GreenlyFrost dates and when to plant in Illinois
United States
In Illinois the last spring frost comes between 17 April (Springfield) and 26 April (Rockford) — the median of 2016–2025 in 4 cities. The first autumn frost comes between 28 October (Rockford) and 6 November (Chicago). The frost-free season lasts 185–202 days. Tomato: plant out into open ground 26 April – 17 July, earliest in Springfield, latest in Chicago.
Frost dates by city in Illinois
| City | Last spring frost | First autumn frost | Frost-free days | Climate zone | Tomato: window |
|---|---|---|---|---|---|
| Springfield | 17 April | 1 November | 198 | 150–209 frost-free days | 26 April – 12 July |
| Chicago | 18 April | 6 November | 202 | 150–209 frost-free days | 9 May – 17 July |
| Peoria | 19 April | 31 October | 195 | 150–209 frost-free days | 2 May – 11 July |
| Rockford | 26 April | 28 October | 185 | 150–209 frost-free days | 13 May – 8 July |
Median of 2016–2025 for the 25 km grid cell of the city: the last day at or below 0 °C in spring and the first in autumn (ERA5 reanalysis via Open-Meteo). In a cold year a frost can come a week or two later — Greenly warns 48 hours ahead.
When to plant in Illinois: windows by crop
The earliest start and the latest end among the cities here, each with its city; every city’s own window is on its page. Each window starts on the Greenly calendar date and ends on the last day that still leaves the crop’s days to harvest (from its card) before the average first autumn frost, plus six weeks of picking for crops harvested over weeks (tomatoes, peppers, aubergines, cucumbers, courgettes, pumpkins, watermelons, beans) — our rule; earlier in the window is safer.
| Crop | Planting out or sowing |
|---|---|
| Tomato | 26 April (Springfield) – 17 July (Chicago) |
| Cucumber | 15 May (Springfield) – 22 August (Chicago) |
| Bell Pepper, Eggplant | 15 May (Springfield) – 17 July (Chicago) |
| Zucchini | 15 May (Springfield) – 20 August (Chicago) |
| Pumpkin | 15 May (Springfield) – 12 July (Chicago) |
| Watermelon | 15 May (Springfield) – 28 July (Chicago) |
| Peas | 27 March (Springfield) – 24 August (Chicago) |
| Garlic, Tulip, Daffodil, Crocus, Hyacinth | Autumn planting: 28 September (Rockford) – 22 December (Springfield) |
Weather by month in Illinois
Across the 4 cities the warmest month is July (average days 27.9 … 29.8 °C) and the coldest January (average nights -8.9 … -5.3 °C). Every city averages below 0 °C at night in December–February.
| Month | Day, °C | Night, °C | Rain, mm |
|---|---|---|---|
| January | -0.9 … 3.1 °C | -8.9 … -5.3 °C | 58 |
| February | 2.3 … 6.7 °C | -7.2 … -3.7 °C | 56 |
| March | 8.7 … 12.8 °C | -0.8 … 2.4 °C | 94 |
| April | 13.9 … 18 °C | 3.6 … 6.8 °C | 100 |
| May | 19.9 … 23.8 °C | 10.2 … 13.2 °C | 119 |
| June | 26.4 … 29.3 °C | 16.4 … 18.7 °C | 95 |
| July | 27.9 … 29.8 °C | 18.5 … 20.5 °C | 112 |
| August | 27.2 … 28.5 °C | 17.5 … 19.3 °C | 82 |
| September | 24.3 … 26.6 °C | 14.7 … 16.5 °C | 73 |
| October | 17.1 … 19.7 °C | 7.9 … 9.9 °C | 95 |
| November | 8.6 … 11.6 °C | 0.5 … 2.6 °C | 56 |
| December | 2.7 … 6.5 °C | -5 … -2.1 °C | 55 |
Day and night: the lowest … highest monthly average among the cities; rain: the middle city (ERA5 via Open-Meteo).
What to plant month by month: the Northeast and Midwest
Other regions: United States
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More about the United States
- Gardening in the United States: climate zones and crops by region
- United States: frost dates by city
- When to plant tomatoes in the United States
- When to plant cucumbers in the United States
- When to plant bell peppers in the United States
- When to plant peas in the United States
- When to plant garlic in the United States
- Gardening by country
