Frost dates for Sweetgrass, MT
At Sweetgrass, the median last spring frost (32°F) is May 15 and the median first fall frost is Sep 25, for a typical growing season of 132 days. For a 90% chance that the last frost has passed, wait until May 28. Nearby ZIP codes are in USDA hardiness zone 4a.
- May 15Last spring frost (median)50% chance of 32°F after this date
- May 28Safer planting date10% chance of 32°F after this date
- Sep 25First fall frost (median)50% chance of 32°F by this date
- 132 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 |
|---|---|---|---|
| 36°F | May 16 | May 29 | Jun 16 |
| 32°F | May 3 | May 15 | May 28 |
| 28°F | Apr 19 | May 4 | May 16 |
| 24°F | Apr 7 | Apr 24 | May 6 |
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 |
|---|---|---|---|
| 36°F | Sep 1 | Sep 15 | Sep 27 |
| 32°F | Sep 11 | Sep 25 | Oct 8 |
| 28°F | Sep 21 | Oct 4 | Oct 19 |
| 24°F | Sep 29 | Oct 15 | Oct 30 |
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 | 82 days | 107 days | 127 days |
| 32°F | 114 days | 132 days | 151 days |
| 28°F | 134 days | 152 days | 172 days |
| 24°F | 152 days | 172 days | 195 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 |
|---|---|---|---|
| 59482 | 4a | -30 to -25°F | 17 km |
| 59454 | 4a | -30 to -25°F | 25 km |
| 59484 | 4a | -30 to -25°F | 31 km |
| 59466 | 4a | -30 to -25°F | 34 km |
| 59435 | 4a | -30 to -25°F | 47 km |
| 59545 | 4a | -30 to -25°F | 55 km |
| 59474 | 4a | -30 to -25°F | 56 km |
| 59444 | 4a | -30 to -25°F | 58 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 USC00248093, at 48.9972, -111.9642, elevation 1070 m (3,510 ft).
- Data quality: Representative (10+ years), 13 years of freeze data.
- Average days per year at or below 32°F: 179.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Sunburst 8 E (21 km)
- Goldbutte 7 N (41 km)
- Dunkirk 19NNE (47 km)
- Cut Bank Muni Airport (53 km)
- Shelby (54 km)
- Del Bonita (60 km)
- Valier (79 km)
- Chester (91 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.