Last spring frost
Jun 5
Frost Dates for St. Marie, MT
First fall frost
Sep 2
Growing season
89 days
Hardiness zone
3b
In an average year around St. Marie, MT, the last spring frost (32°F) is about Jun 5 and the first fall frost is about Sep 2, giving roughly a 89-day growing season. To be safe most years, wait until May 22 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%) |
|---|---|---|---|
| Freeze (32°F) | Jun 23 | Jun 5 | May 22 |
| Hard (28°F) | Jun 6 | May 24 | May 12 |
| Severe (24°F) | May 26 | May 14 | May 2 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Jun 23 | Jun 16 | Jun 11 | Jun 8 | Jun 5 | Jun 2 | May 31 | May 27 | May 22 |
| 28°F | Jun 6 | Jun 1 | May 29 | May 26 | May 24 | May 22 | May 19 | May 17 | May 12 |
| 24°F | May 26 | May 22 | May 19 | May 16 | May 14 | May 11 | May 9 | May 6 | May 2 |
| 20°F | May 16 | May 12 | May 9 | May 6 | May 3 | May 1 | Apr 28 | Apr 25 | Apr 18 |
| 16°F | May 7 | May 2 | Apr 29 | Apr 26 | Apr 22 | Apr 19 | Apr 15 | Apr 11 | Apr 6 |
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%) |
|---|---|---|---|
| Freeze (32°F) | Aug 15 | Sep 2 | Sep 17 |
| Hard (28°F) | Aug 31 | Sep 14 | Sep 25 |
| Severe (24°F) | Sep 10 | Sep 23 | Oct 5 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Aug 15 | Aug 23 | Aug 28 | Aug 31 | Sep 2 | Sep 5 | Sep 8 | Sep 12 | Sep 17 |
| 28°F | Aug 31 | Sep 5 | Sep 8 | Sep 11 | Sep 14 | Sep 17 | Sep 19 | Sep 22 | Sep 25 |
| 24°F | Sep 10 | Sep 15 | Sep 19 | Sep 21 | Sep 23 | Sep 25 | Sep 27 | Sep 30 | Oct 5 |
| 20°F | Sep 18 | Sep 23 | Sep 26 | Sep 29 | Oct 1 | Oct 4 | Oct 8 | Oct 13 | Oct 18 |
| 16°F | Sep 25 | Oct 1 | Oct 5 | Oct 9 | Oct 13 | Oct 17 | Oct 20 | Oct 23 | Oct 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: Opheim 12 Sse, Mt, about 5.3 miles away@if(!empty($station['elevation_ft'])), at 2,936 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 St. Marie, MT?
On average, the last spring frost (32°F) around St. Marie, MT is about Jun 5. In cooler years it can run later, so for a safer bet many gardeners wait until May 22.
When is the first frost in the fall?
The first fall frost around St. Marie, MT typically arrives about Sep 2. 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 Jun 5 on average, or May 22 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 89 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 59244 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026