Last spring frost
Apr 15
Frost Dates for Sullivan, IL
First fall frost
Oct 22
Growing season
187 days
Hardiness zone
6a
In an average year around Sullivan, IL, the last spring frost (32°F) is about Apr 15 and the first fall frost is about Oct 22, giving roughly a 187-day growing season. To be safe most years, wait until Apr 2 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 25 | Apr 13 |
| Freeze (32°F) | Apr 30 | Apr 15 | Apr 2 |
| Hard (28°F) | Apr 18 | Apr 4 | Mar 20 |
| Severe (24°F) | Apr 8 | Mar 26 | Mar 8 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 13 | May 7 | May 2 | Apr 29 | Apr 25 | Apr 23 | Apr 20 | Apr 17 | Apr 13 |
| 32°F | Apr 30 | Apr 24 | Apr 20 | Apr 17 | Apr 15 | Apr 12 | Apr 10 | Apr 7 | Apr 2 |
| 28°F | Apr 18 | Apr 13 | Apr 9 | Apr 6 | Apr 4 | Apr 2 | Mar 31 | Mar 26 | Mar 20 |
| 24°F | Apr 8 | Apr 3 | Mar 31 | Mar 29 | Mar 26 | Mar 22 | Mar 19 | Mar 14 | Mar 8 |
| 20°F | Mar 31 | Mar 26 | Mar 22 | Mar 19 | Mar 15 | Mar 12 | Mar 8 | Mar 3 | Feb 26 |
| 16°F | Mar 22 | Mar 17 | Mar 13 | Mar 9 | Mar 6 | Mar 2 | Feb 27 | Feb 22 | Feb 13 |
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 25 | Oct 11 | Oct 24 |
| Freeze (32°F) | Oct 6 | Oct 22 | Nov 2 |
| Hard (28°F) | Oct 20 | Nov 1 | Nov 16 |
| Severe (24°F) | Nov 1 | Nov 11 | Nov 29 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 25 | Sep 30 | Oct 4 | Oct 7 | Oct 11 | Oct 13 | Oct 16 | Oct 19 | Oct 24 |
| 32°F | Oct 6 | Oct 13 | Oct 17 | Oct 20 | Oct 22 | Oct 24 | Oct 27 | Oct 30 | Nov 2 |
| 28°F | Oct 20 | Oct 25 | Oct 28 | Oct 31 | Nov 1 | Nov 4 | Nov 6 | Nov 10 | Nov 16 |
| 24°F | Nov 1 | Nov 3 | Nov 6 | Nov 8 | Nov 11 | Nov 15 | Nov 19 | Nov 23 | Nov 29 |
| 20°F | Nov 8 | Nov 11 | Nov 15 | Nov 20 | Nov 24 | Nov 27 | Nov 30 | Dec 4 | Dec 10 |
| 16°F | Nov 14 | Nov 22 | Nov 27 | Nov 30 | Dec 3 | Dec 7 | Dec 12 | 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: Sullivan 3S, Il, about 2.6 miles away@if(!empty($station['elevation_ft'])), at 640 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 Sullivan, IL?
On average, the last spring frost (32°F) around Sullivan, IL is about Apr 15. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 2.
When is the first frost in the fall?
The first fall frost around Sullivan, IL typically arrives about Oct 22. 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 15 on average, or Apr 2 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 187 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 61951 Planting calendar → Illinois frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026