Last spring frost
Apr 8
Frost Dates for Prairie du Rocher, IL
First fall frost
Oct 30
Growing season
202 days
Hardiness zone
7a
In an average year around Prairie du Rocher, IL, the last spring frost (32°F) is about Apr 8 and the first fall frost is about Oct 30, giving roughly a 202-day growing season. To be safe most years, wait until Mar 22 before setting out tender plants.
Last spring frost
Dates the last frost of spring typically ends. Plant tender crops after the risk has passed for your comfort level.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | May 5 | Apr 20 | Apr 5 |
| Freeze (32°F) | Apr 25 | Apr 8 | Mar 22 |
| Hard (28°F) | Apr 13 | Mar 28 | Mar 9 |
| Severe (24°F) | Apr 2 | Mar 14 | Feb 26 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 5 | Apr 30 | Apr 26 | Apr 23 | Apr 20 | Apr 17 | Apr 14 | Apr 10 | Apr 5 |
| 32°F | Apr 25 | Apr 19 | Apr 15 | Apr 11 | Apr 8 | Apr 5 | Apr 2 | Mar 29 | Mar 22 |
| 28°F | Apr 13 | Apr 7 | Apr 3 | Mar 31 | Mar 28 | Mar 23 | Mar 19 | Mar 15 | Mar 9 |
| 24°F | Apr 2 | Mar 27 | Mar 22 | Mar 18 | Mar 14 | Mar 10 | Mar 7 | Mar 3 | Feb 26 |
| 20°F | Mar 22 | Mar 16 | Mar 12 | Mar 8 | Mar 4 | Mar 1 | Feb 26 | Feb 20 | Feb 9 |
| 16°F | Mar 14 | Mar 8 | Mar 4 | Feb 28 | Feb 24 | Feb 18 | Feb 11 | Feb 5 | Jan 30 |
First fall frost
Dates the first frost of fall typically arrives. Count back by a crop's days-to-maturity to know your planting cutoff.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | Oct 4 | Oct 18 | Oct 31 |
| Freeze (32°F) | Oct 15 | Oct 30 | Nov 13 |
| Hard (28°F) | Oct 27 | Nov 10 | Nov 30 |
| Severe (24°F) | Nov 5 | Nov 24 | Dec 12 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Oct 4 | Oct 9 | Oct 12 | Oct 15 | Oct 18 | Oct 20 | Oct 23 | Oct 27 | Oct 31 |
| 32°F | Oct 15 | Oct 20 | Oct 24 | Oct 28 | Oct 30 | Nov 1 | Nov 4 | Nov 8 | Nov 13 |
| 28°F | Oct 27 | Nov 1 | Nov 3 | Nov 6 | Nov 10 | Nov 13 | Nov 18 | Nov 23 | Nov 30 |
| 24°F | Nov 5 | Nov 12 | Nov 16 | Nov 20 | Nov 24 | Nov 28 | Dec 1 | Dec 5 | Dec 12 |
| 20°F | Nov 16 | Nov 23 | Nov 28 | Dec 1 | Dec 4 | Dec 8 | Dec 13 | Dec 18 | Dec 24 |
| 16°F | Nov 27 | Dec 1 | Dec 7 | Dec 11 | Dec 16 | Dec 21 | Dec 26 | Dec 30 | Jan 7 |
Growing season is measured at 32°F with about 50% probability. Hardiness zone from the 2023 USDA map; frost normals from NOAA 1991-2020.
Where this data comes from
@endif These dates are computed from the nearest official weather station with long-term records: Kaskaskia Rvr Navigation Lock, Il, about 11.9 miles away@if(!empty($station['elevation_ft'])), at 380 ft elevation. The further the station and the greater the elevation difference from your yard, the more your local frost timing can vary.
How we calculate these dates
Frost and freeze dates come from the NOAA 1991–2020 U.S. Climate Normals, the National Oceanic and Atmospheric Administration's 30-year averages for each weather station. A date shown at the 50% level is the median: in half of years the frost falls on or before it. The 10% and 90% columns show the early and late ends of the range, so you can plant on the odds you're comfortable with. These are long-term averages, not a forecast for this year - always check your local outlook before planting.
Source: NOAA National Centers for Environmental Information, 1991–2020 Climate Normals (public domain). Harman is not affiliated with or endorsed by NOAA.
Frequently asked questions
When is the last frost in Prairie du Rocher, IL?
On average, the last spring frost (32°F) around Prairie du Rocher, IL is about Apr 8. In cooler years it can run later, so for a safer bet many gardeners wait until Mar 22.
When is the first frost in the fall?
The first fall frost around Prairie du Rocher, IL typically arrives about Oct 30. Count back by a crop's days to maturity to find the latest date you can plant it.
When can I plant tomatoes and other tender crops?
Tender crops like tomatoes, peppers and basil are damaged by frost, so wait until the last spring frost has passed - around Apr 8 on average, or Mar 22 to be safe most years.
How long is the growing season here?
The frost-free growing season (between the last spring and first fall 32°F frost) is about 202 days in a typical year.
Are these dates a forecast for this year?
No. They are 30-year averages from the NOAA 1991–2020 Climate Normals, not a prediction for the current year. Use them for planning and always check your local weather forecast before planting or protecting crops.
Local weather for 62277 Planting calendar → Illinois frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026