St. Marys, Ontario climate data is summarized below. Average annual temperatures range from 37.5 °F (low) to 55.1 °F (high). Annual precipitation averages 14.1 Inches. St. Marys is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -23 °F | -19 °F | 50.8 in. |
| February | -23.7 °F | -19.1 °F | 0 in. |
| March | -20.4 °F | -15.2 °F | 482.6 in. |
| April | -17 °F | -10.8 °F | 609.6 in. |
| May | -13.4 °F | -6.7 °F | 1016 in. |
| June | -10.6 °F | -4.2 °F | 1397 in. |
| July | -9.5 °F | -3.1 °F | 1879.6 in. |
| August | -9.8 °F | -3.5 °F | 1727.2 in. |
| September | -12 °F | -5.7 °F | 1016 in. |
| October | -14.9 °F | -9.6 °F | 508 in. |
| November | -18 °F | -13.7 °F | 304.8 in. |
| December | -20.6 °F | -17.2 °F | 127 in. |
| Annual | 37.5 °F | 55.1 °F | 14.13 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |