Last spring frost
May 21
Frost Dates for Forty Mile Colony, MT
First fall frost
Sep 17
Growing season
117 days
Hardiness zone
5a
In an average year around Forty Mile Colony, MT, the last spring frost (32°F) is about May 21 and the first fall frost is about Sep 17, giving roughly a 117-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 26 | Jun 1 | May 21 |
| Freeze (32°F) | Jun 2 | May 21 | May 9 |
| Hard (28°F) | May 24 | May 10 | Apr 27 |
| Severe (24°F) | May 15 | Apr 29 | Apr 12 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 26 | Jun 14 | Jun 8 | Jun 5 | Jun 1 | May 30 | May 28 | May 24 | May 21 |
| 32°F | Jun 2 | May 29 | May 26 | May 24 | May 21 | May 19 | May 15 | May 13 | May 9 |
| 28°F | May 24 | May 19 | May 15 | May 12 | May 10 | May 7 | May 4 | Apr 30 | Apr 27 |
| 24°F | May 15 | May 8 | May 4 | May 1 | Apr 29 | Apr 26 | Apr 22 | Apr 18 | Apr 12 |
| 20°F | May 2 | Apr 27 | Apr 22 | Apr 19 | Apr 15 | Apr 11 | Apr 6 | Apr 2 | Mar 28 |
| 16°F | Apr 20 | Apr 14 | Apr 9 | Apr 5 | Apr 1 | Mar 29 | Mar 25 | Mar 20 | Mar 12 |
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 24 | Sep 6 | Sep 19 |
| Freeze (32°F) | Sep 5 | Sep 17 | Oct 1 |
| Hard (28°F) | Sep 15 | Sep 28 | Oct 9 |
| Severe (24°F) | Sep 24 | Oct 6 | Oct 19 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 24 | Aug 29 | Aug 31 | Sep 3 | Sep 6 | Sep 9 | Sep 12 | Sep 14 | Sep 19 |
| 32°F | Sep 5 | Sep 9 | Sep 12 | Sep 15 | Sep 17 | Sep 19 | Sep 22 | Sep 26 | Oct 1 |
| 28°F | Sep 15 | Sep 19 | Sep 23 | Sep 26 | Sep 28 | Oct 1 | Oct 3 | Oct 5 | Oct 9 |
| 24°F | Sep 24 | Sep 28 | Oct 1 | Oct 3 | Oct 6 | Oct 9 | Oct 12 | Oct 16 | Oct 19 |
| 20°F | Oct 1 | Oct 6 | Oct 9 | Oct 13 | Oct 17 | Oct 19 | Oct 22 | Oct 25 | Oct 29 |
| 16°F | Oct 9 | Oct 16 | Oct 20 | Oct 24 | Oct 27 | Oct 30 | Nov 2 | Nov 6 | 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: Wyola 1 Sw, Mt, about 10.1 miles away@if(!empty($station['elevation_ft'])), at 3,730 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 Forty Mile Colony, MT?
On average, the last spring frost (32°F) around Forty Mile Colony, MT is about May 21. 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 Forty Mile Colony, MT typically arrives about Sep 17. 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 21 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 117 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 59050 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026