Last spring frost
Apr 8
Frost Dates for Flat Rock, IL
First fall frost
Oct 26
Growing season
200 days
Hardiness zone
6b
In an average year around Flat Rock, IL, the last spring frost (32°F) is about Apr 8 and the first fall frost is about Oct 26, giving roughly a 200-day growing season. To be safe most years, wait until Mar 26 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 4 | Apr 19 | Apr 6 |
| Freeze (32°F) | Apr 22 | Apr 8 | Mar 26 |
| Hard (28°F) | Apr 12 | Mar 29 | Mar 14 |
| Severe (24°F) | Apr 1 | Mar 17 | Mar 1 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 4 | Apr 30 | Apr 26 | Apr 22 | Apr 19 | Apr 16 | Apr 13 | Apr 10 | Apr 6 |
| 32°F | Apr 22 | Apr 18 | Apr 14 | Apr 11 | Apr 8 | Apr 5 | Apr 3 | Mar 31 | Mar 26 |
| 28°F | Apr 12 | Apr 7 | Apr 4 | Apr 1 | Mar 29 | Mar 26 | Mar 23 | Mar 19 | Mar 14 |
| 24°F | Apr 1 | Mar 28 | Mar 24 | Mar 20 | Mar 17 | Mar 14 | Mar 11 | Mar 6 | Mar 1 |
| 20°F | Mar 23 | Mar 17 | Mar 14 | Mar 11 | Mar 7 | Mar 4 | Feb 28 | Feb 24 | Feb 15 |
| 16°F | Mar 16 | Mar 10 | Mar 5 | Mar 2 | Feb 28 | Feb 23 | Feb 16 | Feb 10 | Feb 3 |
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 2 | Oct 17 | Oct 29 |
| Freeze (32°F) | Oct 13 | Oct 26 | Nov 7 |
| Hard (28°F) | Oct 25 | Nov 5 | Nov 22 |
| Severe (24°F) | Nov 4 | Nov 17 | Dec 5 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Oct 2 | Oct 8 | Oct 12 | Oct 15 | Oct 17 | Oct 20 | Oct 22 | Oct 25 | Oct 29 |
| 32°F | Oct 13 | Oct 19 | Oct 22 | Oct 24 | Oct 26 | Oct 29 | Oct 31 | Nov 3 | Nov 7 |
| 28°F | Oct 25 | Oct 30 | Nov 1 | Nov 3 | Nov 5 | Nov 8 | Nov 11 | Nov 16 | Nov 22 |
| 24°F | Nov 4 | Nov 7 | Nov 11 | Nov 14 | Nov 17 | Nov 22 | Nov 25 | Nov 30 | Dec 5 |
| 20°F | Nov 11 | Nov 17 | Nov 22 | Nov 26 | Nov 30 | Dec 2 | Dec 6 | Dec 11 | Dec 19 |
| 16°F | Nov 22 | Nov 28 | Dec 1 | Dec 6 | Dec 10 | Dec 14 | Dec 20 | Dec 24 | Dec 30 |
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: Lawrenceville Intl Ap, Il, about 7.9 miles away@if(!empty($station['elevation_ft'])), at 429 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 Flat Rock, IL?
On average, the last spring frost (32°F) around Flat Rock, IL is about Apr 8. In cooler years it can run later, so for a safer bet many gardeners wait until Mar 26.
When is the first frost in the fall?
The first fall frost around Flat Rock, IL typically arrives about Oct 26. 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 26 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 200 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 62427 Planting calendar → Illinois frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026