Last spring frost
May 18
Frost Dates for Zortman, MT
First fall frost
Sep 23
Growing season
126 days
Hardiness zone
4b
In an average year around Zortman, MT, the last spring frost (32°F) is about May 18 and the first fall frost is about Sep 23, giving roughly a 126-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%) |
|---|---|---|---|
| Light (36°F) | Jun 20 | May 31 | May 18 |
| Freeze (32°F) | Jun 1 | May 18 | May 4 |
| Hard (28°F) | May 20 | May 5 | Apr 20 |
| Severe (24°F) | May 8 | Apr 24 | Apr 8 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 20 | Jun 11 | Jun 6 | Jun 2 | May 31 | May 28 | May 25 | May 22 | May 18 |
| 32°F | Jun 1 | May 28 | May 24 | May 21 | May 18 | May 15 | May 11 | May 8 | May 4 |
| 28°F | May 20 | May 14 | May 9 | May 7 | May 5 | May 2 | Apr 30 | Apr 27 | Apr 20 |
| 24°F | May 8 | May 3 | Apr 30 | Apr 28 | Apr 24 | Apr 21 | Apr 17 | Apr 13 | Apr 8 |
| 20°F | Apr 29 | Apr 24 | Apr 19 | Apr 15 | Apr 12 | Apr 9 | Apr 6 | Apr 2 | Mar 27 |
| 16°F | Apr 21 | Apr 14 | Apr 10 | Apr 6 | Apr 3 | Mar 31 | Mar 27 | 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) | Aug 26 | Sep 12 | Sep 26 |
| Freeze (32°F) | Sep 7 | Sep 23 | Oct 5 |
| Hard (28°F) | Sep 17 | Oct 2 | Oct 17 |
| Severe (24°F) | Sep 26 | Oct 12 | Oct 30 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 26 | Sep 1 | Sep 5 | Sep 9 | Sep 12 | Sep 15 | Sep 19 | Sep 22 | Sep 26 |
| 32°F | Sep 7 | Sep 14 | Sep 18 | Sep 21 | Sep 23 | Sep 25 | Sep 28 | Oct 1 | Oct 5 |
| 28°F | Sep 17 | Sep 24 | Sep 27 | Sep 30 | Oct 2 | Oct 5 | Oct 8 | Oct 12 | Oct 17 |
| 24°F | Sep 26 | Oct 1 | Oct 5 | Oct 8 | Oct 12 | Oct 15 | Oct 19 | Oct 24 | Oct 30 |
| 20°F | Oct 1 | Oct 9 | Oct 15 | Oct 19 | Oct 23 | Oct 27 | Oct 30 | Nov 3 | Nov 9 |
| 16°F | Oct 10 | Oct 18 | Oct 24 | Oct 28 | Nov 1 | Nov 4 | Nov 8 | Nov 12 | 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: Zortman, Mt, about 1.5 miles away@if(!empty($station['elevation_ft'])), at 4,035 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 Zortman, MT?
On average, the last spring frost (32°F) around Zortman, MT is about May 18. 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 Zortman, MT typically arrives about Sep 23. 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 18 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 126 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 59546 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026