Autumn Foliage Forecast: How Weather Dictates Canada's Fall Colors

The Chemistry of Fall Colors

Canada's autumn foliage is world-famous, attracting millions of leaf-peepers to Ontario, Quebec, and the Maritimes. But the timing and intensity of this visual show are completely dependent on the weather. As days shorten, deciduous trees stop producing green chlorophyll, revealing yellow and orange carotenoid pigments that were present in the leaves all summer.

The Recipe for Brilliant Reds and Purples

While yellows are constant, the creation of deep red, purple, and crimson colors depends on a specific weather recipe: sunny, warm autumn days followed by cool, crisp nights (but remaining above freezing).

During the day, leaves produce lots of sugars. When the night turns cold, the veins in the leaf constrict, trapping the sugars. This high concentration of sugars triggers the production of anthocyanin pigments, which create the brilliant red tones typical of the Canadian sugar maple. If the autumn is too rainy, overcast, or hit by an early hard freeze, the colors will be duller and leaves will drop prematurely.