Last spring frost
May 18
Frost Dates for Spencer, ID
First fall frost
Sep 30
Growing season
132 days
Hardiness zone
5b
In an average year around Spencer, ID, the last spring frost (32°F) is about May 18 and the first fall frost is about Sep 30, giving roughly a 132-day growing season. To be safe most years, wait until May 3 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 22 | Jun 5 | May 18 |
| Freeze (32°F) | Jun 9 | May 18 | May 3 |
| Hard (28°F) | May 17 | May 2 | Apr 16 |
| Severe (24°F) | May 2 | Apr 17 | Mar 31 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 22 | Jun 17 | Jun 13 | Jun 10 | Jun 5 | Jun 2 | May 30 | May 23 | May 18 |
| 32°F | Jun 9 | Jun 1 | May 27 | May 22 | May 18 | May 14 | May 11 | May 8 | May 3 |
| 28°F | May 17 | May 11 | May 8 | May 4 | May 2 | Apr 29 | Apr 26 | Apr 21 | Apr 16 |
| 24°F | May 2 | Apr 28 | Apr 24 | Apr 20 | Apr 17 | Apr 13 | Apr 10 | Apr 6 | Mar 31 |
| 20°F | Apr 17 | Apr 11 | Apr 7 | Apr 3 | Mar 31 | Mar 27 | Mar 21 | Mar 17 | Mar 10 |
| 16°F | Apr 2 | Mar 28 | Mar 23 | Mar 19 | Mar 16 | Mar 12 | Mar 9 | Mar 5 | Mar 2 |
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 18 | Oct 1 |
| Freeze (32°F) | Sep 14 | Sep 30 | Oct 16 |
| Hard (28°F) | Sep 27 | Oct 12 | Oct 26 |
| Severe (24°F) | Oct 7 | Oct 23 | Nov 6 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 4 | Sep 8 | Sep 12 | Sep 15 | Sep 18 | Sep 20 | Sep 23 | Sep 26 | Oct 1 |
| 32°F | Sep 14 | Sep 19 | Sep 24 | Sep 27 | Sep 30 | Oct 3 | Oct 6 | Oct 10 | Oct 16 |
| 28°F | Sep 27 | Oct 1 | Oct 5 | Oct 8 | Oct 12 | Oct 16 | Oct 19 | Oct 23 | Oct 26 |
| 24°F | Oct 7 | Oct 13 | Oct 17 | Oct 21 | Oct 23 | Oct 26 | Oct 29 | Nov 1 | Nov 6 |
| 20°F | Oct 18 | Oct 23 | Oct 27 | Oct 30 | Nov 1 | Nov 4 | Nov 7 | Nov 11 | Nov 17 |
| 16°F | Oct 26 | Oct 31 | Nov 3 | Nov 7 | Nov 10 | Nov 12 | Nov 15 | Nov 19 | Nov 25 |
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: Dubois Exp Stn, Id, about 8.4 miles away@if(!empty($station['elevation_ft'])), at 5,450 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 Spencer, ID?
On average, the last spring frost (32°F) around Spencer, ID is about May 18. In cooler years it can run later, so for a safer bet many gardeners wait until May 3.
When is the first frost in the fall?
The first fall frost around Spencer, ID typically arrives about Sep 30. 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 18 on average, or May 3 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 132 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 83446 Planting calendar → Idaho frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026