Last spring frost
May 22
Frost Dates for Custer, MT
First fall frost
Sep 22
Growing season
121 days
Hardiness zone
4b
In an average year around Custer, MT, the last spring frost (32°F) is about May 22 and the first fall frost is about Sep 22, giving roughly a 121-day growing season. To be safe most years, wait until May 9 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 19 | Jun 4 | May 21 |
| Freeze (32°F) | Jun 7 | May 22 | May 9 |
| Hard (28°F) | May 25 | May 10 | Apr 26 |
| Severe (24°F) | May 11 | Apr 28 | Apr 11 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 19 | Jun 14 | Jun 10 | Jun 8 | Jun 4 | Jun 1 | May 29 | May 25 | May 21 |
| 32°F | Jun 7 | Jun 1 | May 28 | May 25 | May 22 | May 19 | May 16 | May 13 | May 9 |
| 28°F | May 25 | May 20 | May 16 | May 12 | May 10 | May 7 | May 3 | Apr 30 | Apr 26 |
| 24°F | May 11 | May 6 | May 3 | Apr 30 | Apr 28 | Apr 26 | Apr 22 | Apr 17 | Apr 11 |
| 20°F | May 1 | Apr 27 | Apr 22 | Apr 19 | Apr 15 | Apr 11 | Apr 8 | Apr 4 | Mar 31 |
| 16°F | Apr 22 | Apr 15 | Apr 11 | Apr 7 | Apr 3 | Mar 31 | Mar 28 | Mar 23 | 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%) |
|---|---|---|---|
| Light (36°F) | Aug 26 | Sep 11 | Sep 24 |
| Freeze (32°F) | Sep 7 | Sep 22 | Oct 4 |
| Hard (28°F) | Sep 19 | Oct 2 | Oct 15 |
| Severe (24°F) | Sep 27 | Oct 11 | Oct 24 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 26 | Sep 1 | Sep 5 | Sep 8 | Sep 11 | Sep 13 | Sep 16 | Sep 19 | Sep 24 |
| 32°F | Sep 7 | Sep 13 | Sep 16 | Sep 19 | Sep 22 | Sep 24 | Sep 27 | Sep 30 | Oct 4 |
| 28°F | Sep 19 | Sep 24 | Sep 27 | Sep 29 | Oct 2 | Oct 4 | Oct 7 | Oct 10 | Oct 15 |
| 24°F | Sep 27 | Oct 1 | Oct 4 | Oct 8 | Oct 11 | Oct 14 | Oct 16 | Oct 20 | Oct 24 |
| 20°F | Oct 4 | Oct 10 | Oct 14 | Oct 18 | Oct 20 | Oct 24 | Oct 26 | Oct 30 | Nov 4 |
| 16°F | Oct 13 | Oct 18 | Oct 22 | Oct 26 | Oct 30 | Nov 2 | Nov 5 | Nov 9 | Nov 15 |
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: Pompeys Pillar 15N, Mt, about 11 miles away@if(!empty($station['elevation_ft'])), at 3,390 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 Custer, MT?
On average, the last spring frost (32°F) around Custer, MT is about May 22. In cooler years it can run later, so for a safer bet many gardeners wait until May 9.
When is the first frost in the fall?
The first fall frost around Custer, MT typically arrives about Sep 22. 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 22 on average, or May 9 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 59024 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026