Last spring frost
May 9
Frost Dates for Ammon, ID
First fall frost
Oct 1
Growing season
142 days
Hardiness zone
5b
In an average year around Ammon, ID, the last spring frost (32°F) is about May 9 and the first fall frost is about Oct 1, giving roughly a 142-day growing season. To be safe most years, wait until Apr 27 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 16 | May 30 | May 10 |
| Freeze (32°F) | Jun 2 | May 9 | Apr 27 |
| Hard (28°F) | May 11 | Apr 28 | Apr 11 |
| Severe (24°F) | Apr 29 | Apr 11 | Mar 20 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 16 | Jun 11 | Jun 7 | Jun 3 | May 30 | May 24 | May 19 | May 16 | May 10 |
| 32°F | Jun 2 | May 23 | May 17 | May 13 | May 9 | May 7 | May 4 | May 1 | Apr 27 |
| 28°F | May 11 | May 6 | May 3 | Apr 30 | Apr 28 | Apr 24 | Apr 21 | Apr 17 | Apr 11 |
| 24°F | Apr 29 | Apr 24 | Apr 19 | Apr 15 | Apr 11 | Apr 7 | Apr 3 | Mar 28 | Mar 20 |
| 20°F | Apr 15 | Apr 7 | Apr 2 | Mar 28 | Mar 22 | Mar 16 | Mar 11 | Mar 8 | Mar 4 |
| 16°F | Mar 27 | Mar 17 | Mar 12 | Mar 9 | Mar 6 | Mar 4 | Mar 2 | Feb 27 | Feb 20 |
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 4 | Sep 20 | Oct 3 |
| Freeze (32°F) | Sep 17 | Oct 1 | Oct 15 |
| Hard (28°F) | Sep 27 | Oct 13 | Oct 27 |
| Severe (24°F) | Oct 9 | Oct 25 | Nov 8 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 4 | Sep 9 | Sep 14 | Sep 17 | Sep 20 | Sep 23 | Sep 25 | Sep 29 | Oct 3 |
| 32°F | Sep 17 | Sep 21 | Sep 25 | Sep 28 | Oct 1 | Oct 3 | Oct 6 | Oct 10 | Oct 15 |
| 28°F | Sep 27 | Oct 2 | Oct 6 | Oct 9 | Oct 13 | Oct 16 | Oct 19 | Oct 23 | Oct 27 |
| 24°F | Oct 9 | Oct 14 | Oct 19 | Oct 22 | Oct 25 | Oct 28 | Oct 31 | Nov 3 | Nov 8 |
| 20°F | Oct 20 | Oct 25 | Oct 28 | Nov 1 | Nov 3 | Nov 5 | Nov 9 | Nov 14 | Nov 20 |
| 16°F | Oct 27 | Nov 1 | Nov 5 | Nov 9 | Nov 12 | Nov 16 | Nov 20 | Nov 23 | Nov 28 |
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 - Kifi, Id, about 6.5 miles away@if(!empty($station['elevation_ft'])), at 4,742 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 Ammon, ID?
On average, the last spring frost (32°F) around Ammon, ID is about May 9. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 27.
When is the first frost in the fall?
The first fall frost around Ammon, ID typically arrives about Oct 1. 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 9 on average, or Apr 27 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 142 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 83406 Planting calendar → Idaho frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026