Last spring frost
May 23
Frost Dates for Harlowton, MT
First fall frost
Sep 16
Growing season
113 days
Hardiness zone
4b
In an average year around Harlowton, MT, the last spring frost (32°F) is about May 23 and the first fall frost is about Sep 16, giving roughly a 113-day growing season. To be safe most years, wait until May 12 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 30 | Jun 9 | May 24 |
| Freeze (32°F) | Jun 6 | May 23 | May 12 |
| Hard (28°F) | May 23 | May 12 | May 1 |
| Severe (24°F) | May 15 | May 3 | Apr 17 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 30 | Jun 21 | Jun 16 | Jun 12 | Jun 9 | Jun 4 | May 31 | May 29 | May 24 |
| 32°F | Jun 6 | May 31 | May 29 | May 26 | May 23 | May 21 | May 18 | May 15 | May 12 |
| 28°F | May 23 | May 19 | May 17 | May 14 | May 12 | May 9 | May 7 | May 4 | May 1 |
| 24°F | May 15 | May 10 | May 7 | May 5 | May 3 | May 1 | Apr 28 | Apr 23 | Apr 17 |
| 20°F | May 6 | May 1 | Apr 27 | Apr 24 | Apr 20 | Apr 17 | Apr 13 | Apr 10 | Apr 4 |
| 16°F | Apr 26 | Apr 20 | Apr 16 | Apr 12 | Apr 9 | Apr 5 | Apr 2 | Mar 31 | Mar 24 |
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 16 |
| Freeze (32°F) | Sep 1 | Sep 16 | Sep 28 |
| Hard (28°F) | Sep 12 | Sep 27 | Oct 8 |
| Severe (24°F) | Sep 23 | Oct 6 | Oct 19 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 20 | Aug 26 | Aug 30 | Sep 1 | Sep 4 | Sep 6 | Sep 10 | Sep 12 | Sep 16 |
| 32°F | Sep 1 | Sep 6 | Sep 10 | Sep 13 | Sep 16 | Sep 18 | Sep 21 | Sep 24 | Sep 28 |
| 28°F | Sep 12 | Sep 18 | Sep 21 | Sep 24 | Sep 27 | Sep 30 | Oct 2 | Oct 4 | Oct 8 |
| 24°F | Sep 23 | Sep 28 | Oct 1 | Oct 3 | Oct 6 | Oct 8 | Oct 12 | Oct 15 | Oct 19 |
| 20°F | Oct 2 | Oct 6 | Oct 10 | Oct 14 | Oct 16 | Oct 19 | Oct 22 | Oct 25 | Oct 29 |
| 16°F | Oct 7 | Oct 14 | Oct 18 | Oct 22 | Oct 26 | Oct 29 | Nov 1 | Nov 5 | Nov 9 |
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: Harlowton, Mt, about 2.6 miles away@if(!empty($station['elevation_ft'])), at 4,222 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 Harlowton, MT?
On average, the last spring frost (32°F) around Harlowton, MT is about May 23. In cooler years it can run later, so for a safer bet many gardeners wait until May 12.
When is the first frost in the fall?
The first fall frost around Harlowton, MT typically arrives about Sep 16. 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 23 on average, or May 12 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 113 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 59036 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026