Last spring frost
May 16
Frost Dates for Trego, MT
First fall frost
Sep 18
Growing season
121 days
Hardiness zone
5a
In an average year around Trego, MT, the last spring frost (32°F) is about May 16 and the first fall frost is about Sep 18, giving roughly a 121-day growing season. To be safe most years, wait until May 3 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 23 | Jun 2 | May 18 |
| Freeze (32°F) | Jun 3 | May 16 | May 3 |
| Hard (28°F) | May 20 | May 1 | Apr 14 |
| Severe (24°F) | May 4 | Apr 12 | Mar 24 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 23 | Jun 15 | Jun 10 | Jun 6 | Jun 2 | May 31 | May 28 | May 23 | May 18 |
| 32°F | Jun 3 | May 28 | May 23 | May 20 | May 16 | May 13 | May 9 | May 7 | May 3 |
| 28°F | May 20 | May 13 | May 8 | May 4 | May 1 | Apr 28 | Apr 25 | Apr 20 | Apr 14 |
| 24°F | May 4 | Apr 26 | Apr 21 | Apr 16 | Apr 12 | Apr 8 | Apr 4 | Apr 1 | Mar 24 |
| 20°F | Apr 14 | Apr 6 | Apr 1 | Mar 30 | Mar 25 | Mar 21 | Mar 16 | Mar 11 | Mar 5 |
| 16°F | Mar 28 | Mar 22 | Mar 17 | Mar 13 | Mar 9 | Mar 5 | Mar 2 | Feb 26 | Feb 18 |
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 4 | Sep 22 |
| Freeze (32°F) | Sep 1 | Sep 18 | Oct 5 |
| Hard (28°F) | Sep 15 | Oct 2 | Oct 19 |
| Severe (24°F) | Sep 30 | Oct 19 | Nov 5 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 20 | Aug 25 | Aug 30 | Sep 2 | Sep 4 | Sep 7 | Sep 10 | Sep 14 | Sep 22 |
| 32°F | Sep 1 | Sep 6 | Sep 10 | Sep 14 | Sep 18 | Sep 21 | Sep 26 | Sep 30 | Oct 5 |
| 28°F | Sep 15 | Sep 22 | Sep 26 | Sep 30 | Oct 2 | Oct 5 | Oct 10 | Oct 14 | Oct 19 |
| 24°F | Sep 30 | Oct 4 | Oct 9 | Oct 14 | Oct 19 | Oct 23 | Oct 26 | Oct 31 | Nov 5 |
| 20°F | Oct 8 | Oct 17 | Oct 22 | Oct 27 | Oct 31 | Nov 3 | Nov 8 | Nov 14 | Nov 21 |
| 16°F | Oct 22 | Oct 30 | Nov 2 | Nov 7 | Nov 11 | Nov 16 | Nov 21 | Nov 27 | Dec 4 |
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: Fortine 1 N, Mt, about 7.2 miles away@if(!empty($station['elevation_ft'])), at 3,000 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 Trego, MT?
On average, the last spring frost (32°F) around Trego, MT is about May 16. In cooler years it can run later, so for a safer bet many gardeners wait until May 3.
When is the first frost in the fall?
The first fall frost around Trego, MT typically arrives about Sep 18. 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 16 on average, or May 3 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 59933 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026