Frost dates for Heron 2 NW, MT
At Heron 2 NW, the median last spring frost (32°F) is May 20 and the median first fall frost is Sep 20, for a typical growing season of 119 days. For a 90% chance that the last frost has passed, wait until Jun 5. Nearby ZIP codes are in USDA hardiness zones 6a to 6b.
- May 20Last spring frost (median)50% chance of 32°F after this date
- Jun 5Safer planting date10% chance of 32°F after this date
- Sep 20First fall frost (median)50% chance of 32°F by this date
- 119 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 4 | May 20 | Jun 5 |
| 28°F | Apr 18 | May 2 | May 16 |
| 24°F | Mar 22 | Apr 14 | May 2 |
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 1 | Sep 20 | Oct 5 |
| 28°F | Sep 22 | Oct 7 | Oct 27 |
| 24°F | Oct 3 | Oct 27 | 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 | 42 days | 72 days | 101 days |
| 32°F | 93 days | 119 days | 145 days |
| 28°F | 133 days | 157 days | 183 days |
| 24°F | 163 days | 193 days | 225 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 |
|---|---|---|---|
| 59844 | 6b | -5 to 0°F | 10 km |
| 59853 | 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 USC00244084, at 48.0800, -116.0014, elevation 683 m (2,240 ft).
- Data quality: Representative (10+ years), 19 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
- Trout Creek Ranger Station (37 km)
- Troy (45 km)
- Libby (47 km)
- Libby 1 NE Ranger Station (50 km)
- Libby 32 SSE (60 km)
- Libby Dam (base) (63 km)
- Thompson Falls Ph (72 km)
- Haugan 1 W (88 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.