Garrison, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 27.2 °F (low) to 56 °F (high). Annual precipitation averages 10.6 Inches. Garrison is located in USDA Plant Hardiness Zone 4b. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Aug. 11 - Aug. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 11.3 °F | 32.7 °F | 0.5 in. |
| February | 13.2 °F | 36.4 °F | 0.46 in. |
| March | 20.2 °F | 45.8 °F | 0.43 in. |
| April | 26.5 °F | 54.6 °F | 0.84 in. |
| May | 33.8 °F | 64.0 °F | 1.72 in. |
| June | 40.5 °F | 71.3 °F | 1.91 in. |
| July | 45.1 °F | 82.8 °F | 1.04 in. |
| August | 43.1 °F | 82.1 °F | 0.99 in. |
| September | 35.8 °F | 71.7 °F | 0.98 in. |
| October | 27.0 °F | 56.9 °F | 0.7 in. |
| November | 18.2 °F | 41.3 °F | 0.52 in. |
| December | 11.2 °F | 31.8 °F | 0.49 in. |
| Annual | 27.2 °F | 56 °F | 10.58 in. |
| Average First Frost Date | Aug. 11 - Aug. 20 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 4b : -25°F to -20°F |
| Plant Hardiness Zone - 2012 | Zone 4a : -30°F to -25°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |