Frost dates in USDA zone 1b (-55 to -50°F)
USDA zone 1b covers places whose coldest winter night averages -55 to -50°F. Only 5 NOAA weather stations are matched to ZIP codes in zone 1b, and 2 of them have frost dates: too few stations for a zone-wide median. Each station’s own dates are listed below. Missing dates here mean incomplete records, not a lack of frost. Enter your ZIP for your local date.
- 11ZIP codes in zone 1b-55 to -50°F average coldest night
- 2Stations with frost datesof 5 NOAA stations matched
Every NOAA station in zone 1b
| Station | Last spring frost | First fall frost |
|---|---|---|
| Bettles Airport, AK | May 22 | Aug 24 |
| Tanana Calhoun Mem Airport, AK | May 29 | Aug 28 |
| Chicken, AK | any month | any month |
| Manley Hot Springs 15NE, AK | any month | any month |
| Tok, AK | any month | any month |
Zone 1b by state
| State | Stations | With frost dates |
|---|---|---|
| Alaska | 5 | 2 |
Neighboring zones
- Zone 1a (-60 to -55°F): too few stations for a median
- Zone 2a (-50 to -45°F): too few stations for a median
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.