Frost dates for Troy, MT
At Troy, the median last spring frost (32°F) is May 14 and the median first fall frost is Sep 25, for a typical growing season of 132 days. For a 90% chance that the last frost has passed, wait until May 31. Nearby ZIP codes are in USDA hardiness zone 6a.
- May 14Last spring frost (median)50% chance of 32°F after this date
- May 31Safer planting date10% chance of 32°F after this date
- Sep 25First fall frost (median)50% chance of 32°F by this date
- 132 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 18 | Jun 2 | Jun 27 |
| 32°F | Apr 29 | May 14 | May 31 |
| 28°F | Apr 14 | Apr 29 | May 14 |
| 24°F | Mar 19 | Apr 8 | Apr 29 |
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 7 | Sep 24 |
| 32°F | Sep 9 | Sep 25 | Oct 9 |
| 28°F | Sep 21 | Oct 6 | Oct 30 |
| 24°F | Oct 1 | Oct 24 | Nov 16 |
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 | 70 days | 96 days | 119 days |
| 32°F | 108 days | 132 days | 155 days |
| 28°F | 140 days | 162 days | 190 days |
| 24°F | 167 days | 198 days | 229 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 |
|---|---|---|---|
| 59935 | 6a | -10 to -5°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 USC00248390, at 48.4806, -115.9061, elevation 588 m (1,929 ft).
- Data quality: Representative (10+ years), 21 years of freeze data.
- Average days per year at or below 32°F: 152.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Libby (28 km)
- Libby 1 NE Ranger Station (28 km)
- Libby Dam (base) (44 km)
- Heron 2 NW (45 km)
- Yaak 9nne (56 km)
- Trout Creek Ranger Station (71 km)
- Eureka Ranger Station (77 km)
- Libby 32 SSE (77 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.