
| Carbondale Southern Il Airport, Illinois | April 9 | October 22 | Not stated by the source |
| Rockford Greater Rockford Airport, Illinois | April 24 | October 17 | Not stated by the source |
Worked example
Take Chicago O'Hare International Airport, Illinois. NOAA’s 1991-2020 normals put the 50% last spring 32°F night on April 16 and the 50% first fall one on October 28.
Northern Illinois’ window for summer squash runs from May 24 to June 30. The opening falls 38 days after the median last freeze, and the close 75 days after it.
None of this predicts next spring. It only lines up two published sets of dates; swap in your own station from the frost dates tool and the same steps apply.
Before you rely on these dates
The page tells readers to always confirm dates with the seed packet or plant tag.
Weather in the week of planting matters more than the printed date; use it to nudge the start earlier or later.
Sources (4)
- University of Illinois Extension (Kelly Allsup), When to Plant, Home Vegetable Gardening, updated August 2025 extension.illinois.edu
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991-2020, annual and seasonal normals (frost/freeze probabilities by station) ncei.noaa.gov
- University of New Hampshire Cooperative Extension, Growing Vegetables: When to Plant Your Vegetable Garden (fact sheet) extension.unh.edu
- University of Minnesota Extension, Starting seeds indoors (page reviewed 2026) extension.umn.edu