Why do leaves change colour in the fall?

The science behind those beautiful fall shades.

Autumn is often referred to as the "picture perfect" season as splashes of colour fill trees across the country.

Pockets of vibrant reds and yellow are slowly beginning to show, but green currently remains dominant.

Travel and tourism officials say colours usually begin to peak by the end of September and into October.

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"Temperature plays a factor in the coloration of trees, but the most important factor is called the photo period, or the length of daylight," says horticulturalist Bob Osbourne of Cornhill, N.B.

"The shortening of the number of hours of daylight is the trigger for trees to start shutting down. A cork-like layer forms between the leaf stem and the branch, which will eventually fall off and it also protects the tree from any kind of fungal or bacterial infection. As it shuts down, it begins to stop water and nutrients from coming into the leaf and the chlorophyll dies."


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Osbourne says this phenomenon is what allows us to see the various pigments that have been there all along, but are only visible now because of the lack of green.

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Stress is a big factor in coloration. It may be a matter of too much or too little water, or other nutritional factors. You can spot a stressed tree by its leaves, which change color before the others.

Fall leaves pixabay

(Source: Pixabay)

Tree stress can affect whether the fall show will be sensational or not. Things like drought, extreme temperatures and air pollution can make the leaves dull.

"The most famous of all the trees is the Sugar Maple," adds Osbourne, "with its combination of orange, yellow, and sometimes red. One of the most spectacular of the trees is the native Red Maple with its vivid red color. These are the trees we most associate with fall."

We'd love to see the fall colours that are popping up in your neighbourhood! You can upload your images directly to our website.

This article was originally published in 2018, and has been updated.