Frost dates for Fort Benton, MT
At Fort Benton, the median last spring frost (32°F) is May 15 and the median first fall frost is Sep 22, for a typical growing season of 129 days. For a 90% chance that the last frost has passed, wait until May 30. Nearby ZIP codes are in USDA hardiness zone 4a.
- May 15Last spring frost (median)50% chance of 32°F after this date
- May 30Safer planting date10% chance of 32°F after this date
- Sep 22First fall frost (median)50% chance of 32°F by this date
- 129 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 15 | May 28 | Jun 13 |
| 32°F | May 3 | May 15 | May 30 |
| 28°F | Apr 21 | May 4 | May 14 |
| 24°F | Apr 7 | Apr 23 | 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 | Aug 30 | Sep 12 | Sep 25 |
| 32°F | Sep 7 | Sep 22 | Oct 4 |
| 28°F | Sep 18 | Oct 1 | Oct 14 |
| 24°F | Sep 27 | Oct 10 | Oct 24 |
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 | 83 days | 106 days | 124 days |
| 32°F | 109 days | 129 days | 146 days |
| 28°F | 131 days | 149 days | 168 days |
| 24°F | 150 days | 170 days | 189 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 |
|---|---|---|---|
| 59442 | 4a | -30 to -25°F | 11 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 USC00243113, at 47.8303, -110.6611, elevation 807 m (2,649 ft).
- Data quality: Standard (24+ years of data), 24 years of freeze data.
- Average days per year at or below 32°F: 172.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Loma (17 km)
- Highwood 7 NE (21 km)
- Shonkin 7 S (34 km)
- Geraldine (39 km)
- Carter 14 W (42 km)
- Rudyard 19s (50 km)
- Brady 27 ENE (the Knees) (52 km)
- Big Sandy (56 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.