Montana frost dates: B
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Babb | Jun 3 | Sep 8 | 97 d | 4b |
| Bainville | May 21 | Sep 20 | 122 d | 4a |
| Baker | May 16 | Sep 26 | 133 d | 4b |
| Ballantine | May 12 | Sep 25 | 136 d | 4b |
| Basin | Jun 7 | Sep 8 | 93 d | 5a |
| Bearcreek | Jun 3 | Sep 19 | 108 d | 5a |
| Belfry | May 18 | Sep 22 | 127 d | 5a |
| Belgrade | May 26 | Sep 16 | 113 d | 5a |
| Belt | May 22 | Sep 23 | 124 d | 4b |
| Biddle | May 14 | Sep 28 | 137 d | 4b |
| Big Arm | May 15 | Sep 28 | 136 d | 6a |
| Big Sandy | May 17 | Sep 25 | 131 d | 4a |
| Big Sky | frost any month | 4b | ||
| Big Timber | May 14 | Sep 26 | 135 d | 5a |
| Bigfork | May 12 | Sep 25 | 136 d | 5b |
| Bighorn | May 26 | Sep 16 | 113 d | 4b |
| Billings | May 4 | Oct 7 | 156 d | 5a |
| Birney | May 23 | Sep 15 | 115 d | 4b |
| Black Eagle | May 6 | Oct 1 | 148 d | 4b |
| Bloomfield | May 3 | Oct 4 | 154 d | 4a |
| Bonner | May 10 | Oct 1 | 144 d | 5b |
| Boulder | Jun 7 | Sep 8 | 93 d | 5a |
| Box Elder | May 15 | Sep 24 | 132 d | 4a |
| Boyd | May 29 | Sep 15 | 109 d | 5a |
| Boyes | May 16 | Sep 28 | 135 d | 4b |
| Bozeman | May 26 | Sep 22 | 119 d | 5a |
| Brady | May 20 | Sep 19 | 122 d | 4a |
| Bridger | May 14 | Sep 24 | 133 d | 5a |
| Broadus | May 16 | Sep 28 | 135 d | 4b |
| Broadview | May 22 | Sep 15 | 116 d | 4b |
| Brockton | May 24 | Sep 14 | 113 d | 4a |
| Brockway | May 20 | Sep 19 | 122 d | 4a |
| Browning | Jun 8 | Sep 5 | 89 d | 4a |
| Brusett | May 18 | Sep 20 | 125 d | 4b |
| Buffalo | Jun 1 | Sep 15 | 106 d | 4b |
| Busby | May 23 | Sep 15 | 115 d | 5a |
| Butte | Jun 13 | Sep 5 | 84 d | 4a |
| Bynum | May 16 | Sep 27 | 134 d | 4b |