Last spring frost
May 6
Frost Dates for Riggins, ID
First fall frost
Oct 9
Growing season
155 days
Hardiness zone
6b
In an average year around Riggins, ID, the last spring frost (32°F) is about May 6 and the first fall frost is about Oct 9, giving roughly a 155-day growing season. To be safe most years, wait until Apr 23 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) | Jun 20 | May 29 | May 11 |
| Freeze (32°F) | May 26 | May 6 | Apr 23 |
| Hard (28°F) | May 5 | Apr 18 | Mar 31 |
| Severe (24°F) | Apr 17 | Mar 27 | Mar 6 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 20 | Jun 13 | Jun 6 | Jun 2 | May 29 | May 23 | May 20 | May 16 | May 11 |
| 32°F | May 26 | May 16 | May 12 | May 9 | May 6 | May 3 | May 1 | Apr 29 | Apr 23 |
| 28°F | May 5 | Apr 30 | Apr 25 | Apr 22 | Apr 18 | Apr 13 | Apr 10 | Apr 5 | Mar 31 |
| 24°F | Apr 17 | Apr 7 | Apr 3 | Mar 31 | Mar 27 | Mar 21 | Mar 16 | Mar 11 | Mar 6 |
| 20°F | Mar 23 | Mar 15 | Mar 11 | Mar 6 | Mar 3 | Feb 28 | Feb 25 | Feb 20 | Feb 12 |
| 16°F | Mar 7 | Mar 1 | Feb 26 | Feb 21 | Feb 17 | Feb 12 | Feb 7 | Feb 1 | Jan 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 10 | Sep 27 | Oct 12 |
| Freeze (32°F) | Sep 24 | Oct 9 | Oct 26 |
| Hard (28°F) | Oct 7 | Oct 22 | Nov 7 |
| Severe (24°F) | Oct 18 | Nov 3 | Nov 23 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 10 | Sep 15 | Sep 19 | Sep 23 | Sep 27 | Sep 29 | Oct 2 | Oct 6 | Oct 12 |
| 32°F | Sep 24 | Sep 29 | Oct 2 | Oct 5 | Oct 9 | Oct 12 | Oct 15 | Oct 19 | Oct 26 |
| 28°F | Oct 7 | Oct 12 | Oct 16 | Oct 19 | Oct 22 | Oct 26 | Oct 29 | Nov 2 | Nov 7 |
| 24°F | Oct 18 | Oct 24 | Oct 28 | Nov 1 | Nov 3 | Nov 7 | Nov 12 | Nov 16 | Nov 23 |
| 20°F | Oct 30 | Nov 3 | Nov 9 | Nov 14 | Nov 19 | Nov 23 | Nov 27 | Dec 1 | Dec 8 |
| 16°F | Nov 7 | Nov 15 | Nov 23 | Nov 27 | Dec 1 | Dec 5 | Dec 11 | Dec 19 | Dec 31 |
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: Riggins, Id, about 6.3 miles away@if(!empty($station['elevation_ft'])), at 2,056 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 Riggins, ID?
On average, the last spring frost (32°F) around Riggins, ID is about May 6. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 23.
When is the first frost in the fall?
The first fall frost around Riggins, ID typically arrives about Oct 9. 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 6 on average, or Apr 23 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 155 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 83549 Planting calendar → Idaho frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026