Irwin, Idaho has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 31.4 °F (low) to 56 °F (high). Annual precipitation averages 19.7 Inches. Irwin is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around Jun. 1 - Jun. 10 through the first frost around Sep.1 - Sep. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 13.6 °F | 29.8 °F | 1.84 in. |
| February | 15.4 °F | 33.7 °F | 1.27 in. |
| March | 23.1 °F | 44.1 °F | 1.54 in. |
| April | 30.0 °F | 54.1 °F | 1.85 in. |
| May | 37.8 °F | 64.5 °F | 2.44 in. |
| June | 44.5 °F | 73.6 °F | 2.09 in. |
| July | 50.5 °F | 83.9 °F | 1.13 in. |
| August | 49.3 °F | 83.1 °F | 1.17 in. |
| September | 41.6 °F | 73.4 °F | 1.59 in. |
| October | 32.8 °F | 58.6 °F | 1.71 in. |
| November | 23.4 °F | 42.3 °F | 1.62 in. |
| December | 15.2 °F | 30.3 °F | 1.47 in. |
| Annual | 31.4 °F | 56 °F | 19.72 in. |
| Average First Frost Date | Sep.1 - Sep. 10 |
| Average Last Frost Date | Jun. 1 - Jun. 10 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Extreme Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |