Frost dates for Chinook, MT
At Chinook, the median last spring frost (32°F) is May 14 and the median first fall frost is Sep 23, for a typical growing season of 130 days. For a 90% chance that the last frost has passed, wait until May 27. Nearby ZIP codes are in USDA hardiness zone 3b.
- May 14Last spring frost (median)50% chance of 32°F after this date
- May 27Safer planting date10% chance of 32°F after this date
- Sep 23First fall frost (median)50% chance of 32°F by this date
- 130 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 13 | May 25 | Jun 8 |
| 32°F | May 1 | May 14 | May 27 |
| 28°F | Apr 19 | May 3 | May 16 |
| 24°F | Apr 9 | Apr 23 | May 6 |
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 1 | Sep 12 | Sep 24 |
| 32°F | Sep 11 | Sep 23 | Oct 3 |
| 28°F | Sep 20 | Oct 1 | Oct 11 |
| 24°F | Sep 25 | Oct 8 | Oct 21 |
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 | 91 days | 109 days | 126 days |
| 32°F | 112 days | 130 days | 148 days |
| 28°F | 131 days | 149 days | 167 days |
| 24°F | 148 days | 167 days | 186 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 |
|---|---|---|---|
| 59523 | 3b | -35 to -30°F | 7 km |
| 59547 | 3b | -35 to -30°F | 22 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 USC00241722, at 48.5883, -109.2267, elevation 738 m (2,420 ft).
- Data quality: Standard (24+ years of data), 28 years of freeze data.
- Average days per year at or below 32°F: 195.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Harlem (32 km)
- Fort Belknap 2 SW (36 km)
- Havre City Co Airport (40 km)
- Fort Assiniboine (43 km)
- Chinook 35se (49 km)
- Harlem 20s (56 km)
- Turner 11n (76 km)
- Gildford (79 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.