Last spring frost
May 15
Frost Dates for Brockway, MT
First fall frost
Sep 25
Growing season
132 days
Hardiness zone
4a
In an average year around Brockway, MT, the last spring frost (32°F) is about May 15 and the first fall frost is about Sep 25, giving roughly a 132-day growing season. To be safe most years, wait until May 2 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 10 | May 26 | May 13 |
| Freeze (32°F) | May 27 | May 15 | May 2 |
| Hard (28°F) | May 19 | May 5 | Apr 20 |
| Severe (24°F) | May 11 | Apr 24 | Apr 9 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 10 | Jun 4 | May 31 | May 29 | May 26 | May 23 | May 20 | May 18 | May 13 |
| 32°F | May 27 | May 23 | May 20 | May 18 | May 15 | May 13 | May 10 | May 7 | May 2 |
| 28°F | May 19 | May 14 | May 11 | May 8 | May 5 | May 2 | Apr 30 | Apr 26 | Apr 20 |
| 24°F | May 11 | May 5 | May 1 | Apr 27 | Apr 24 | Apr 20 | Apr 16 | Apr 13 | Apr 9 |
| 20°F | May 1 | Apr 24 | Apr 19 | Apr 16 | Apr 12 | Apr 9 | Apr 5 | Apr 2 | Mar 26 |
| 16°F | Apr 19 | Apr 13 | Apr 9 | Apr 5 | Apr 2 | Mar 30 | Mar 25 | Mar 18 | Mar 11 |
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 30 | Sep 16 | Sep 29 |
| Freeze (32°F) | Sep 10 | Sep 25 | Oct 6 |
| Hard (28°F) | Sep 21 | Oct 3 | Oct 17 |
| Severe (24°F) | Sep 30 | Oct 12 | Oct 26 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 30 | Sep 4 | Sep 9 | Sep 12 | Sep 16 | Sep 19 | Sep 21 | Sep 24 | Sep 29 |
| 32°F | Sep 10 | Sep 16 | Sep 20 | Sep 23 | Sep 25 | Sep 27 | Sep 30 | Oct 3 | Oct 6 |
| 28°F | Sep 21 | Sep 25 | Sep 29 | Oct 1 | Oct 3 | Oct 6 | Oct 9 | Oct 12 | Oct 17 |
| 24°F | Sep 30 | Oct 3 | Oct 6 | Oct 9 | Oct 12 | Oct 15 | Oct 19 | Oct 22 | Oct 26 |
| 20°F | Oct 7 | Oct 12 | Oct 16 | Oct 19 | Oct 22 | Oct 25 | Oct 28 | Oct 31 | Nov 5 |
| 16°F | Oct 16 | Oct 21 | Oct 25 | Oct 29 | Nov 1 | Nov 3 | Nov 6 | Nov 11 | Nov 17 |
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: Rock Springs, Mt, about 15 miles away@if(!empty($station['elevation_ft'])), at 3,024 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 Brockway, MT?
On average, the last spring frost (32°F) around Brockway, MT is about May 15. In cooler years it can run later, so for a safer bet many gardeners wait until May 2.
When is the first frost in the fall?
The first fall frost around Brockway, MT typically arrives about Sep 25. 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 15 on average, or May 2 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 132 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 59312 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026