Frost dates / Illinois / Rockford
Frost dates in Rockford, IL
The station on this page is ROCKFORD GTR ROCKFORD AP, IL US (USW00094822), 4.8 miles from the city point. Population in the Census 2024 estimate is 147,486.
Zone 5bUSDA average annual extreme minimum -15°F to -10°F, from ZIP 61104 (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 10 | Earlier date. A last 32°F on or after this day is common. |
| 50% | April 24 | Median. |
| 10% | May 8 | 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 2 | Earlier date. A first 32°F on or before this day is uncommon. |
| 50% | October 17 | Median. |
| 90% | October 30 | 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% | 192 |
| 50% | 174 |
| 90% | 154 |
28°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | April 27 | April 13 | March 31 |
| First, on or before | October 14 | October 28 | November 10 |
| Season at least | 217 days | 197 days | 177 days |
36°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | May 19 | May 4 | April 21 |
| First, on or before | September 23 | October 6 | October 20 |
| Season at least | 173 days | 153 days | 134 days |
Station ROCKFORD GTR ROCKFORD AP, IL US (USW00094822). Completeness flag S, 30 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 222.5 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 Rockford · Zone 5b
Check a crop against this station
Rockford, IL
Zone 5bAverage annual extreme minimum -15°F to -10°F
Outside the median season Growing seasonTicks: 10%, 50%, and 90%
Illinois. Nearest qualified station: ROCKFORD GTR ROCKFORD AP, IL US (USW00094822), 4.8 miles. Median last 32°F April 24. Median first 32°F October 17. The 50% growing-season length is 174 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 8 | April 24 | April 10 |
| First 32°F on or before | October 2 | October 17 | October 30 |
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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