Last spring frost
May 19
Frost Dates for Ayers Ranch Colony, MT
First fall frost
Sep 25
Growing season
128 days
Hardiness zone
4b
In an average year around Ayers Ranch Colony, MT, the last spring frost (32°F) is about May 19 and the first fall frost is about Sep 25, giving roughly a 128-day growing season. To be safe most years, wait until May 7 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 | Jun 1 | May 20 |
| Freeze (32°F) | Jun 1 | May 19 | May 7 |
| Hard (28°F) | May 22 | May 7 | Apr 23 |
| Severe (24°F) | May 10 | Apr 26 | Apr 5 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 16 | Jun 10 | Jun 7 | Jun 4 | Jun 1 | May 31 | May 28 | May 24 | May 20 |
| 32°F | Jun 1 | May 29 | May 25 | May 22 | May 19 | May 16 | May 14 | May 11 | May 7 |
| 28°F | May 22 | May 16 | May 12 | May 10 | May 7 | May 4 | May 2 | Apr 29 | Apr 23 |
| 24°F | May 10 | May 5 | May 1 | Apr 29 | Apr 26 | Apr 22 | Apr 17 | Apr 12 | Apr 5 |
| 20°F | Apr 30 | Apr 25 | Apr 20 | Apr 15 | Apr 11 | Apr 8 | Apr 4 | Mar 31 | Mar 24 |
| 16°F | Apr 19 | Apr 13 | Apr 8 | Apr 4 | Apr 1 | Mar 29 | Mar 25 | Mar 20 | Mar 13 |
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 2 | Sep 16 | Sep 27 |
| Freeze (32°F) | Sep 11 | Sep 25 | Oct 9 |
| Hard (28°F) | Sep 22 | Oct 5 | Oct 21 |
| Severe (24°F) | Sep 29 | Oct 16 | Oct 31 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 2 | Sep 7 | Sep 10 | Sep 13 | Sep 16 | Sep 18 | Sep 21 | Sep 23 | Sep 27 |
| 32°F | Sep 11 | Sep 17 | Sep 20 | Sep 23 | Sep 25 | Sep 28 | Sep 30 | Oct 4 | Oct 9 |
| 28°F | Sep 22 | Sep 26 | Sep 29 | Oct 1 | Oct 5 | Oct 8 | Oct 12 | Oct 16 | Oct 21 |
| 24°F | Sep 29 | Oct 4 | Oct 9 | Oct 13 | Oct 16 | Oct 19 | Oct 22 | Oct 26 | Oct 31 |
| 20°F | Oct 8 | Oct 15 | Oct 19 | Oct 23 | Oct 26 | Oct 29 | Nov 1 | Nov 5 | Nov 11 |
| 16°F | Oct 18 | Oct 25 | Oct 28 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 14 | Nov 19 |
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: Lewistown 11Ese, Mt, about 14.4 miles away@if(!empty($station['elevation_ft'])), at 5,028 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 Ayers Ranch Colony, MT?
On average, the last spring frost (32°F) around Ayers Ranch Colony, MT is about May 19. In cooler years it can run later, so for a safer bet many gardeners wait until May 7.
When is the first frost in the fall?
The first fall frost around Ayers Ranch Colony, 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 19 on average, or May 7 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 128 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 59441 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026