Casselman, Ontario climate data is summarized below. Average annual temperatures range from 1.6 °C (low) to 12.5 °C (high). Annual precipitation averages 529 Millimeters. Casselman 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 | -14.02 °C | -4.44 °C | 0 mm |
| February | -13.74 °C | -3.62 °C | 0 mm |
| March | -7.38 °C | 3.21 °C | 17 mm |
| April | 0.99 °C | 12.73 °C | 47 mm |
| May | 7.7 °C | 20.66 °C | 54 mm |
| June | 13.06 °C | 25.1 °C | 76 mm |
| July | 15.55 °C | 27.74 °C | 102 mm |
| August | 14.62 °C | 26.73 °C | 84 mm |
| September | 9.68 °C | 22.13 °C | 65 mm |
| October | 4.13 °C | 14.25 °C | 52 mm |
| November | -2.1 °C | 6.88 °C | 25 mm |
| December | -9.08 °C | -1.53 °C | 7 mm |
| Annual | 1.6 °C | 12.5 °C | 529 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 |
| Current Drought Conditions | Normal |