Growing season
67 days
Frost Dates for Helmville, MT
Hardiness zone
5a
Frost normals for this location are limited (the nearest long-term weather station may not report all thresholds). The available data is shown below.
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%) |
|---|---|---|---|
| Hard (28°F) | Jun 18 | May 26 | May 11 |
| Severe (24°F) | May 23 | May 9 | Apr 30 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 28°F | Jun 18 | Jun 7 | Jun 2 | May 30 | May 26 | May 23 | May 20 | May 16 | May 11 |
| 24°F | May 23 | May 18 | May 15 | May 12 | May 9 | May 7 | May 5 | May 3 | Apr 30 |
| 20°F | May 9 | May 6 | May 3 | May 1 | Apr 30 | Apr 27 | Apr 24 | Apr 19 | Apr 12 |
| 16°F | Apr 30 | Apr 26 | Apr 22 | Apr 18 | Apr 13 | Apr 8 | Apr 3 | Mar 31 | Mar 23 |
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%) |
|---|---|---|---|
| Hard (28°F) | Aug 30 | Sep 11 | Sep 25 |
| Severe (24°F) | Sep 9 | Sep 24 | Oct 6 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 28°F | Aug 30 | Sep 2 | Sep 5 | Sep 8 | Sep 11 | Sep 14 | Sep 17 | Sep 20 | Sep 25 |
| 24°F | Sep 9 | Sep 15 | Sep 19 | Sep 21 | Sep 24 | Sep 27 | Sep 30 | Oct 2 | Oct 6 |
| 20°F | Sep 20 | Sep 24 | Sep 28 | Oct 1 | Oct 3 | Oct 6 | Oct 9 | Oct 13 | Oct 18 |
| 16°F | Sep 28 | Oct 2 | Oct 6 | Oct 10 | Oct 15 | Oct 19 | Oct 22 | Oct 26 | Nov 1 |
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: Ovando 9 Sse, Mt, about 2.8 miles away@if(!empty($station['elevation_ft'])), at 4,325 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
How long is the growing season here?
The frost-free growing season (between the last spring and first fall 32°F frost) is about 67 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 59843 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026