Last spring frost
May 9
Frost Dates for Helena West Side, MT
First fall frost
Sep 30
Growing season
142 days
Hardiness zone
4b
In an average year around Helena West Side, MT, the last spring frost (32°F) is about May 9 and the first fall frost is about Sep 30, giving roughly a 142-day growing season. To be safe most years, wait until Apr 29 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 13 | May 25 | May 10 |
| Freeze (32°F) | May 25 | May 9 | Apr 29 |
| Hard (28°F) | May 11 | Apr 29 | Apr 15 |
| Severe (24°F) | May 1 | Apr 16 | Apr 1 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 13 | Jun 6 | Jun 1 | May 28 | May 25 | May 21 | May 18 | May 15 | May 10 |
| 32°F | May 25 | May 19 | May 15 | May 12 | May 9 | May 7 | May 4 | May 2 | Apr 29 |
| 28°F | May 11 | May 6 | May 3 | May 1 | Apr 29 | Apr 26 | Apr 22 | Apr 19 | Apr 15 |
| 24°F | May 1 | Apr 28 | Apr 24 | Apr 20 | Apr 16 | Apr 13 | Apr 10 | Apr 5 | Apr 1 |
| 20°F | Apr 21 | Apr 15 | Apr 10 | Apr 6 | Apr 4 | Mar 31 | Mar 28 | Mar 21 | Mar 13 |
| 16°F | Apr 11 | Apr 4 | Mar 31 | Mar 27 | Mar 22 | Mar 17 | Mar 12 | Mar 8 | Mar 3 |
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 4 | Sep 19 | Oct 1 |
| Freeze (32°F) | Sep 17 | Sep 30 | Oct 11 |
| Hard (28°F) | Sep 25 | Oct 7 | Oct 22 |
| Severe (24°F) | Oct 1 | Oct 18 | Nov 2 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 4 | Sep 8 | Sep 12 | Sep 16 | Sep 19 | Sep 22 | Sep 25 | Sep 28 | Oct 1 |
| 32°F | Sep 17 | Sep 22 | Sep 25 | Sep 27 | Sep 30 | Oct 1 | Oct 4 | Oct 7 | Oct 11 |
| 28°F | Sep 25 | Sep 30 | Oct 2 | Oct 5 | Oct 7 | Oct 10 | Oct 14 | Oct 17 | Oct 22 |
| 24°F | Oct 1 | Oct 6 | Oct 11 | Oct 15 | Oct 18 | Oct 21 | Oct 25 | Oct 28 | Nov 2 |
| 20°F | Oct 11 | Oct 19 | Oct 24 | Oct 27 | Oct 30 | Nov 1 | Nov 5 | Nov 9 | Nov 13 |
| 16°F | Oct 22 | Oct 28 | Nov 1 | Nov 4 | Nov 7 | Nov 10 | Nov 14 | Nov 17 | Nov 23 |
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: Helena Rgnl Ap, Mt, about 7.2 miles away@if(!empty($station['elevation_ft'])), at 3,828 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 Helena West Side, MT?
On average, the last spring frost (32°F) around Helena West Side, MT is about May 9. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 29.
When is the first frost in the fall?
The first fall frost around Helena West Side, MT typically arrives about Sep 30. 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 9 on average, or Apr 29 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 142 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 59636 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026