Frost dates in USDA zone 3b (-35 to -30°F)
USDA zone 3b covers places whose coldest winter night averages -35 to -30°F. Across the 73 NOAA weather stations in zone 3b, the median last spring frost is May 19, but it ranges from May 14 to May 31 for most stations. Enter your ZIP for your local date.
- May 19Median last spring frostmost stations May 14–May 31
- Sep 22Median first fall frost50% chance of 32°F by this date
- 125 daysMedian growing seasonbetween 32°F freezes
- 230ZIP codes in zone 3b-35 to -30°F average coldest night
- 73Stations with frost datesof 75 NOAA stations matched
Earliest and latest last-frost stations in zone 3b
| Station | Last spring frost | First fall frost |
|---|---|---|
| Agassiz Refuge, MN | May 6 | Oct 3 |
| Hallock, MN | May 8 | Sep 30 |
| Leech Lake, MN | May 8 | Oct 1 |
| Red Lake Falls, MN | May 9 | Sep 30 |
| Cook 8NE, MN | May 12 | Oct 1 |
| Eveleth Wastewater Plant, MN | May 12 | Oct 1 |
| Red Lake Indian Agcy, MN | May 12 | Oct 4 |
| Baudette, MN | May 14 | Sep 26 |
| Chinook, MT | May 14 | Sep 23 |
| Ely, MN | May 14 | Sep 29 |
| Station | Last spring frost | First fall frost |
|---|---|---|
| McKinley Park, AK | Jun 6 | Aug 25 |
| Opheim 12 SSE, MT | Jun 5 | Sep 2 |
| Hibbing Chisholm Hibbing Airport, MN | Jun 5 | Sep 8 |
| Delta Junction 20 SE, AK | Jun 5 | Aug 21 |
| Orr 3E, MN | Jun 2 | Sep 15 |
| Opheim 10 N, MT | Jun 1 | Sep 7 |
| Celina 2E, MN | Jun 1 | Sep 15 |
| Isabella 14W, MN | May 31 | Sep 15 |
| Two Rivers, AK | May 30 | Sep 2 |
| Clayton Lake, ME | May 30 | Sep 13 |
Zone 3b by state
| State | Stations | Median last frost | Median first frost |
|---|---|---|---|
| Alaska | 8 | May 27 | Sep 1 |
| Maine | 1 | May 30 | Sep 13 |
| Minnesota | 37 | May 18 | Sep 26 |
| Montana | 12 | May 18 | Sep 19 |
| North Dakota | 17 | May 17 | Sep 23 |
Neighboring zones
- Zone 3a (-40 to -35°F): too few stations for a median
- Zone 4a (-30 to -25°F): median last frost May 14
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.