Frost dates for Ryegate 18 NNW, MT
At Ryegate 18 NNW, the median last spring frost (32°F) is May 26 and the median first fall frost is Sep 18, for a typical growing season of 112 days. For a 90% chance that the last frost has passed, wait until Jun 13. Nearby ZIP codes are in USDA hardiness zone 4b.
- May 26Last spring frost (median)50% chance of 32°F after this date
- Jun 13Safer planting date10% chance of 32°F after this date
- Sep 18First fall frost (median)50% chance of 32°F by this date
- 112 daysGrowing season (median)between 32°F freezes
USDA hardiness zone 4b (-25 to -20°F) · see all zone 4b frost dates
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 13 | Jul 3 |
| 32°F | May 12 | May 26 | Jun 13 |
| 28°F | May 1 | May 11 | May 28 |
| 24°F | Apr 17 | Apr 30 | May 11 |
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 19 | Sep 6 | Sep 21 |
| 32°F | Sep 1 | Sep 18 | Oct 1 |
| 28°F | Sep 14 | Sep 29 | Oct 12 |
| 24°F | Sep 22 | Oct 7 | Oct 23 |
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 | 59 days | 83 days | 104 days |
| 32°F | 90 days | 112 days | 133 days |
| 28°F | 117 days | 139 days | 157 days |
| 24°F | 140 days | 159 days | 178 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 |
|---|---|---|---|
| 59078 | 4b | -25 to -20°F | 17 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 USC00247263, at 46.5333, -109.3442, elevation 1353 m (4,440 ft).
- Data quality: Standard (24+ years of data), 29 years of freeze data.
- Average days per year at or below 32°F: 200.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Judith Gap 13 E (13 km)
- Ryegate (27 km)
- Judith Gap (35 km)
- Harlowton (40 km)
- Lewistown 11 SSE (42 km)
- Harlowton 11NW (46 km)
- Roundup 15 SW (52 km)
- Lewistown 11ESE (53 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.