Frost dates for Darby, MT
At Darby, the median last spring frost (32°F) is May 24 and the median first fall frost is Sep 23, for a typical growing season of 118 days. For a 90% chance that the last frost has passed, wait until Jun 14. Nearby ZIP codes are in USDA hardiness zones 5a to 5b.
- May 24Last spring frost (median)50% chance of 32°F after this date
- Jun 14Safer planting date10% chance of 32°F after this date
- Sep 23First fall frost (median)50% chance of 32°F by this date
- 118 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 28 | Jun 16 | Jul 4 |
| 32°F | May 6 | May 24 | Jun 14 |
| 28°F | Apr 23 | May 5 | May 21 |
| 24°F | Apr 5 | Apr 24 | May 7 |
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 20 | Sep 6 | Sep 25 |
| 32°F | Sep 4 | Sep 23 | Oct 8 |
| 28°F | Sep 18 | Oct 4 | Oct 19 |
| 24°F | Oct 1 | Oct 16 | Nov 2 |
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 | 55 days | 80 days | 105 days |
| 32°F | 92 days | 118 days | 143 days |
| 28°F | 124 days | 150 days | 170 days |
| 24°F | 153 days | 174 days | 200 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 |
|---|---|---|---|
| 59827 | 5b | -15 to -10°F | 16 km |
| 59840 | 5b | -15 to -10°F | 17 km |
| 59829 | 5b | -15 to -10°F | 31 km |
| 59871 | 5a | -20 to -15°F | 32 km |
| 59841 | 5b | -15 to -10°F | 34 km |
| 59828 | 5b | -15 to -10°F | 37 km |
| 59875 | 5b | -15 to -10°F | 45 km |
| 83463 | 5b | -15 to -10°F | 53 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 USC00242221, at 46.0264, -114.1764, elevation 1183 m (3,880 ft).
- Data quality: Representative (10+ years), 19 years of freeze data.
- Average days per year at or below 32°F: 167.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Hamilton (24 km)
- Sula 3 ENE (28 km)
- Western Ag Rsch Center (34 km)
- Sula 14 NE (36 km)
- Stevensville (55 km)
- Wisdom (72 km)
- Phillipsburg Ranger Station (75 km)
- Jackson (94 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.