John Wheeler: Here is a fall color leaf primer just in time for fall

FARGO — The appearance of autumn colors in trees is not caused by cool temperatures or frost, as many people believe. Rather, leaves turning red and yellow is mainly a function of the growing hours of darkness. Deciduous plants reach a certain threshold in the fall in which the nights are so long that cells in the interface between the leaf and the stem stop dividing and growing as they did during summer, creating a growing layer of essentially non-functional cells that can no longer transport minerals from the tree roots to the leaves.

Likewise, carbohydrates are no longer passed from the leaves back into the tree. This process happens at the same time every year and has nothing to do with weather. Varying weather conditions year to year can have a large effect on the intensity of the colors and on how long the dying leaves remain on the trees.



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