Last spring frost
May 24
Frost Dates for Chester, MT
First fall frost
Sep 17
Growing season
115 days
Hardiness zone
4a
In an average year around Chester, MT, the last spring frost (32°F) is about May 24 and the first fall frost is about Sep 17, giving roughly a 115-day growing season. To be safe most years, wait until May 11 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) | Jul 1 | Jun 9 | May 24 |
| Freeze (32°F) | Jun 9 | May 24 | May 11 |
| Hard (28°F) | May 26 | May 11 | Apr 30 |
| Severe (24°F) | May 12 | May 2 | Apr 18 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jul 1 | Jun 23 | Jun 18 | Jun 13 | Jun 9 | Jun 5 | Jun 2 | May 29 | May 24 |
| 32°F | Jun 9 | Jun 2 | May 29 | May 27 | May 24 | May 22 | May 19 | May 16 | May 11 |
| 28°F | May 26 | May 20 | May 16 | May 13 | May 11 | May 9 | May 7 | May 4 | Apr 30 |
| 24°F | May 12 | May 8 | May 6 | May 4 | May 2 | Apr 30 | Apr 27 | Apr 23 | Apr 18 |
| 20°F | May 4 | Apr 30 | Apr 28 | Apr 24 | Apr 20 | Apr 17 | Apr 13 | Apr 9 | Apr 4 |
| 16°F | Apr 27 | Apr 21 | Apr 16 | Apr 13 | Apr 9 | Apr 6 | Apr 3 | Apr 1 | Mar 25 |
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 20 | Sep 5 | Sep 18 |
| Freeze (32°F) | Sep 3 | Sep 17 | Sep 29 |
| Hard (28°F) | Sep 14 | Sep 26 | Oct 8 |
| Severe (24°F) | Sep 20 | Oct 3 | Oct 15 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 20 | Aug 26 | Aug 31 | Sep 3 | Sep 5 | Sep 8 | Sep 11 | Sep 14 | Sep 18 |
| 32°F | Sep 3 | Sep 7 | Sep 11 | Sep 15 | Sep 17 | Sep 19 | Sep 22 | Sep 24 | Sep 29 |
| 28°F | Sep 14 | Sep 19 | Sep 22 | Sep 24 | Sep 26 | Sep 29 | Oct 1 | Oct 3 | Oct 8 |
| 24°F | Sep 20 | Sep 25 | Sep 28 | Oct 1 | Oct 3 | Oct 5 | Oct 8 | Oct 11 | Oct 15 |
| 20°F | Sep 27 | Oct 1 | Oct 5 | Oct 8 | Oct 11 | Oct 14 | Oct 17 | Oct 20 | Oct 24 |
| 16°F | Oct 3 | Oct 8 | Oct 13 | Oct 16 | Oct 19 | Oct 22 | Oct 25 | Oct 27 | Oct 31 |
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: Chester, Mt, about 11.4 miles away@if(!empty($station['elevation_ft'])), at 3,132 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 Chester, MT?
On average, the last spring frost (32°F) around Chester, MT is about May 24. In cooler years it can run later, so for a safer bet many gardeners wait until May 11.
When is the first frost in the fall?
The first fall frost around Chester, 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 24 on average, or May 11 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 115 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 59461 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026