Last spring frost
May 23
Frost Dates for Sweetwater, ID
First fall frost
Sep 25
Growing season
121 days
Hardiness zone
7b
In an average year around Sweetwater, ID, the last spring frost (32°F) is about May 23 and the first fall frost is about Sep 25, giving roughly a 121-day growing season. To be safe most years, wait until May 4 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) | Jul 12 | Jun 18 | Jun 1 |
| Freeze (32°F) | Jun 19 | May 23 | May 4 |
| Hard (28°F) | May 24 | May 2 | Apr 13 |
| Severe (24°F) | May 1 | Apr 10 | Mar 21 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jul 12 | Jul 2 | Jun 27 | Jun 23 | Jun 18 | Jun 14 | Jun 10 | Jun 5 | Jun 1 |
| 32°F | Jun 19 | Jun 6 | Jun 1 | May 28 | May 23 | May 19 | May 15 | May 10 | May 4 |
| 28°F | May 24 | May 14 | May 10 | May 6 | May 2 | Apr 29 | Apr 26 | Apr 21 | Apr 13 |
| 24°F | May 1 | Apr 26 | Apr 19 | Apr 14 | Apr 10 | Apr 5 | Apr 1 | Mar 28 | Mar 21 |
| 20°F | Apr 7 | Apr 2 | Mar 28 | Mar 25 | Mar 21 | Mar 17 | Mar 13 | Mar 9 | Mar 4 |
| 16°F | Mar 27 | Mar 20 | Mar 15 | Mar 11 | Mar 7 | Mar 4 | Mar 1 | Feb 24 | Feb 15 |
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) | Aug 21 | Sep 8 | Sep 26 |
| Freeze (32°F) | Sep 7 | Sep 25 | Oct 10 |
| Hard (28°F) | Sep 21 | Oct 9 | Oct 28 |
| Severe (24°F) | Oct 5 | Oct 26 | Nov 13 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 21 | Aug 30 | Sep 2 | Sep 5 | Sep 8 | Sep 12 | Sep 17 | Sep 21 | Sep 26 |
| 32°F | Sep 7 | Sep 12 | Sep 17 | Sep 21 | Sep 25 | Sep 28 | Oct 1 | Oct 4 | Oct 10 |
| 28°F | Sep 21 | Sep 29 | Oct 2 | Oct 5 | Oct 9 | Oct 14 | Oct 17 | Oct 21 | Oct 28 |
| 24°F | Oct 5 | Oct 12 | Oct 18 | Oct 22 | Oct 26 | Oct 29 | Nov 1 | Nov 5 | Nov 13 |
| 20°F | Oct 18 | Oct 26 | Nov 1 | Nov 5 | Nov 8 | Nov 12 | Nov 16 | Nov 21 | Nov 27 |
| 16°F | Oct 27 | Nov 5 | Nov 10 | Nov 15 | Nov 19 | Nov 23 | Nov 27 | Dec 2 | Dec 10 |
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: Winchester, Id, about 10.6 miles away@if(!empty($station['elevation_ft'])), at 3,972 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 Sweetwater, ID?
On average, the last spring frost (32°F) around Sweetwater, ID is about May 23. In cooler years it can run later, so for a safer bet many gardeners wait until May 4.
When is the first frost in the fall?
The first fall frost around Sweetwater, ID typically arrives about Sep 25. 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 May 23 on average, or May 4 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 121 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 83540 Planting calendar → Idaho frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026