Frost dates for Harlowton 11nw, MT
At Harlowton 11nw, the median last spring frost (32°F) is May 30 and the median first fall frost is Sep 20, for a typical growing season of 110 days. For a 90% chance that the last frost has passed, wait until Jun 17. Nearby ZIP codes are in USDA hardiness zone 4b.
- May 30Last spring frost (median)50% chance of 32°F after this date
- Jun 17Safer planting date10% chance of 32°F after this date
- Sep 20First fall frost (median)50% chance of 32°F by this date
- 110 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 31 | Jun 16 | Jul 3 |
| 32°F | May 16 | May 30 | Jun 17 |
| 28°F | May 3 | May 15 | May 29 |
| 24°F | Apr 19 | May 3 | May 16 |
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 8 | Sep 23 |
| 32°F | Sep 4 | Sep 20 | Oct 3 |
| 28°F | Sep 15 | Oct 1 | Oct 13 |
| 24°F | Sep 24 | Oct 9 | Oct 22 |
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 | 58 days | 82 days | 102 days |
| 32°F | 88 days | 110 days | 131 days |
| 28°F | 115 days | 136 days | 155 days |
| 24°F | 137 days | 157 days | 176 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 |
|---|---|---|---|
| 59036 | 4b | -25 to -20°F | 14 km |
| 59453 | 4b | -25 to -20°F | 22 km |
| 59418 | 4b | -25 to -20°F | 26 km |
| 59085 | 4b | -25 to -20°F | 34 km |
| 59452 | 4b | -25 to -20°F | 36 km |
| 59464 | 4b | -25 to -20°F | 38 km |
| 59078 | 4b | -25 to -20°F | 39 km |
| 59053 | 4b | -25 to -20°F | 40 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 USC00243947, at 46.5786, -109.9478, elevation 1510 m (4,954 ft).
- Data quality: Estimated from nearby stations (2+ years), 9 years of freeze data.
- Average days per year at or below 32°F: 190.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Harlowton (18 km)
- Judith Gap (19 km)
- Martinsdale 3 NNW (31 km)
- Judith Gap 13 E (36 km)
- Lewistown 42 WSW (43 km)
- Ryegate 18 NNW (46 km)
- Moccasin Exp. Station (53 km)
- Melville 4 W (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.