Last spring frost
Apr 24
Frost Dates for Thornburg, IA
First fall frost
Oct 15
Growing season
172 days
Hardiness zone
5b
In an average year around Thornburg, IA, the last spring frost (32°F) is about Apr 24 and the first fall frost is about Oct 15, giving roughly a 172-day growing season. To be safe most years, wait until Apr 9 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 17 | May 4 | Apr 20 |
| Freeze (32°F) | May 9 | Apr 24 | Apr 9 |
| Hard (28°F) | Apr 30 | Apr 12 | Mar 31 |
| Severe (24°F) | Apr 21 | Apr 3 | Mar 18 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 17 | May 12 | May 9 | May 7 | May 4 | May 2 | Apr 30 | Apr 26 | Apr 20 |
| 32°F | May 9 | May 4 | May 1 | Apr 28 | Apr 24 | Apr 21 | Apr 18 | Apr 14 | Apr 9 |
| 28°F | Apr 30 | Apr 24 | Apr 19 | Apr 15 | Apr 12 | Apr 9 | Apr 6 | Apr 3 | Mar 31 |
| 24°F | Apr 21 | Apr 13 | Apr 9 | Apr 6 | Apr 3 | Apr 1 | Mar 30 | Mar 24 | Mar 18 |
| 20°F | Apr 10 | Apr 4 | Mar 31 | Mar 29 | Mar 25 | Mar 22 | Mar 18 | Mar 13 | Mar 7 |
| 16°F | Mar 31 | Mar 26 | Mar 23 | Mar 19 | Mar 15 | Mar 12 | Mar 8 | Mar 5 | Mar 1 |
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 21 | Oct 5 | Oct 17 |
| Freeze (32°F) | Sep 30 | Oct 15 | Oct 27 |
| Hard (28°F) | Oct 12 | Oct 24 | Nov 4 |
| Severe (24°F) | Oct 21 | Nov 1 | Nov 15 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 21 | Sep 26 | Sep 29 | Oct 2 | Oct 5 | Oct 7 | Oct 10 | Oct 13 | Oct 17 |
| 32°F | Sep 30 | Oct 5 | Oct 9 | Oct 12 | Oct 15 | Oct 17 | Oct 20 | Oct 23 | Oct 27 |
| 28°F | Oct 12 | Oct 16 | Oct 20 | Oct 22 | Oct 24 | Oct 26 | Oct 29 | Nov 1 | Nov 4 |
| 24°F | Oct 21 | Oct 25 | Oct 28 | Oct 30 | Nov 1 | Nov 4 | Nov 7 | Nov 10 | Nov 15 |
| 20°F | Oct 31 | Nov 2 | Nov 5 | Nov 8 | Nov 11 | Nov 14 | Nov 18 | Nov 22 | Nov 27 |
| 16°F | Nov 5 | Nov 10 | Nov 14 | Nov 17 | Nov 21 | Nov 24 | Nov 28 | Dec 1 | Dec 6 |
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: Sigourney, Ia, about 9.7 miles away@if(!empty($station['elevation_ft'])), at 732 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 Thornburg, IA?
On average, the last spring frost (32°F) around Thornburg, IA is about Apr 24. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 9.
When is the first frost in the fall?
The first fall frost around Thornburg, IA typically arrives about Oct 15. 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 24 on average, or Apr 9 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 172 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 50136 Planting calendar → Iowa frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026