Frost dates for Great Falls WFO, MT
At Great Falls WFO, the median last spring frost (32°F) is May 12 and the median first fall frost is Sep 28, for a typical growing season of 138 days. For a 90% chance that the last frost has passed, wait until May 27. Nearby ZIP codes are in USDA hardiness zone 4b.
- May 12Last spring frost (median)50% chance of 32°F after this date
- May 27Safer planting date10% chance of 32°F after this date
- Sep 28First fall frost (median)50% chance of 32°F by this date
- 138 daysGrowing season (median)between 32°F freezes
USDA hardiness zone 4b (-25 to -20°F) · see all zone 4b frost dates
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 13 | May 27 | Jun 11 |
| 32°F | Apr 30 | May 12 | May 27 |
| 28°F | Apr 14 | Apr 30 | May 13 |
| 24°F | Apr 2 | Apr 19 | May 4 |
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 3 | Sep 19 | Oct 2 |
| 32°F | Sep 14 | Sep 28 | Oct 11 |
| 28°F | Sep 26 | Oct 8 | Oct 23 |
| 24°F | Oct 2 | Oct 18 | Nov 3 |
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 | 90 days | 112 days | 132 days |
| 32°F | 117 days | 138 days | 157 days |
| 28°F | 142 days | 161 days | 181 days |
| 24°F | 161 days | 183 days | 206 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 100% |
| 28°F | 100% |
| 24°F | 100% |
ZIP codes whose closest station is this one
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 59405 | 4b | -25 to -20°F | 20 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 USC00243753, at 47.4614, -111.3850, elevation 1129 m (3,705 ft).
- Data quality: Representative (10+ years), 24 years of freeze data.
- Average days per year at or below 32°F: 157.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Great Falls Intl Airport (1 km)
- Great Falls 16st (8 km)
- Sun River 4 S (27 km)
- Cascade 5 S (36 km)
- Carter 14 W (39 km)
- Fairfield (48 km)
- Raynesford 2 NNW (51 km)
- Cascade 20 SSE (54 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.