Hardiness zone 3b · 30 stations
Zone 3b Frost Dates: Typical Last Frost May 20
Summary across 30 NOAA weather stations whose location falls in zone 3b*. Dates are the typical (50% chance) frost dates of each station; the table shows the spread between stations.
- Typical last frostMay 20median station, 50% chance
- Typical first fall frostSeptember 21median station, 50% chance
- Frost-free season124 daysmedian station
- Weather stations30dates differ by weeks inside a zone
| 25% of stations | Median station | 75% of stations | |
|---|---|---|---|
| Last frost in spring (32°F) | May 15 | May 20 | May 25 |
| First frost in fall (32°F) | September 16 | September 21 | September 25 |
| Frost-free days | 112 | 124 | 131 |
Frost dates inside one zone differ by weeks: the zone describes winter cold, not the length of the season. Use the station closest to you.
Stations in zone 3b
| Station | Zone | Last spring frost (50%) | First fall frost (50%) | Frost-free days |
|---|---|---|---|---|
| Bottineau, ND | 3b | May 15 | September 25 | 133 |
| Bredette, MT | 3b | May 20 | September 21 | 122 |
| Cass Lake, MN | 3b | May 17 | September 26 | 131 |
| Chinook, MT | 3b | May 14 | September 23 | 130 |
| Content, MT | 3b | May 25 | September 14 | 110 |
| Culbertson, MT | 3b | May 21 | September 20 | 120 |
| Drake, ND | 3b | May 12 | September 27 | 137 |
| Floodwood, MN | 3b | May 29 | September 18 | 111 |
| Fortuna, ND | 3b | May 19 | September 17 | 121 |
| Gildford, MT | 3b | May 24 | September 15 | 114 |
| Itasca University of Minn, MN | 3b | May 18 | September 27 | 130 |
| Leech Lake, MN | 3b | May 8 | October 1 | 144 |
| Loma, MT | 3b | May 20 | September 19 | 120 |
| Opheim, MT | 3b | June 5 | September 2 | 89 |
| Park Rapids, MN | 3b | May 14 | September 25 | 133 |
| Pembina, ND | 3b | May 16 | September 29 | 137 |
| Red Lake Falls, MN | 3b | May 9 | September 30 | 142 |
| Rugby, ND | 3b | May 17 | September 23 | 129 |
| Saco, MT | 3b | May 15 | September 23 | 130 |
| Scobey, MT | 3b | May 28 | September 6 | 99 |
| Simpson 6 N - Wildhorse, MT | 3b | May 26 | September 15 | 110 |
| Taylor Park, CO | 3b | June 20 | September 8 | 80 |
| Thorhult, MN | 3b | May 24 | September 20 | 120 |
| Towner, ND | 3b | May 17 | September 22 | 126 |
| Upham, ND | 3b | May 15 | September 25 | 132 |
| Van Buren, ME | 3b | May 23 | September 26 | 125 |
| Westby, MT | 3b | May 18 | September 21 | 125 |
| Willow City, ND | 3b | May 21 | September 20 | 119 |
| near Hibbing, MN | 3b | June 5 | September 8 | 93 |
| near International Falls, MN | 3b | May 29 | September 13 | 109 |
Sources
- NOAA NCEI U.S. Climate Normals 1991–2020
- PRISM Group, Oregon State University — 2023 USDA Plant Hardiness Zone GIS datasets
* zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.