Frost dates for St. Mary, MT
At St. Mary, the median last spring frost (32°F) is Jun 12 and the median first fall frost is Sep 5, for a typical growing season of 82 days. For a 90% chance that the last frost has passed, wait until Jul 3. Nearby ZIP codes are in USDA hardiness zones 4a to 5b.
- Jun 12Last spring frost (median)50% chance of 32°F after this date
- Jul 3Safer planting date10% chance of 32°F after this date
- Sep 5First fall frost (median)50% chance of 32°F by this date
- 82 daysGrowing season (median)between 32°F freezes
Last spring frost dates
The chance that the temperature drops to the threshold after each date.
| Temperature | 90% chance later | 50% chance later | 10% chance later |
|---|---|---|---|
| 32°F | May 27 | Jun 12 | Jul 3 |
| 28°F | May 6 | May 19 | Jun 7 |
| 24°F | Apr 21 | May 4 | May 19 |
First fall frost dates
The chance that the temperature has dropped to the threshold by each date.
| Temperature | 10% chance by | 50% chance by | 90% chance by |
|---|---|---|---|
| 32°F | Aug 18 | Sep 5 | Sep 21 |
| 28°F | Sep 1 | Sep 20 | Oct 4 |
| 24°F | Sep 13 | Oct 2 | Oct 21 |
Growing season length
Days between the last spring and first fall freeze, as the length reached in 9, 5 and 1 out of 10 years.
| Temperature | 9 in 10 years at least | 5 in 10 years at least | 1 in 10 years at least |
|---|---|---|---|
| 36°F | 30 days | 50 days | 72 days |
| 32°F | 55 days | 82 days | 105 days |
| 28°F | 95 days | 121 days | 143 days |
| 24°F | 125 days | 150 days | 173 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 100% |
| 28°F | 100% |
| 24°F | 100% |
Closest ZIP codes (each is closer to another station)
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 59434 | 4b | -25 to -20°F | 7 km |
| 59411 | 4b | -25 to -20°F | 17 km |
| 59417 | 4a | -30 to -25°F | 38 km |
| 59916 | 5a | -20 to -15°F | 51 km |
| 59913 | 5a | -20 to -15°F | 52 km |
| 59427 | 4a | -30 to -25°F | 56 km |
| 59926 | 5b | -15 to -10°F | 60 km |
| 59936 | 5a | -20 to -15°F | 61 km |
Not the official USDA Plant Hardiness Zone Map. Zones shown here are the values published in the USDA-ARS / Oregon State University PRISM Group 2023 ZIP code tables, shown unaltered. For the official interactive map, visit planthardiness.ars.usda.gov.
About this station
- NOAA station ID USC00247292, at 48.7386, -113.4294, elevation 1390 m (4,560 ft).
- Data quality: Representative (10+ years), 18 years of freeze data.
- Average days per year at or below 32°F: 188.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- St. Mary 1 SSW (0 km)
- E Glacier (36 km)
- W Glacier (49 km)
- Del Bonita (55 km)
- Hungry Horse Dam (62 km)
- Polebridge (63 km)
- Whitefish (78 km)
- Kalispell Glacier Airport (78 km)
Sources
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, annual/seasonal normals by station (version 1.0.1), file dated June 27, 2023; documentation.
- USDA Agricultural Research Service and PRISM Group, Oregon State University, 2023 Plant Hardiness Zone Map ZIP code tables (CONUS, Alaska, Hawaii, Puerto Rico), files dated November 17, 2023. Data © PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed November 1, 2026. Official map: planthardiness.ars.usda.gov.
- U.S. Census Bureau, 2024 Gazetteer ZIP Code Tabulation Areas file, dated August 30, 2024 (ZIP centroids used to find the nearest station).
Freeze dates are 30-year probabilities, not forecasts. Check your local forecast before planting tender plants.