Frost dates / Illinois / Decatur
Frost dates in Decatur, IL
The station on this page is DECATUR WTP, IL US (USC00112193), 2.1 miles from the city point. Population in the Census 2024 estimate is 68,763.
Zone 6aUSDA average annual extreme minimum -10°F to -5°F, from ZIP 62523 (1.3 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 1 | Earlier date. A last 32°F on or after this day is common. |
| 50% | April 14 | Median. |
| 10% | May 1 | 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 11 | Earlier date. A first 32°F on or before this day is uncommon. |
| 50% | October 24 | Median. |
| 90% | November 5 | 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% | 211 |
| 50% | 192 |
| 90% | 171 |
28°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | April 17 | April 3 | March 18 |
| First, on or before | October 20 | November 3 | November 19 |
| Season at least | 237 days | 214 days | 195 days |
36°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | May 13 | April 26 | April 12 |
| First, on or before | September 30 | October 14 | October 27 |
| Season at least | 190 days | 170 days | 147 days |
Station DECATUR WTP, IL US (USC00112193). Completeness flag R, 24 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 194.8 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 Decatur · Zone 6a
Check a crop against this station
Decatur, 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: DECATUR WTP, IL US (USC00112193), 2.1 miles. Median last 32°F April 14. Median first 32°F October 24. The 50% growing-season length is 192 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 1 | April 14 | April 1 |
| First 32°F on or before | October 11 | October 24 | November 5 |
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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