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The Science Behind Fall Color

Image by cmtyers courtesy iStock. 

Ever wonder what makes the green leaves of summer turn yellow, orange, and red in the fall?

All leaf colors come from three different pigments: Chlorophyll (green), carotenoid (yellow, orange, brown), and anthocyanin (red). In the growing season, leaves carry carotenoids but their more dominant chlorophyll covers it with green. Anthocyanins, on the other hand, are only present in certain types of plants under specific conditions during autumn.

In the fall, as the days grow shorter, trees and shrubs react by producing less and less chlorophyll in preparation for dormancy. When they finally stop producing chlorophyll altogether, the green fades and the carotenoids and anthocyanins are revealed in the golds, oranges, yellows, and rusty browns of autumn.

Image by soleg courtesy iStock. 

As for the bright reds, their vividness is tied more to weather, specifically temperature. The best autumn reds occur when warm, dry sunny days are followed followed by brisk, but not freezing, nights. These temperature swings cause the leaves to produce a lot of sugar during the day. Chilly nighttime temperatures trigger the anthocyanins to carry the sugars out of the leaves and into the rest of the tree for nourishment — that's when you get the brightest reds.

Photo by Kat Cervoni.


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