Frost dates / Illinois / Downers Grove
Frost dates in Downers Grove, IL
The station on this page is LISLE-MORTON ARBORETUM, IL US (USC00115097), 2.9 miles from the city point. Population in the Census 2024 estimate is 50,552.
Zone 6aUSDA average annual extreme minimum -10°F to -5°F, from ZIP 60515 (1.1 miles from the city point used on this page).
Outside the median season Growing seasonTicks: 10%, 50%, and 90%
Last 32°F in spring
NCEI’s probability is the chance the last spring minimum of 32°F or lower occurs on or after the date.
| Probability | Date | How to read it |
|---|---|---|
| 90% | April 14 | Earlier date. A last 32°F on or after this day is common. |
| 50% | April 28 | Median. |
| 10% | May 14 | Later date. A last 32°F on or after this day is uncommon. |
First 32°F in fall
NCEI’s probability is the chance the first autumn minimum of 32°F or lower occurs on or before the date.
| Probability | Date | How to read it |
|---|---|---|
| 10% | October 1 | Earlier date. A first 32°F on or before this day is uncommon. |
| 50% | October 16 | Median. |
| 90% | October 29 | Later date. A first 32°F on or before this day is common. |
32°F growing-season length
Days between the last spring 32°F and the first autumn 32°F. The figure is a length the season meets or exceeds at that probability, from NCEI, not a subtraction done on this site.
| Probability the season is at least this long | Days |
|---|---|
| 10% | 188 |
| 50% | 169 |
| 90% | 148 |
28°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | May 4 | April 16 | April 2 |
| First, on or before | October 12 | October 26 | November 8 |
| Season at least | 210 days | 190 days | 169 days |
36°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | May 25 | May 9 | April 25 |
| First, on or before | September 22 | October 6 | October 18 |
| Season at least | 167 days | 148 days | 129 days |
Station LISLE-MORTON ARBORETUM, IL US (USC00115097). Completeness flag R, 13 years used for the 32°F dates. NCEI’s annual probability of at least one day at or below 32°F is 100%. Freeze dates are conditional on that threshold occurring. Elevation 207.3 m. The same probability language applies to the 28°F and 36°F rows when those dates passed the same quality checks.
Planting calendar for Downers Grove · Zone 6a
Check a crop against this station
Downers Grove, IL
Zone 6aAverage annual extreme minimum -10°F to -5°F
Outside the median season Growing seasonTicks: 10%, 50%, and 90%
Illinois. Nearest qualified station: LISLE-MORTON ARBORETUM, IL US (USC00115097), 2.9 miles. Median last 32°F April 28. Median first 32°F October 16. The 50% growing-season length is 169 days.
Open the ZIP page · Crop guides for Illinois
32°F probabilities at this station
| 10% | 50% | 90% | |
|---|---|---|---|
| Last 32°F on or after | May 14 | April 28 | April 14 |
| First 32°F on or before | October 1 | October 16 | October 29 |
Column order matches the NCEI file: for the last freeze, 10% is the later date and 90% is the earlier date. How to read the probabilities.
Planting calendar
Windows below are from University of Illinois Extension, When to Plant, retrieved 2026-10-10. The planting chart is divided into Northern Illinois, Central Illinois, and Southern Illinois. Median dates for the last 32-degree freeze on the same page: Northern Illinois April 8-29; Central Illinois April 8-15; Southern Illinois April 1-8. The page also says southern Illinois can generally start crops two weeks earlier than central Illinois and northern Illinois about two weeks later; the crop rows below are the chart dates, not a recalculation of that sentence.
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