Frost dates for Boulder, MT
At Boulder, the median last spring frost (32°F) is Jun 7 and the median first fall frost is Sep 8, for a typical growing season of 90 days. For a 90% chance that the last frost has passed, wait until Jun 25. Nearby ZIP codes are in USDA hardiness zone 5a.
- Jun 7Last spring frost (median)50% chance of 32°F after this date
- Jun 25Safer planting date10% chance of 32°F after this date
- Sep 8First fall frost (median)50% chance of 32°F by this date
- 90 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 24 | Jun 7 | Jun 25 |
| 28°F | May 7 | May 21 | Jun 5 |
| 24°F | Apr 24 | May 8 | May 22 |
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 | Aug 25 | Sep 8 | Sep 21 |
| 28°F | Sep 6 | Sep 21 | Oct 3 |
| 24°F | Sep 15 | Sep 30 | Oct 12 |
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 | 39 days | 63 days | 83 days |
| 32°F | 68 days | 90 days | 110 days |
| 28°F | 99 days | 121 days | 141 days |
| 24°F | 124 days | 144 days | 163 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 |
|---|---|---|---|
| 59632 | 5a | -20 to -15°F | 11 km |
| 59631 | 5a | -20 to -15°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 USC00241008, at 46.2311, -112.1139, elevation 1495 m (4,904 ft).
- Data quality: Standard (24+ years of data), 26 years of freeze data.
- Average days per year at or below 32°F: 205.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Tizer Gardens (17 km)
- Butte Bert Mooney Airport (42 km)
- Helena Regional Airport (43 km)
- Elliston (44 km)
- Austin 1 W (48 km)
- Townsend 3se (51 km)
- Deer Lodge 3 W (55 km)
- Trident (58 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.