Frost dates for Helena Regional Airport, MT
At Helena Regional Airport, the median last spring frost (32°F) is May 9 and the median first fall frost is Sep 30, for a typical growing season of 142 days. For a 90% chance that the last frost has passed, wait until May 25. Nearby ZIP codes are in USDA hardiness zones 4b to 5a.
- May 9Last spring frost (median)50% chance of 32°F after this date
- May 25Safer planting date10% chance of 32°F after this date
- Sep 30First fall frost (median)50% chance of 32°F by this date
- 142 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 10 | May 25 | Jun 13 |
| 32°F | Apr 29 | May 9 | May 25 |
| 28°F | Apr 15 | Apr 29 | May 11 |
| 24°F | Apr 1 | Apr 16 | May 1 |
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 4 | Sep 19 | Oct 1 |
| 32°F | Sep 17 | Sep 30 | Oct 11 |
| 28°F | Sep 25 | Oct 7 | Oct 22 |
| 24°F | Oct 1 | Oct 18 | 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 | 93 days | 114 days | 135 days |
| 32°F | 121 days | 142 days | 159 days |
| 28°F | 145 days | 162 days | 182 days |
| 24°F | 163 days | 184 days | 207 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 |
|---|---|---|---|
| 59623 | 5a | -20 to -15°F | 6 km |
| 59636 | 4b | -25 to -20°F | 12 km |
| 59635 | 5a | -20 to -15°F | 13 km |
| 59602 | 4b | -25 to -20°F | 14 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 USW00024144, at 46.6056, -111.9636, elevation 1167 m (3,828 ft).
- Data quality: Standard (24+ years of data), 25 years of freeze data.
- Average days per year at or below 32°F: 170.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Austin 1 W (23 km)
- Tizer Gardens (26 km)
- Elliston (36 km)
- Holter Dam (43 km)
- Boulder (43 km)
- Adel (50 km)
- Townsend 3se (52 km)
- Cascade 20 SSE (52 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.