Frost dates for Midwest, WY
At Midwest, the median last spring frost (32°F) is May 22 and the median first fall frost is Sep 22, for a typical growing season of 121 days. For a 90% chance that the last frost has passed, wait until Jun 6. Nearby ZIP codes are in USDA hardiness zone 5a.
- May 22Last spring frost (median)50% chance of 32°F after this date
- Jun 6Safer planting date10% chance of 32°F after this date
- Sep 22First fall frost (median)50% chance of 32°F by this date
- 121 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 22 | Jun 5 | Jun 23 |
| 32°F | May 8 | May 22 | Jun 6 |
| 28°F | Apr 28 | May 9 | May 24 |
| 24°F | Apr 14 | Apr 30 | May 12 |
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 29 | Sep 12 | Sep 24 |
| 32°F | Sep 8 | Sep 22 | Oct 5 |
| 28°F | Sep 17 | Oct 2 | Oct 17 |
| 24°F | Sep 27 | Oct 13 | Oct 26 |
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 | 72 days | 95 days | 116 days |
| 32°F | 100 days | 121 days | 140 days |
| 28°F | 122 days | 143 days | 165 days |
| 24°F | 146 days | 165 days | 187 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 |
|---|---|---|---|
| 82643 | 5a | -20 to -15°F | 1 km |
| 82635 | 5a | -20 to -15°F | 5 km |
| 82640 | 5a | -20 to -15°F | 21 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 USC00486195, at 43.4131, -106.2772, elevation 1481 m (4,860 ft).
- Data quality: Standard (24+ years of data), 26 years of freeze data.
- Average days per year at or below 32°F: 187.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Kaycee (44 km)
- Wright 12w (57 km)
- Casper Natrona Co Airport (59 km)
- Casper Wastewater Plant (61 km)
- Casper 1sw (62 km)
- Powder River #2 (71 km)
- Powder River (school) (71 km)
- Billy Creek (87 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.