Frost dates in USDA zone 5a (-20 to -15°F)
USDA zone 5a covers places whose coldest winter night averages -20 to -15°F. Across the 577 NOAA weather stations in zone 5a, the median last spring frost is May 9, but it ranges from Apr 28 to May 29 for most stations. Enter your ZIP for your local date.
- May 9Median last spring frostmost stations Apr 28–May 29
- Oct 2Median first fall frost50% chance of 32°F by this date
- 147 daysMedian growing seasonbetween 32°F freezes
- 2,426ZIP codes in zone 5a-20 to -15°F average coldest night
- 577Stations with frost datesof 602 NOAA stations matched
Earliest and latest last-frost stations in zone 5a
| Station | Last spring frost | First fall frost |
|---|---|---|
| La Crescent Dam 7, MN | Apr 15 | Oct 28 |
| Dubuque L&d 11, IA | Apr 19 | Oct 21 |
| Guttenberg L&d 10, IA | Apr 21 | Oct 18 |
| Skagway Airport, AK | Apr 21 | Oct 17 |
| Tipton, IA | Apr 21 | Oct 15 |
| Winona, MN | Apr 21 | Oct 19 |
| Genoa Dam 8, WI | Apr 22 | Oct 17 |
| Steuben 4 SE, WI | Apr 22 | Oct 20 |
| Hastings Dam 2, MN | Apr 23 | Oct 19 |
| La Crosse Muni Airport, WI | Apr 23 | Oct 17 |
| Station | Last spring frost | First fall frost |
|---|---|---|
| Taylor Park, CO | Jun 20 | Sep 8 |
| Afton, WY | Jun 20 | Sep 1 |
| Bern, ID | Jun 17 | Sep 3 |
| Twin Lakes Rsvr, CO | Jun 16 | Sep 10 |
| Hill City 1 W, ID | Jun 16 | Sep 6 |
| Ennis, MT | Jun 16 | Sep 4 |
| Tetonia Exp. Station, ID | Jun 15 | Sep 7 |
| Yampa, CO | Jun 14 | Sep 11 |
| Kremmling, CO | Jun 14 | Sep 5 |
| Chilly Barton Flat, ID | Jun 14 | Sep 8 |
Zone 5a by state
| State | Stations | Median last frost | Median first frost |
|---|---|---|---|
| Alaska | 13 | May 16 | Sep 22 |
| Colorado | 32 | Jun 2 | Sep 19 |
| Idaho | 21 | May 29 | Sep 14 |
| Illinois | 5 | Apr 27 | Oct 9 |
| Iowa | 87 | Apr 30 | Oct 7 |
| Maine | 26 | May 9 | Oct 1 |
| Michigan | 45 | May 22 | Oct 1 |
| Minnesota | 38 | Apr 30 | Oct 8 |
| Montana | 46 | May 23 | Sep 19 |
| Nebraska | 47 | May 7 | Oct 2 |
| Nevada | 3 | Jun 9 | Sep 12 |
| New Hampshire | 13 | May 14 | Sep 29 |
| New York | 17 | May 5 | Oct 4 |
| South Dakota | 59 | May 6 | Oct 3 |
| Utah | 3 | May 28 | Sep 26 |
| Vermont | 25 | May 13 | Oct 1 |
| Wisconsin | 56 | May 2 | Oct 7 |
| Wyoming | 66 | May 19 | Sep 24 |
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.