Frost dates in USDA zone 4b (-25 to -20°F)
USDA zone 4b covers places whose coldest winter night averages -25 to -20°F. Across the 354 NOAA weather stations in zone 4b, the median last spring frost is May 11, but it ranges from May 3 to May 31 for most stations. Enter your ZIP for your local date.
- May 11Median last spring frostmost stations May 3–May 31
- Sep 30Median first fall frost50% chance of 32°F by this date
- 140 daysMedian growing seasonbetween 32°F freezes
- 1,284ZIP codes in zone 4b-25 to -20°F average coldest night
- 354Stations with frost datesof 376 NOAA stations matched
Earliest and latest last-frost stations in zone 4b
| Station | Last spring frost | First fall frost |
|---|---|---|
| Stillwater 1 SE, MN | Apr 22 | Oct 19 |
| Minnesota City Dam 5, MN | Apr 23 | Oct 16 |
| Red Wing Dam 3, MN | Apr 23 | Oct 14 |
| Eau Claire 3SW, WI | Apr 24 | Oct 14 |
| Gaylord, MN | Apr 24 | Oct 15 |
| Farmington 3 NW, MN | Apr 27 | Oct 10 |
| Alma Dam 4, WI | Apr 28 | Oct 14 |
| Basin, WY | Apr 28 | Oct 10 |
| Hutchinson 1 N, MN | Apr 28 | Oct 8 |
| Austin Waste Water Plant Facility, MN | Apr 30 | Oct 5 |
| Station | Last spring frost | First fall frost |
|---|---|---|
| Cochetopa Creek, CO | Jun 21 | Sep 5 |
| Moose, WY | Jun 20 | Aug 31 |
| Tizer Gardens, MT | Jun 18 | Sep 5 |
| Lewistown 42 WSW, MT | Jun 15 | Sep 4 |
| Hebgen Dam, MT | Jun 15 | Sep 10 |
| Monteview, ID | Jun 12 | Sep 7 |
| Gibson Dam, MT | Jun 12 | Sep 8 |
| Soldotna 5SSW, AK | Jun 11 | Sep 1 |
| Melville 4 W, MT | Jun 9 | Sep 7 |
| Elliston, MT | Jun 9 | Sep 8 |
Zone 4b by state
| State | Stations | Median last frost | Median first frost |
|---|---|---|---|
| Alaska | 12 | May 20 | Sep 16 |
| Colorado | 5 | Jun 10 | Sep 11 |
| Idaho | 1 | Jun 12 | Sep 7 |
| Iowa | 2 | May 8 | Oct 1 |
| Maine | 11 | May 23 | Sep 24 |
| Michigan | 13 | May 26 | Sep 25 |
| Minnesota | 57 | May 4 | Oct 4 |
| Montana | 74 | May 21 | Sep 21 |
| New Hampshire | 7 | May 21 | Sep 22 |
| New York | 13 | May 10 | Oct 1 |
| North Dakota | 15 | May 11 | Sep 28 |
| South Dakota | 75 | May 7 | Oct 1 |
| Vermont | 10 | May 14 | Sep 30 |
| Wisconsin | 55 | May 9 | Oct 2 |
| Wyoming | 26 | May 21 | Sep 23 |
Neighboring zones
How these numbers are made
Each ZIP code gets one zone from the USDA-ARS / PRISM 2023 ZIP tables, which use the map’s 1991–2020 average of the coldest winter night of each year. A ZIP code can span more than one zone, and USDA’s own lookup works by ZIP or address. Each NOAA station is assigned the most common zone among the closest ZIP codes listed on its station page (a tie goes to the zone of the closest of those ZIPs). Frost dates are NOAA’s 50%-probability (median) 32°F dates for each station, the same values shown on the station pages. Zone figures are the median, 10th and 90th percentile across stations; a first fall frost in January to June is counted in the following year.
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.
Sources
- 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.
- 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.
- 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.