Frost dates in USDA zone 4a (-30 to -25°F)
USDA zone 4a covers places whose coldest winter night averages -30 to -25°F. Across the 244 NOAA weather stations in zone 4a, the median last spring frost is May 14, but it ranges from May 7 to May 25 for most stations. Enter your ZIP for your local date.
- May 14Median last spring frostmost stations May 7–May 25
- Sep 27Median first fall frost50% chance of 32°F by this date
- 134 daysMedian growing seasonbetween 32°F freezes
- 814ZIP codes in zone 4a-30 to -25°F average coldest night
- 244Stations with frost datesof 256 NOAA stations matched
Earliest and latest last-frost stations in zone 4a
| Station | Last spring frost | First fall frost |
|---|---|---|
| Gull Lake Dam, MN | May 2 | Oct 6 |
| Hillsboro 3 N, ND | May 2 | Oct 9 |
| Enderlin 2W, ND | May 3 | Oct 5 |
| Glasgow #2, MT | May 3 | Oct 1 |
| Glendive, MT | May 3 | Oct 4 |
| White Rock Dam, MN | May 3 | Oct 3 |
| Breckenridge 3E, MN | May 4 | Oct 3 |
| Detroit Lakes 12E, MN | May 4 | Oct 3 |
| Fargo 3S, ND | May 4 | Oct 2 |
| Fergus Falls, MN | May 4 | Oct 5 |
| Station | Last spring frost | First fall frost |
|---|---|---|
| Moose 1 NNE, WY | Jun 24 | Aug 28 |
| Randolph, UT | Jun 14 | Sep 1 |
| La Barge, WY | Jun 9 | Sep 4 |
| E Glacier, MT | Jun 8 | Sep 5 |
| Laramie 2 NW, WY | Jun 7 | Sep 4 |
| Amasa 1W, MI | Jun 6 | Sep 9 |
| Nome Muni Airport, AK | Jun 5 | Sep 3 |
| Stambaugh 2SSE, MI | Jun 4 | Sep 10 |
| Intricate Bay, AK | Jun 4 | Sep 12 |
| Sun River 4 S, MT | Jun 2 | Sep 10 |
Zone 4a by state
| State | Stations | Median last frost | Median first frost |
|---|---|---|---|
| Alaska | 7 | May 31 | Sep 12 |
| Colorado | 1 | n/a | n/a |
| Idaho | 1 | n/a | n/a |
| Maine | 6 | May 19 | Sep 25 |
| Michigan | 4 | Jun 4 | Sep 10 |
| Minnesota | 43 | May 10 | Sep 30 |
| Montana | 47 | May 19 | Sep 22 |
| New Hampshire | 3 | May 25 | Sep 24 |
| New York | 6 | May 19 | Sep 26 |
| North Dakota | 88 | May 12 | Sep 27 |
| South Dakota | 9 | May 9 | Sep 28 |
| Utah | 1 | Jun 14 | Sep 1 |
| Vermont | 3 | May 19 | Sep 28 |
| Wisconsin | 27 | May 19 | Sep 28 |
| Wyoming | 10 | Jun 7 | Sep 4 |
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.