Frost dates for Libby 1 NE Ranger Station, MT
At Libby 1 NE Ranger Station, the median last spring frost (32°F) is May 23 and the median first fall frost is Sep 21, for a typical growing season of 120 days. For a 90% chance that the last frost has passed, wait until Jun 7. Nearby ZIP codes are in USDA hardiness zones 5a to 6b.
- May 23Last spring frost (median)50% chance of 32°F after this date
- Jun 7Safer planting date10% chance of 32°F after this date
- Sep 21First fall frost (median)50% chance of 32°F by this date
- 120 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 |
|---|---|---|---|
| 32°F | May 9 | May 23 | Jun 7 |
| 28°F | Apr 23 | May 7 | May 20 |
| 24°F | Mar 29 | Apr 19 | May 5 |
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 |
|---|---|---|---|
| 32°F | Sep 3 | Sep 21 | Oct 8 |
| 28°F | Sep 20 | Oct 5 | Oct 29 |
| 24°F | Oct 1 | Oct 23 | Nov 18 |
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 | 58 days | 85 days | 110 days |
| 32°F | 96 days | 120 days | 143 days |
| 28°F | 130 days | 152 days | 177 days |
| 24°F | 158 days | 186 days | 217 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 |
|---|---|---|---|
| 59923 | 6a | -10 to -5°F | 22 km |
| 59935 | 6a | -10 to -5°F | 35 km |
| 59853 | 6a | -10 to -5°F | 44 km |
| 83836 | 6b | -5 to 0°F | 51 km |
| 59934 | 5a | -20 to -15°F | 52 km |
| 59844 | 6b | -5 to 0°F | 54 km |
| 83845 | 6b | -5 to 0°F | 56 km |
| 83811 | 6b | -5 to 0°F | 56 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 USC00245015, at 48.4036, -115.5392, elevation 639 m (2,096 ft).
- Data quality: Representative (10+ years), 24 years of freeze data.
- Average days per year at or below 32°F: 174.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Libby (3 km)
- Libby Dam (base) (16 km)
- Troy (28 km)
- Heron 2 NW (50 km)
- Libby 32 SSE (55 km)
- Trout Creek Ranger Station (60 km)
- Yaak 9nne (61 km)
- Fortine 1 N (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.