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Seasonal Spectrum: The Science of Changing Colours

Posted on October 18, 2015


Ever wonder why a chokecherry leaf turns dark purple, but an aspen turns bright sunny yellow? Or why some falls are more colourful than others? The science behind these seasonal changes makes the colours all that more interesting.

Most plants have green leaves due to the abundance of a pigment called chlorophyll. Chlorophyll captures sunlight energy to drive photosynthesis, which is the plant’s way of making sugar. The reason chlorophyll looks green? From a white light spectrum, chlorophyll absorbs purple, blue, red, and yellow lights well, but not green light, which gets reflected to our eyes.

In every leaf, there are other pigments that exist, which vary depending on the species of plant. Pigments known as carotenoids appear yellow and orange – carrot colours! Pigments known as anthocyanins produce deep reds, purples, and blues, and thrive when there is high sugar content in the leaf. So what determines the timing of colour season? In fact, the change begins with the shortening day length after summer solstice. Cells at the leaf base begin to slowly cut off the transport of water and nutrients, preparing the leaf to drop. Though cold weather doesn’t trigger the change, weather conditions influence the intensity of colour. Sunny, warm days, followed by cool nights produce the most brilliance because leaves continue producing sugars. The moment there is a frost, colour dims.

And the curtain call? High winds, rain or snow will strip off the leaves and change the scene, sometimes overnight. But remember, those stark branches have their own kind of beauty on a sunny winter day.

by Barrett Miller & Katrina Froese