When to plant onion in Illinois
Illinois spans USDA zones 5a–7a, so the right time to plant onion shifts by weeks across the state. The window to transplant seedlings outdoors runs from about March 7 in Granite City to April 3 in McHenry — below are local dates for 226 cities, each computed from its own frost dates.
| City | Zone | Last frost | First frost | Transplant out |
|---|---|---|---|---|
| Chicago | 6a | April 10 | November 5 | March 13 – March 27 |
| Aurora | 5b | April 19 | October 23 | March 22 – April 5 |
| Naperville | 5b | April 28 | October 16 | March 31 – April 14 |
| Joliet | 5b | April 19 | October 25 | March 22 – April 5 |
| Rockford | 5b | April 24 | October 17 | March 27 – April 10 |
| Elgin | 5b | April 26 | October 18 | March 29 – April 12 |
| Springfield | 6a | April 15 | October 22 | March 18 – April 1 |
| Peoria | 6a | April 16 | October 23 | March 19 – April 2 |
Onion in Illinois: FAQ
When can I plant onion in Illinois?
Across Illinois, the time to transplant onion spans roughly March 7 in Granite City to April 3 in McHenry, each following that city's local frost dates.
Does the onion planting date vary across Illinois?
Yes. Illinois publishes 226 cities with their own frost dates, so the right onion planting window shifts by weeks between the warmest and coldest parts of the state — use your city's page for the exact dates.
How long does onion take to grow in Illinois?
Onion takes about 90–110 days to reach harvest once planted — check that this fits inside your city's frost-free season on its place page.
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