Washington, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.1 °F (low) to 60.6 °F (high). Annual precipitation averages 37.9 Inches. Washington is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 13.8 °F | 30.8 °F | 1.33 in. |
| February | 17.5 °F | 35.5 °F | 1.55 in. |
| March | 28.0 °F | 48.4 °F | 2.3 in. |
| April | 39.1 °F | 62.3 °F | 3.64 in. |
| May | 51.2 °F | 72.8 °F | 5.14 in. |
| June | 61.3 °F | 81.9 °F | 5.13 in. |
| July | 64.6 °F | 85.1 °F | 4.13 in. |
| August | 62.1 °F | 83.6 °F | 4.23 in. |
| September | 53.1 °F | 77.4 °F | 3.52 in. |
| October | 41.4 °F | 64.7 °F | 2.99 in. |
| November | 29.9 °F | 49.0 °F | 2.23 in. |
| December | 19.7 °F | 36.1 °F | 1.71 in. |
| Annual | 40.1 °F | 60.6 °F | 37.9 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5b : -15°F to -10°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |