U.S. Frost Dates by Weather Station

Hardiness zone 4a · 106 stations

Zone 4a Frost Dates: Typical Last Frost May 15

Summary across 106 NOAA weather stations whose location falls in zone 4a*. Dates are the typical (50% chance) frost dates of each station; the table shows the spread between stations.

  • Typical last frostMay 15median station, 50% chance
  • Typical first fall frostSeptember 26median station, 50% chance
  • Frost-free season133 daysmedian station
  • Weather stations106dates differ by weeks inside a zone
Typical (50%) frost dates across stations in zone 4a
25% of stationsMedian station75% of stations
Last frost in spring (32°F)May 11May 15May 20
First frost in fall (32°F)September 22September 26September 29
Frost-free days123133139

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 4a

Weather stations with their typical (50% chance) frost dates
StationZoneLast spring frost (50%)First fall frost (50%)Frost-free days
Aberdeen, SD4aMay 9September 26139
Argyle, MN4aMay 15September 26134
Bergland Dam, MI4aMay 26September 27123
Bismarck, ND4aMay 14September 27136
Brainerd, MN4aMay 8September 29141
Bridgewater, ME4aMay 25September 18115
Britton, SD4aMay 13September 26135
Brockway, MT4aMay 20September 19121
Bruno, MN4aMay 18September 26129
Busby, MT4aMay 23September 15113
Cavalier, ND4aMay 12September 30139
Circle, MT4aMay 17September 22126
Cloquet, MN4aMay 26September 23118
Cohagen, MT4aMay 25September 14110
Colebrook, NH4aMay 31September 21112
Columbia, SD4aMay 7September 29145
Conrad, MT4aMay 20September 19119
Crosby, ND4aMay 17September 23128
Cut Bank, MT4aMay 24September 15113
Eveleth, MN4aMay 12October 1141
First Connecticut Lake, NH4aMay 25September 24121
Forman, ND4aMay 7October 1146
Fort Assiniboine, MT4aMay 15September 24130
Fort Kent, ME4aMay 23September 24121
Fort Peck Power Plant, MT4aMay 8October 1145
Fullerton, ND4aMay 9September 28141
Garrison, ND4aMay 12September 30140
Georgetown, MN4aMay 9October 1143
Glasgow, MT4aMay 6September 30145
Glasgow #2, MT4aMay 3October 1151
Glendive, MT4aMay 3October 4153
Gordon, WI4aMay 22September 26126
Grand Forks (Airport), ND4aMay 16September 27133
Grantsburg, WI4aMay 11October 1141
Grassy Butte, ND4aMay 18September 23125
Harvey, ND4aMay 12September 26135
Havre, MT4aMay 15September 23129
Hazelton, ND4aMay 12September 27136
Heart Butte Dam, ND4aMay 14September 29136
Hebron, ND4aMay 19September 21123
Hysham 25 SSE, MT4aMay 26September 16111
Ingomar, MT4aMay 21September 19120
Island Pond, VT4aMay 19September 29129
Keene, ND4aMay 20September 20121
Kenmare, ND4aMay 15September 23129
Kremmling, CO4aJune 14September 580
Lake Placid, NY4aMay 27September 22118
Lancaster, NH4aMay 18September 30132
Long Prairie, MN4aMay 13September 27134
Malta, MT4aMay 16September 23128
Mandan, ND4aMay 13September 27136
Max, ND4aMay 10September 28141
Maybell, CO4aJune 16September 983
McHenry, ND4aMay 12September 28139
McLeod, ND4aMay 6September 29145
Medora, ND4aMay 19September 20123
Mellette, SD4aMay 11September 27137
Mildred, MT4aMay 20September 22124
Millegan, MT4aJune 11September 686
Minot (Airport), ND4aMay 10October 1144
Mizpah, MT4aMay 22September 15117
Moose Lake, MN4aMay 12October 2144
Mora, MN4aMay 9September 30142
Napoleon, ND4aMay 13September 28136
New England, ND4aMay 16September 24128
New Salem, ND4aMay 18September 24127
Petersburg, ND4aMay 11September 28138
Plevna, MT4aMay 21September 20121
Pollock, SD4aMay 8September 30143
Powderville, MT4aMay 13September 25132
Pretty Rock, ND4aMay 20September 23124
Redbird, WY4aMay 16September 24130
Ridgeway, MT4aMay 17September 22127
Rothsay, MN4aMay 5October 3149
Savage, MT4aMay 12September 25135
Sidney, MT4aMay 11September 27137
Spooner, WI4aMay 14September 28135
St. Croix Falls, WI4aMay 4October 8156
Stambaugh, MI4aJune 4September 1095
Steele, ND4aMay 15September 25133
Streeter, ND4aMay 13September 30139
Summit, SD4aMay 14September 28136
Sun River, MT4aJune 2September 1096
Tensleep 16 SSE, WY4aMay 22September 18117
Terry, MT4aMay 12September 28137
Turner, MT4aMay 19September 21124
Turtle Lake, ND4aMay 14September 26136
Valentine, MT4aMay 18September 21124
Vida, MT4aMay 16September 22129
Wild River State Park, MN4aMay 19September 25128
Williston (Sloulin Airport), ND4aMay 21September 21123
Winter, WI4aMay 19September 27129
Wolf Ridge Elc, MN4aMay 15October 1137
Wright, MN4aMay 31September 18107
near Butte, MT4aJune 13September 583
near Casselton, ND4aMay 9October 1143
near Crookston, MN4aMay 11September 27138
near Crosslake, MN4aMay 6October 7152
near East Gull Lake, MN4aMay 2October 6156
near Fargo, ND4aMay 10September 30143
near Granby, CO4aJune 13September 784
near Grand Forks, ND4aMay 10October 1144
near Grand Rapids, MN4aMay 17September 24130
near Minot, ND4aMay 13September 27136
near Williston, ND4aMay 13September 26135

Sources

* zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.