St. Marys, Ontario climate data is summarized below. Average annual temperatures range from 3.1 °C (low) to 12.9 °C (high). Annual precipitation averages 359 Millimeters. 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 | -9.32 °C | -2.11 °C | 2 mm |
| February | -10.66 °C | -2.41 °C | 0 mm |
| March | -4.7 °C | 4.66 °C | 19 mm |
| April | 1.46 °C | 12.65 °C | 24 mm |
| May | 7.88 °C | 19.93 °C | 40 mm |
| June | 12.9 °C | 24.42 °C | 55 mm |
| July | 14.89 °C | 26.41 °C | 74 mm |
| August | 14.42 °C | 25.73 °C | 68 mm |
| September | 10.32 °C | 21.77 °C | 40 mm |
| October | 5.21 °C | 14.67 °C | 20 mm |
| November | -0.42 °C | 7.42 °C | 12 mm |
| December | -5.05 °C | 1.05 °C | 5 mm |
| Annual | 3.1 °C | 12.9 °C | 359 mm |
| 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 : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |