Frost dates for Holter Dam, MT
At Holter Dam, the median last spring frost (32°F) is May 19 and the median first fall frost is Sep 24, for a typical growing season of 124 days. For a 90% chance that the last frost has passed, wait until Jun 8. Nearby ZIP codes are in USDA hardiness zones 4b to 5a.
- May 19Last spring frost (median)50% chance of 32°F after this date
- Jun 8Safer planting date10% chance of 32°F after this date
- Sep 24First fall frost (median)50% chance of 32°F by this date
- 124 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 24 | Jun 10 | Jun 30 |
| 32°F | May 6 | May 19 | Jun 8 |
| 28°F | Apr 24 | May 6 | May 19 |
| 24°F | Apr 8 | Apr 27 | May 11 |
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 24 | Sep 8 | Sep 24 |
| 32°F | Sep 6 | Sep 24 | Oct 7 |
| 28°F | Sep 20 | Oct 4 | Oct 21 |
| 24°F | Sep 30 | Oct 16 | 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 | 66 days | 90 days | 112 days |
| 32°F | 101 days | 124 days | 143 days |
| 28°F | 129 days | 149 days | 169 days |
| 24°F | 149 days | 172 days | 197 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 |
|---|---|---|---|
| 59648 | 4b | -25 to -20°F | 18 km |
| 59421 | 4b | -25 to -20°F | 26 km |
| 59602 | 4b | -25 to -20°F | 30 km |
| 59633 | 4b | -25 to -20°F | 32 km |
| 59640 | 4b | -25 to -20°F | 37 km |
| 59636 | 4b | -25 to -20°F | 42 km |
| 59623 | 5a | -20 to -15°F | 45 km |
| 59477 | 4b | -25 to -20°F | 50 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 USC00244241, at 46.9908, -112.0117, elevation 1063 m (3,487 ft).
- Data quality: Standard (24+ years of data), 30 years of freeze data.
- Average days per year at or below 32°F: 136.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Adel (26 km)
- Rogers Pass 9 NNE (31 km)
- Cascade 20 SSE (33 km)
- Cascade 5 S (34 km)
- Austin 1 W (43 km)
- Helena Regional Airport (43 km)
- Lincoln Ranger Station (49 km)
- Elliston (57 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.