Last spring frost
May 23
Frost Dates for Corvallis, MT
First fall frost
Sep 21
Growing season
118 days
Hardiness zone
5b
In an average year around Corvallis, MT, the last spring frost (32°F) is about May 23 and the first fall frost is about Sep 21, giving roughly a 118-day growing season. To be safe most years, wait until May 8 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) | Jul 1 | Jun 12 | May 27 |
| Freeze (32°F) | Jun 10 | May 23 | May 8 |
| Hard (28°F) | May 19 | May 5 | Apr 22 |
| Severe (24°F) | May 8 | Apr 23 | Apr 3 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jul 1 | Jun 24 | Jun 20 | Jun 16 | Jun 12 | Jun 10 | Jun 5 | Jun 2 | May 27 |
| 32°F | Jun 10 | Jun 3 | May 31 | May 27 | May 23 | May 19 | May 16 | May 12 | May 8 |
| 28°F | May 19 | May 14 | May 11 | May 8 | May 5 | May 3 | Apr 30 | Apr 27 | Apr 22 |
| 24°F | May 8 | May 4 | Apr 30 | Apr 27 | Apr 23 | Apr 20 | Apr 15 | Apr 10 | Apr 3 |
| 20°F | Apr 24 | Apr 17 | Apr 12 | Apr 7 | Apr 3 | Mar 31 | Mar 26 | Mar 19 | Mar 10 |
| 16°F | Apr 5 | Mar 30 | Mar 24 | Mar 18 | Mar 13 | Mar 7 | Mar 4 | Feb 28 | Feb 22 |
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 24 | Sep 8 | Sep 22 |
| Freeze (32°F) | Sep 5 | Sep 21 | Oct 4 |
| Hard (28°F) | Sep 16 | Oct 2 | Oct 16 |
| Severe (24°F) | Sep 28 | Oct 14 | Oct 29 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 24 | Aug 30 | Sep 1 | Sep 4 | Sep 8 | Sep 11 | Sep 14 | Sep 17 | Sep 22 |
| 32°F | Sep 5 | Sep 11 | Sep 15 | Sep 18 | Sep 21 | Sep 24 | Sep 27 | Oct 1 | Oct 4 |
| 28°F | Sep 16 | Sep 22 | Sep 26 | Sep 29 | Oct 2 | Oct 4 | Oct 7 | Oct 10 | Oct 16 |
| 24°F | Sep 28 | Oct 3 | Oct 7 | Oct 11 | Oct 14 | Oct 17 | Oct 20 | Oct 24 | Oct 29 |
| 20°F | Oct 7 | Oct 15 | Oct 20 | Oct 22 | Oct 27 | Oct 30 | Nov 1 | Nov 5 | Nov 11 |
| 16°F | Oct 20 | Oct 27 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 13 | Nov 19 | Nov 25 |
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: Western Ag Rsch Ctr, Mt, about 6 miles away@if(!empty($station['elevation_ft'])), at 3,608 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 Corvallis, MT?
On average, the last spring frost (32°F) around Corvallis, MT is about May 23. In cooler years it can run later, so for a safer bet many gardeners wait until May 8.
When is the first frost in the fall?
The first fall frost around Corvallis, MT typically arrives about Sep 21. 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 8 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 118 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 59828 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026