Northern Rockies

ZONE 1. Coldest Winters in the West and Western Prairie States
Growing season: early June through Aug., but with some variation―the longest seasons are usually found near this zone's large bodies of water. Frost can come any night of the year. Winters are snowy and intensely cold, due to latitude, elevation, and/or influence of continental air mass. There's some summer rainfall.

ZONE 2. Second-coldest Western Climate
Growing season: early May through Sept. Winters are cold (lows run from -3 degrees to -34 degrees F/-19 degrees to -37 degrees C), but less so than in Zone 1. In northern and interior areas, lower elevations fall into Zone 2, higher areas into Zone 1.

ZONE 3. West's Mildest High-elevation and Interior Regions
Growing season: early May to late Sept.―shorter than in Zone 2, but offset by milder winters (lows from 13 degrees to -24 degrees F/-11 degrees to -31 degrees C). This is fine territory for plants needing winter chill and dry, hot summers.