Waltham, Minnesota has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 1.9 °C (low) to 12 °C (high). Annual precipitation averages 906.8 Millimeters. Waltham 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 | -14.2 °C | -5.5 °C | 25.4 mm |
| February | -12.2 °C | -3.1 °C | 25.65 mm |
| March | -5.4 °C | 3.8 °C | 47.5 mm |
| April | 1.5 °C | 12.4 °C | 90.17 mm |
| May | 8.4 °C | 19.4 °C | 118.62 mm |
| June | 14.4 °C | 25 °C | 141.73 mm |
| July | 16.2 °C | 26.7 °C | 117.6 mm |
| August | 14.7 °C | 25.5 °C | 106.68 mm |
| September | 10.1 °C | 22.1 °C | 95.5 mm |
| October | 3.1 °C | 14.4 °C | 67.06 mm |
| November | -3.7 °C | 5.4 °C | 38.61 mm |
| December | -10.3 °C | -2.3 °C | 32.26 mm |
| Annual | 1.9 °C | 12 °C | 906.78 mm |
| 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 : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 4b : -31.7°C to -28.9°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |