Frost dates for Libby Dam (base), MT
At Libby Dam (base), the median last spring frost (32°F) is May 11 and the median first fall frost is Oct 1, for a typical growing season of 142 days. For a 90% chance that the last frost has passed, wait until May 27. Nearby ZIP codes are in USDA hardiness zone 6a.
- May 11Last spring frost (median)50% chance of 32°F after this date
- May 27Safer planting date10% chance of 32°F after this date
- Oct 1First fall frost (median)50% chance of 32°F by this date
- 142 daysGrowing season (median)between 32°F freezes
USDA hardiness zone 6a (-10 to -5°F) · see all zone 6a 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 15 | May 30 | Jul 2 |
| 32°F | Apr 30 | May 11 | May 27 |
| 28°F | Apr 10 | Apr 28 | May 11 |
| 24°F | Mar 17 | Apr 6 | Apr 28 |
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 20 | Oct 3 |
| 32°F | Sep 14 | Oct 1 | Oct 18 |
| 28°F | Sep 29 | Oct 16 | Nov 1 |
| 24°F | Oct 11 | Oct 30 | Nov 19 |
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 | 84 days | 111 days | 131 days |
| 32°F | 119 days | 142 days | 160 days |
| 28°F | 148 days | 170 days | 191 days |
| 24°F | 175 days | 202 days | 230 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 |
|---|---|---|---|
| 59923 | 6a | -10 to -5°F | 16 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 USC00245011, at 48.4100, -115.3181, elevation 643 m (2,110 ft).
- Data quality: Standard (24+ years of data), 27 years of freeze data.
- Average days per year at or below 32°F: 178.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Libby 1 NE Ranger Station (16 km)
- Libby (18 km)
- Troy (44 km)
- Libby 32 SSE (51 km)
- Fortine 1 N (51 km)
- Olney (57 km)
- Eureka Ranger Station (57 km)
- Heron 2 NW (63 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.