Janesville, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 37.3 °F (low) to 57.3 °F (high). Annual precipitation averages 37.2 Inches. Janesville is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 8.9 °F | 26.1 °F | 1.02 in. |
| February | 13.2 °F | 30.5 °F | 1.12 in. |
| March | 24.6 °F | 43.8 °F | 1.97 in. |
| April | 36.5 °F | 58.5 °F | 3.95 in. |
| May | 49.0 °F | 70.4 °F | 4.9 in. |
| June | 59.8 °F | 80.2 °F | 5.78 in. |
| July | 62.9 °F | 83.1 °F | 4.53 in. |
| August | 60.0 °F | 81.1 °F | 4.11 in. |
| September | 51.2 °F | 75.2 °F | 3.5 in. |
| October | 38.8 °F | 61.7 °F | 2.84 in. |
| November | 26.5 °F | 45.4 °F | 1.89 in. |
| December | 15.9 °F | 32.0 °F | 1.54 in. |
| Annual | 37.3 °F | 57.3 °F | 37.15 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 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 | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |