Last spring frost
May 12
Frost Dates for St. Ignatius, MT
First fall frost
Oct 2
Growing season
142 days
Hardiness zone
6a
In an average year around St. Ignatius, MT, the last spring frost (32°F) is about May 12 and the first fall frost is about Oct 2, giving roughly a 142-day growing season. To be safe most years, wait until Apr 28 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 24 | Jun 1 | May 12 |
| Freeze (32°F) | Jun 1 | May 12 | Apr 28 |
| Hard (28°F) | May 17 | Apr 30 | Apr 9 |
| Severe (24°F) | May 1 | Apr 7 | Mar 14 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 24 | Jun 15 | Jun 10 | Jun 5 | Jun 1 | May 29 | May 25 | May 20 | May 12 |
| 32°F | Jun 1 | May 24 | May 19 | May 15 | May 12 | May 9 | May 6 | May 1 | Apr 28 |
| 28°F | May 17 | May 10 | May 5 | May 2 | Apr 30 | Apr 27 | Apr 24 | Apr 19 | Apr 9 |
| 24°F | May 1 | Apr 26 | Apr 19 | Apr 13 | Apr 7 | Apr 2 | Mar 29 | Mar 23 | Mar 14 |
| 20°F | Apr 13 | Apr 1 | Mar 27 | Mar 22 | Mar 17 | Mar 13 | Mar 9 | Mar 5 | Mar 1 |
| 16°F | Mar 24 | Mar 17 | Mar 12 | Mar 7 | Mar 5 | Mar 2 | Feb 26 | Feb 20 | Feb 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) | Sep 4 | Sep 22 | Oct 4 |
| Freeze (32°F) | Sep 19 | Oct 2 | Oct 14 |
| Hard (28°F) | Sep 29 | Oct 12 | Oct 27 |
| Severe (24°F) | Oct 7 | Oct 24 | Nov 10 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 4 | Sep 10 | Sep 14 | Sep 19 | Sep 22 | Sep 25 | Sep 27 | Oct 1 | Oct 4 |
| 32°F | Sep 19 | Sep 24 | Sep 28 | Sep 30 | Oct 2 | Oct 4 | Oct 6 | Oct 10 | Oct 14 |
| 28°F | Sep 29 | Oct 2 | Oct 6 | Oct 9 | Oct 12 | Oct 15 | Oct 19 | Oct 22 | Oct 27 |
| 24°F | Oct 7 | Oct 13 | Oct 18 | Oct 21 | Oct 24 | Oct 27 | Oct 31 | Nov 3 | Nov 10 |
| 20°F | Oct 19 | Oct 25 | Oct 30 | Nov 1 | Nov 4 | Nov 8 | Nov 13 | Nov 19 | Nov 25 |
| 16°F | Oct 28 | Nov 2 | Nov 6 | Nov 10 | Nov 15 | Nov 20 | Nov 25 | Nov 29 | Dec 6 |
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: Saint Ignatius, Mt, about 4.8 miles away@if(!empty($station['elevation_ft'])), at 2,915 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 St. Ignatius, MT?
On average, the last spring frost (32°F) around St. Ignatius, MT is about May 12. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 28.
When is the first frost in the fall?
The first fall frost around St. Ignatius, MT typically arrives about Oct 2. 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 12 on average, or Apr 28 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 59865 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026