Why Leaves Change Colour

Tree with red leaves

The switch from summer to fall can be bittersweet. But two things that help ease the transition are the return of pumpkin spice and amazing fall tree colour. We witness this phenomenon every year, but do you know why the leaves change? It’s all in the chemistry.

During the summer, plants transform sunlight, water and carbon dioxide into sugar they use for food. This process is known as photosynthesis.

Plants rely on a pigment called chlorophyll to transform light into food. Chlorophyll absorbs red and blue light during photosynthesis, but reflects green and white light, which is why the leaves look green.

As trees prepare to lose their leaves for winter, they slowly stop producing chlorophyll. Without chlorophyll to absorb red and blue light, the leaves change colour. What colour the leaves change depends on the chemical composition of the tree, as well as weather conditions. For example, lots of sunshine in September can produce more vibrant reds and purples in some trees like red and white oaks.

Of course, not all trees lose their green. The thin, waxy needles you see on conifers like white spruce allow photosynthesis to continue even when the snow starts to fall. One exception is the tamarack. Although it has thin needles like other conifers, they turn yellow in fall and eventually drop, leaving branches bare during winter.

Our watershed is renowned for its show of fall colour. Visit cvc.ca regularly to find out when the colours are at their finest. You can also bring fall colour home by planting a variety of trees and shrubs. Download our plant list to help you choose the plants best suited for your yard.

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