Last spring frost
May 12
Frost Dates for Marion, MT
First fall frost
Sep 25
Growing season
134 days
Hardiness zone
5b
In an average year around Marion, MT, the last spring frost (32°F) is about May 12 and the first fall frost is about Sep 25, giving roughly a 134-day growing season. To be safe most years, wait until Apr 30 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 16 | May 30 | May 16 |
| Freeze (32°F) | May 26 | May 12 | Apr 30 |
| Hard (28°F) | May 11 | Apr 28 | Apr 10 |
| Severe (24°F) | Apr 26 | Apr 8 | Mar 21 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 16 | Jun 9 | Jun 5 | Jun 1 | May 30 | May 27 | May 24 | May 20 | May 16 |
| 32°F | May 26 | May 21 | May 18 | May 15 | May 12 | May 10 | May 7 | May 4 | Apr 30 |
| 28°F | May 11 | May 7 | May 4 | May 1 | Apr 28 | Apr 25 | Apr 21 | Apr 16 | Apr 10 |
| 24°F | Apr 26 | Apr 20 | Apr 15 | Apr 12 | Apr 8 | Apr 4 | Apr 2 | Mar 30 | Mar 21 |
| 20°F | Apr 13 | Apr 6 | Apr 2 | Mar 31 | Mar 26 | Mar 21 | Mar 15 | Mar 10 | Mar 5 |
| 16°F | Apr 1 | Mar 26 | Mar 20 | Mar 14 | Mar 10 | Mar 7 | Mar 3 | Feb 28 | Feb 21 |
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 25 | Sep 8 | Sep 25 |
| Freeze (32°F) | Sep 9 | Sep 25 | Oct 8 |
| Hard (28°F) | Sep 23 | Oct 5 | Oct 22 |
| Severe (24°F) | Oct 1 | Oct 18 | Nov 4 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 25 | Aug 30 | Sep 3 | Sep 5 | Sep 8 | Sep 13 | Sep 16 | Sep 20 | Sep 25 |
| 32°F | Sep 9 | Sep 16 | Sep 20 | Sep 23 | Sep 25 | Sep 27 | Sep 30 | Oct 3 | Oct 8 |
| 28°F | Sep 23 | Sep 27 | Oct 1 | Oct 3 | Oct 5 | Oct 9 | Oct 12 | Oct 16 | Oct 22 |
| 24°F | Oct 1 | Oct 5 | Oct 9 | Oct 14 | Oct 18 | Oct 22 | Oct 26 | Oct 29 | Nov 4 |
| 20°F | Oct 10 | Oct 17 | Oct 22 | Oct 27 | Oct 31 | Nov 3 | Nov 7 | Nov 13 | Nov 22 |
| 16°F | Oct 21 | Oct 29 | Nov 1 | Nov 6 | Nov 11 | Nov 16 | Nov 21 | Nov 26 | 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: Creston, Mt, about 25.4 miles away@if(!empty($station['elevation_ft'])), at 2,935 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 Marion, MT?
On average, the last spring frost (32°F) around Marion, MT is about May 12. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 30.
When is the first frost in the fall?
The first fall frost around Marion, MT typically arrives about Sep 25. 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 12 on average, or Apr 30 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 134 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 59920 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026