Marathon, Ontario climate data is summarized below. Average annual temperatures range from 28 °F (low) to 43.5 °F (high). Annual precipitation averages 21.8 Inches. Marathon is located in USDA Plant Hardiness Zone 3b. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -28.5 °F | -22.6 °F | 0 in. |
| February | -28.5 °F | -22 °F | 0 in. |
| March | -24.2 °F | -18.1 °F | 0 in. |
| April | -20 °F | -14.9 °F | 736.6 in. |
| May | -16 °F | -11.1 °F | 2006.6 in. |
| June | -13.3 °F | -8.6 °F | 2286 in. |
| July | -11.8 °F | -7.8 °F | 2184.4 in. |
| August | -11 °F | -7 °F | 1879.6 in. |
| September | -12.9 °F | -8.9 °F | 2082.8 in. |
| October | -16 °F | -12.5 °F | 1930.4 in. |
| November | -19.9 °F | -16.3 °F | 965.2 in. |
| December | -26.3 °F | -21 °F | 0 in. |
| Annual | 28 °F | 43.5 °F | 21.81 in. |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 3b : -35°F to -30°F |
| Current Drought Conditions | Normal |