Frost dates for Roundup, MT
At Roundup, the median last spring frost (32°F) is May 20 and the median first fall frost is Sep 21, for a typical growing season of 124 days. For a 90% chance that the last frost has passed, wait until Jun 2. Nearby ZIP codes are in USDA hardiness zone 4b.
- May 20Last spring frost (median)50% chance of 32°F after this date
- Jun 2Safer planting date10% chance of 32°F after this date
- Sep 21First fall frost (median)50% chance of 32°F by this date
- 124 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 19 | Jun 2 | Jun 18 |
| 32°F | May 6 | May 20 | Jun 2 |
| 28°F | Apr 24 | May 7 | May 21 |
| 24°F | Apr 9 | Apr 25 | May 10 |
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 24 | Sep 10 | Sep 23 |
| 32°F | Sep 5 | Sep 21 | Oct 4 |
| 28°F | Sep 18 | Oct 1 | Oct 15 |
| 24°F | Sep 26 | Oct 13 | Oct 25 |
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 | 76 days | 98 days | 117 days |
| 32°F | 101 days | 124 days | 142 days |
| 28°F | 127 days | 146 days | 165 days |
| 24°F | 150 days | 169 days | 190 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 |
|---|---|---|---|
| 59072 | 4b | -25 to -20°F | 10 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 USC00247214, at 46.4381, -108.5319, elevation 975 m (3,200 ft).
- Data quality: Standard (24+ years of data), 25 years of freeze data.
- Average days per year at or below 32°F: 163.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Roundup 15 SW (20 km)
- Flatwillow 4 ENE (44 km)
- Pompeys Pillar 15n (49 km)
- Melstone (54 km)
- Ryegate (58 km)
- Huntley Exp. Station (61 km)
- Ryegate 18 NNW (63 km)
- Grass Range (69 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.