Last spring frost
May 21
Frost Dates for South Glastonbury, MT
First fall frost
Sep 27
Growing season
128 days
Hardiness zone
5a
In an average year around South Glastonbury, MT, the last spring frost (32°F) is about May 21 and the first fall frost is about Sep 27, giving roughly a 128-day growing season. To be safe most years, wait until May 5 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 22 | Jun 6 | May 20 |
| Freeze (32°F) | Jun 9 | May 21 | May 5 |
| Hard (28°F) | May 23 | May 6 | Apr 20 |
| Severe (24°F) | May 9 | Apr 23 | Apr 4 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 22 | Jun 16 | Jun 12 | Jun 8 | Jun 6 | Jun 3 | Jun 1 | May 27 | May 20 |
| 32°F | Jun 9 | Jun 3 | May 29 | May 25 | May 21 | May 18 | May 14 | May 11 | May 5 |
| 28°F | May 23 | May 16 | May 12 | May 9 | May 6 | May 3 | Apr 30 | Apr 26 | Apr 20 |
| 24°F | May 9 | May 3 | Apr 30 | Apr 27 | Apr 23 | Apr 20 | Apr 15 | Apr 11 | Apr 4 |
| 20°F | Apr 27 | Apr 22 | Apr 17 | Apr 11 | Apr 7 | Apr 4 | Apr 1 | Mar 28 | Mar 18 |
| 16°F | Apr 16 | Apr 7 | Apr 3 | Mar 31 | Mar 26 | Mar 21 | Mar 17 | Mar 11 | Mar 4 |
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 2 | Sep 16 | Sep 30 |
| Freeze (32°F) | Sep 12 | Sep 27 | Oct 11 |
| Hard (28°F) | Sep 25 | Oct 7 | Oct 23 |
| Severe (24°F) | Oct 2 | Oct 18 | Nov 3 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 2 | Sep 7 | Sep 10 | Sep 13 | Sep 16 | Sep 20 | Sep 22 | Sep 25 | Sep 30 |
| 32°F | Sep 12 | Sep 19 | Sep 23 | Sep 25 | Sep 27 | Sep 30 | Oct 2 | Oct 6 | Oct 11 |
| 28°F | Sep 25 | Sep 30 | Oct 2 | Oct 5 | Oct 7 | Oct 11 | Oct 14 | Oct 18 | Oct 23 |
| 24°F | Oct 2 | Oct 7 | Oct 11 | Oct 14 | Oct 18 | Oct 22 | Oct 25 | Oct 29 | Nov 3 |
| 20°F | Oct 13 | Oct 19 | Oct 23 | Oct 27 | Oct 30 | Nov 1 | Nov 5 | Nov 8 | Nov 14 |
| 16°F | Oct 22 | Oct 27 | Oct 31 | Nov 3 | Nov 6 | Nov 10 | Nov 13 | 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: Gardiner, Mt, about 18.5 miles away@if(!empty($station['elevation_ft'])), at 5,275 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 South Glastonbury, MT?
On average, the last spring frost (32°F) around South Glastonbury, MT is about May 21. In cooler years it can run later, so for a safer bet many gardeners wait until May 5.
When is the first frost in the fall?
The first fall frost around South Glastonbury, MT typically arrives about Sep 27. 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 21 on average, or May 5 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 128 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 59027 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026