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 stations	Median station	75% of stations
Last frost in spring (32°F)	May 11	May 15	May 20
First frost in fall (32°F)	September 22	September 26	September 29
Frost-free days	123	133	139

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
Station	Zone	Last spring frost (50%)	First fall frost (50%)	Frost-free days
Aberdeen, SD	4a	May 9	September 26	139
Argyle, MN	4a	May 15	September 26	134
Bergland Dam, MI	4a	May 26	September 27	123
Bismarck, ND	4a	May 14	September 27	136
Brainerd, MN	4a	May 8	September 29	141
Bridgewater, ME	4a	May 25	September 18	115
Britton, SD	4a	May 13	September 26	135
Brockway, MT	4a	May 20	September 19	121
Bruno, MN	4a	May 18	September 26	129
Busby, MT	4a	May 23	September 15	113
Cavalier, ND	4a	May 12	September 30	139
Circle, MT	4a	May 17	September 22	126
Cloquet, MN	4a	May 26	September 23	118
Cohagen, MT	4a	May 25	September 14	110
Colebrook, NH	4a	May 31	September 21	112
Columbia, SD	4a	May 7	September 29	145
Conrad, MT	4a	May 20	September 19	119
Crosby, ND	4a	May 17	September 23	128
Cut Bank, MT	4a	May 24	September 15	113
Eveleth, MN	4a	May 12	October 1	141
First Connecticut Lake, NH	4a	May 25	September 24	121
Forman, ND	4a	May 7	October 1	146
Fort Assiniboine, MT	4a	May 15	September 24	130
Fort Kent, ME	4a	May 23	September 24	121
Fort Peck Power Plant, MT	4a	May 8	October 1	145
Fullerton, ND	4a	May 9	September 28	141
Garrison, ND	4a	May 12	September 30	140
Georgetown, MN	4a	May 9	October 1	143
Glasgow, MT	4a	May 6	September 30	145
Glasgow #2, MT	4a	May 3	October 1	151
Glendive, MT	4a	May 3	October 4	153
Gordon, WI	4a	May 22	September 26	126
Grand Forks (Airport), ND	4a	May 16	September 27	133
Grantsburg, WI	4a	May 11	October 1	141
Grassy Butte, ND	4a	May 18	September 23	125
Harvey, ND	4a	May 12	September 26	135
Havre, MT	4a	May 15	September 23	129
Hazelton, ND	4a	May 12	September 27	136
Heart Butte Dam, ND	4a	May 14	September 29	136
Hebron, ND	4a	May 19	September 21	123
Hysham 25 SSE, MT	4a	May 26	September 16	111
Ingomar, MT	4a	May 21	September 19	120
Island Pond, VT	4a	May 19	September 29	129
Keene, ND	4a	May 20	September 20	121
Kenmare, ND	4a	May 15	September 23	129
Kremmling, CO	4a	June 14	September 5	80
Lake Placid, NY	4a	May 27	September 22	118
Lancaster, NH	4a	May 18	September 30	132
Long Prairie, MN	4a	May 13	September 27	134
Malta, MT	4a	May 16	September 23	128
Mandan, ND	4a	May 13	September 27	136
Max, ND	4a	May 10	September 28	141
Maybell, CO	4a	June 16	September 9	83
McHenry, ND	4a	May 12	September 28	139
McLeod, ND	4a	May 6	September 29	145
Medora, ND	4a	May 19	September 20	123
Mellette, SD	4a	May 11	September 27	137
Mildred, MT	4a	May 20	September 22	124
Millegan, MT	4a	June 11	September 6	86
Minot (Airport), ND	4a	May 10	October 1	144
Mizpah, MT	4a	May 22	September 15	117
Moose Lake, MN	4a	May 12	October 2	144
Mora, MN	4a	May 9	September 30	142
Napoleon, ND	4a	May 13	September 28	136
New England, ND	4a	May 16	September 24	128
New Salem, ND	4a	May 18	September 24	127
Petersburg, ND	4a	May 11	September 28	138
Plevna, MT	4a	May 21	September 20	121
Pollock, SD	4a	May 8	September 30	143
Powderville, MT	4a	May 13	September 25	132
Pretty Rock, ND	4a	May 20	September 23	124
Redbird, WY	4a	May 16	September 24	130
Ridgeway, MT	4a	May 17	September 22	127
Rothsay, MN	4a	May 5	October 3	149
Savage, MT	4a	May 12	September 25	135
Sidney, MT	4a	May 11	September 27	137
Spooner, WI	4a	May 14	September 28	135
St. Croix Falls, WI	4a	May 4	October 8	156
Stambaugh, MI	4a	June 4	September 10	95
Steele, ND	4a	May 15	September 25	133
Streeter, ND	4a	May 13	September 30	139
Summit, SD	4a	May 14	September 28	136
Sun River, MT	4a	June 2	September 10	96
Tensleep 16 SSE, WY	4a	May 22	September 18	117
Terry, MT	4a	May 12	September 28	137
Turner, MT	4a	May 19	September 21	124
Turtle Lake, ND	4a	May 14	September 26	136
Valentine, MT	4a	May 18	September 21	124
Vida, MT	4a	May 16	September 22	129
Wild River State Park, MN	4a	May 19	September 25	128
Williston (Sloulin Airport), ND	4a	May 21	September 21	123
Winter, WI	4a	May 19	September 27	129
Wolf Ridge Elc, MN	4a	May 15	October 1	137
Wright, MN	4a	May 31	September 18	107
near Butte, MT	4a	June 13	September 5	83
near Casselton, ND	4a	May 9	October 1	143
near Crookston, MN	4a	May 11	September 27	138
near Crosslake, MN	4a	May 6	October 7	152
near East Gull Lake, MN	4a	May 2	October 6	156
near Fargo, ND	4a	May 10	September 30	143
near Granby, CO	4a	June 13	September 7	84
near Grand Forks, ND	4a	May 10	October 1	144
near Grand Rapids, MN	4a	May 17	September 24	130
near Minot, ND	4a	May 13	September 27	136
near Williston, ND	4a	May 13	September 26	135

Sources

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