Last spring frost
May 7
Frost Dates for Fort Smith, MT
First fall frost
Oct 5
Growing season
151 days
Hardiness zone
5a
In an average year around Fort Smith, MT, the last spring frost (32°F) is about May 7 and the first fall frost is about Oct 5, giving roughly a 151-day growing season. To be safe most years, wait until Apr 20 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 4 | May 20 | May 5 |
| Freeze (32°F) | May 24 | May 7 | Apr 20 |
| Hard (28°F) | May 13 | Apr 24 | Apr 5 |
| Severe (24°F) | May 2 | Apr 10 | Mar 23 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 4 | May 29 | May 26 | May 23 | May 20 | May 16 | May 13 | May 10 | May 5 |
| 32°F | May 24 | May 18 | May 14 | May 10 | May 7 | May 4 | Apr 30 | Apr 27 | Apr 20 |
| 28°F | May 13 | May 6 | May 1 | Apr 28 | Apr 24 | Apr 19 | Apr 15 | Apr 12 | Apr 5 |
| 24°F | May 2 | Apr 24 | Apr 19 | Apr 14 | Apr 10 | Apr 7 | Apr 3 | Mar 30 | Mar 23 |
| 20°F | Apr 20 | Apr 11 | Apr 6 | Apr 3 | Mar 31 | Mar 27 | Mar 22 | Mar 17 | Mar 9 |
| 16°F | Apr 9 | Apr 3 | Mar 30 | Mar 25 | Mar 21 | Mar 16 | Mar 11 | Mar 5 | Feb 26 |
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 9 | Sep 26 | Oct 9 |
| Freeze (32°F) | Sep 20 | Oct 5 | Oct 22 |
| Hard (28°F) | Sep 30 | Oct 14 | Nov 1 |
| Severe (24°F) | Oct 5 | Oct 26 | Nov 12 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 9 | Sep 15 | Sep 19 | Sep 23 | Sep 26 | Sep 28 | Oct 1 | Oct 4 | Oct 9 |
| 32°F | Sep 20 | Sep 26 | Sep 30 | Oct 2 | Oct 5 | Oct 8 | Oct 12 | Oct 15 | Oct 22 |
| 28°F | Sep 30 | Oct 3 | Oct 7 | Oct 11 | Oct 14 | Oct 18 | Oct 23 | Oct 27 | Nov 1 |
| 24°F | Oct 5 | Oct 12 | Oct 17 | Oct 22 | Oct 26 | Oct 30 | Nov 2 | Nov 6 | Nov 12 |
| 20°F | Oct 17 | Oct 24 | Oct 29 | Nov 1 | Nov 5 | Nov 8 | Nov 12 | Nov 18 | Nov 23 |
| 16°F | Oct 26 | Nov 1 | Nov 5 | Nov 9 | Nov 12 | Nov 16 | Nov 21 | Nov 25 | Nov 30 |
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: Yellowtail Dam, Mt, about 0.8 miles away@if(!empty($station['elevation_ft'])), at 3,305 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 Fort Smith, MT?
On average, the last spring frost (32°F) around Fort Smith, MT is about May 7. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 20.
When is the first frost in the fall?
The first fall frost around Fort Smith, MT typically arrives about Oct 5. 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 7 on average, or Apr 20 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 151 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 59035 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026