Last spring frost
Jun 9
Frost Dates for Firth, ID
First fall frost
Sep 11
Growing season
93 days
Hardiness zone
5b
In an average year around Firth, ID, the last spring frost (32°F) is about Jun 9 and the first fall frost is about Sep 11, giving roughly a 93-day growing season. To be safe most years, wait until May 19 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%) |
|---|---|---|---|
| Freeze (32°F) | Jun 26 | Jun 9 | May 19 |
| Hard (28°F) | Jun 10 | May 19 | May 4 |
| Severe (24°F) | May 22 | May 5 | Apr 20 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Jun 26 | Jun 21 | Jun 16 | Jun 13 | Jun 9 | Jun 6 | Jun 1 | May 27 | May 19 |
| 28°F | Jun 10 | Jun 4 | May 30 | May 24 | May 19 | May 16 | May 12 | May 9 | May 4 |
| 24°F | May 22 | May 15 | May 11 | May 8 | May 5 | May 2 | Apr 30 | Apr 25 | Apr 20 |
| 20°F | May 8 | May 2 | Apr 28 | Apr 24 | Apr 20 | Apr 17 | Apr 13 | Apr 8 | Apr 3 |
| 16°F | Apr 26 | Apr 19 | Apr 14 | Apr 10 | Apr 6 | Apr 3 | Mar 30 | Mar 24 | Mar 16 |
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%) |
|---|---|---|---|
| Freeze (32°F) | Aug 27 | Sep 11 | Sep 27 |
| Hard (28°F) | Sep 4 | Sep 23 | Oct 10 |
| Severe (24°F) | Sep 14 | Oct 2 | Oct 20 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Aug 27 | Sep 1 | Sep 4 | Sep 8 | Sep 11 | Sep 14 | Sep 18 | Sep 22 | Sep 27 |
| 28°F | Sep 4 | Sep 11 | Sep 15 | Sep 19 | Sep 23 | Sep 26 | Sep 29 | Oct 4 | Oct 10 |
| 24°F | Sep 14 | Sep 21 | Sep 26 | Sep 29 | Oct 2 | Oct 6 | Oct 10 | Oct 15 | Oct 20 |
| 20°F | Sep 25 | Oct 1 | Oct 5 | Oct 9 | Oct 13 | Oct 17 | Oct 21 | Oct 25 | Oct 31 |
| 16°F | Oct 4 | Oct 11 | Oct 16 | Oct 20 | Oct 24 | Oct 28 | Nov 1 | Nov 5 | Nov 11 |
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: Idaho Falls 16 Se, Id, about 11.6 miles away@if(!empty($station['elevation_ft'])), at 5,828 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 Firth, ID?
On average, the last spring frost (32°F) around Firth, ID is about Jun 9. In cooler years it can run later, so for a safer bet many gardeners wait until May 19.
When is the first frost in the fall?
The first fall frost around Firth, ID typically arrives about Sep 11. 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 Jun 9 on average, or May 19 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 93 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 83236 Planting calendar → Idaho frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026