Last spring frost
May 17
Frost Dates for Swan Lake, MT
First fall frost
Sep 19
Growing season
121 days
Hardiness zone
5b
In an average year around Swan Lake, MT, the last spring frost (32°F) is about May 17 and the first fall frost is about Sep 19, giving roughly a 121-day growing season. To be safe most years, wait until May 4 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 4 | May 17 | May 4 |
| Hard (28°F) | May 13 | Apr 30 | Apr 15 |
| Severe (24°F) | Apr 30 | Apr 12 | Mar 24 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Jun 4 | May 28 | May 24 | May 20 | May 17 | May 15 | May 12 | May 9 | May 4 |
| 28°F | May 13 | May 9 | May 5 | May 3 | Apr 30 | Apr 29 | Apr 26 | Apr 21 | Apr 15 |
| 24°F | Apr 30 | Apr 26 | Apr 21 | Apr 16 | Apr 12 | Apr 8 | Apr 4 | Mar 31 | Mar 24 |
| 20°F | Apr 16 | Apr 9 | Apr 4 | Mar 31 | Mar 28 | Mar 23 | Mar 19 | Mar 13 | Mar 7 |
| 16°F | Apr 4 | Mar 30 | Mar 24 | Mar 19 | Mar 15 | Mar 11 | Mar 7 | Mar 3 | 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%) |
|---|---|---|---|
| Freeze (32°F) | Sep 4 | Sep 19 | Oct 2 |
| Hard (28°F) | Sep 14 | Oct 1 | Oct 18 |
| Severe (24°F) | Sep 26 | Oct 17 | Nov 3 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Sep 4 | Sep 10 | Sep 14 | Sep 16 | Sep 19 | Sep 22 | Sep 25 | Sep 28 | Oct 2 |
| 28°F | Sep 14 | Sep 21 | Sep 25 | Sep 28 | Oct 1 | Oct 4 | Oct 8 | Oct 13 | Oct 18 |
| 24°F | Sep 26 | Oct 2 | Oct 7 | Oct 12 | Oct 17 | Oct 21 | Oct 25 | Oct 29 | Nov 3 |
| 20°F | Oct 11 | Oct 18 | Oct 23 | Oct 27 | Oct 30 | Nov 2 | Nov 6 | Nov 11 | Nov 19 |
| 16°F | Oct 20 | Oct 27 | Nov 1 | Nov 4 | Nov 8 | Nov 13 | Nov 19 | Nov 25 | Dec 1 |
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: Swan Lake, Mt, about 4.2 miles away@if(!empty($station['elevation_ft'])), at 3,100 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 Swan Lake, MT?
On average, the last spring frost (32°F) around Swan Lake, MT is about May 17. In cooler years it can run later, so for a safer bet many gardeners wait until May 4.
When is the first frost in the fall?
The first fall frost around Swan Lake, MT typically arrives about Sep 19. 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 17 on average, or May 4 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 59911 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026