Irmo, South Carolina has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 11.6 °C (low) to 23.8 °C (high). Annual precipitation averages 1216.9 Millimeters. Irmo is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Mar. 21 - Mar. 31 through the first frost around Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 1.4 °C | 13.3 °C | 101.6 mm |
| February | 2.7 °C | 15.3 °C | 94.74 mm |
| March | 6.2 °C | 19.5 °C | 107.7 mm |
| April | 10.8 °C | 24.2 °C | 77.22 mm |
| May | 15.8 °C | 28.2 °C | 83.06 mm |
| June | 20.2 °C | 31.6 °C | 122.43 mm |
| July | 21.9 °C | 33.3 °C | 107.19 mm |
| August | 21.5 °C | 32.4 °C | 124.97 mm |
| September | 18.3 °C | 29.7 °C | 107.95 mm |
| October | 11.8 °C | 24.4 °C | 96.27 mm |
| November | 5.8 °C | 18.8 °C | 89.66 mm |
| December | 2.7 °C | 14.6 °C | 104.14 mm |
| Annual | 11.6 °C | 23.8 °C | 1216.93 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 40-50 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°C |
| Plant Hardiness Zone - 2012 | Zone 8a : -12.2°C to -9.4°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |