According to the 2023 USDA Hardiness Zone Map Rocky Boy's Agency, Montana is in Zones 4a (-30°F to -25°F) and 4b (-25°F to -20°F). There is no change from the 2012 USDA Hardiness Zone Map which has Rocky Boy's Agency in also in Zones 4a (-30°F to -25°F) and 4b (-25°F to -20°F).