Last spring frost
Apr 14
Frost Dates for Harristown, IL
First fall frost
Oct 24
Growing season
192 days
Hardiness zone
6a
In an average year around Harristown, IL, the last spring frost (32°F) is about Apr 14 and the first fall frost is about Oct 24, giving roughly a 192-day growing season. To be safe most years, wait until Apr 1 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 13 | Apr 26 | Apr 12 |
| Freeze (32°F) | May 1 | Apr 14 | Apr 1 |
| Hard (28°F) | Apr 17 | Apr 3 | Mar 18 |
| Severe (24°F) | Apr 7 | Mar 24 | Mar 7 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 13 | May 7 | May 3 | Apr 29 | Apr 26 | Apr 22 | Apr 19 | Apr 17 | Apr 12 |
| 32°F | May 1 | Apr 24 | Apr 20 | Apr 16 | Apr 14 | Apr 10 | Apr 8 | Apr 5 | Apr 1 |
| 28°F | Apr 17 | Apr 11 | Apr 7 | Apr 5 | Apr 3 | Mar 31 | Mar 29 | Mar 25 | Mar 18 |
| 24°F | Apr 7 | Apr 2 | Mar 31 | Mar 28 | Mar 24 | Mar 21 | Mar 17 | Mar 12 | Mar 7 |
| 20°F | Mar 31 | Mar 26 | Mar 22 | Mar 18 | Mar 15 | Mar 11 | Mar 7 | Mar 3 | Feb 26 |
| 16°F | Mar 22 | Mar 17 | Mar 13 | Mar 10 | Mar 6 | Mar 3 | Feb 28 | Feb 23 | Feb 14 |
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) | Sep 30 | Oct 14 | Oct 27 |
| Freeze (32°F) | Oct 11 | Oct 24 | Nov 5 |
| Hard (28°F) | Oct 20 | Nov 3 | Nov 19 |
| Severe (24°F) | Oct 30 | Nov 12 | Dec 1 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 30 | Oct 5 | Oct 8 | Oct 11 | Oct 14 | Oct 16 | Oct 19 | Oct 22 | Oct 27 |
| 32°F | Oct 11 | Oct 15 | Oct 18 | Oct 21 | Oct 24 | Oct 27 | Oct 30 | Nov 1 | Nov 5 |
| 28°F | Oct 20 | Oct 26 | Oct 30 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 12 | Nov 19 |
| 24°F | Oct 30 | Nov 3 | Nov 6 | Nov 9 | Nov 12 | Nov 15 | Nov 19 | Nov 25 | Dec 1 |
| 20°F | Nov 7 | Nov 11 | Nov 15 | Nov 19 | Nov 23 | Nov 27 | Dec 1 | Dec 5 | Dec 12 |
| 16°F | Nov 13 | Nov 19 | Nov 24 | Nov 29 | Dec 2 | Dec 6 | Dec 11 | Dec 18 | Dec 23 |
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: Decatur Wtp, Il, about 7.7 miles away@if(!empty($station['elevation_ft'])), at 639 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 Harristown, IL?
On average, the last spring frost (32°F) around Harristown, IL is about Apr 14. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 1.
When is the first frost in the fall?
The first fall frost around Harristown, IL typically arrives about Oct 24. 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 14 on average, or Apr 1 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 192 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 62537 Planting calendar → Illinois frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026