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Fall Color in Trees

Q. What accounts for the different autumn color displays in trees and shrubs?

A. Certain plants have a built-in genetic code that is responsible for their intense fall color displays. But even with these genes in place, trees and shrubs need ideal weather conditions to show off their best color. Trees should also be healthy, well-watered all season and located in full sun.

During the growing season, chlorophyll, indicated by the green color in leaves, is produced by the food-manufacturing process known as photosynthesis. As daylight decreases, so does the manufacture of chlorophyll. With less green present, the two yellow-to-orange pigments, carotin and xanthophyll, begin to dominate leaf color.

For the magnificent reds and burgundies to appear, a plant must have an appropriate genetic predisposition to these colors as well as a weather pattern of warm, sunny days followed by cool nights below 45 degrees F. During these warm days, sugars are manufactured in the leaves. When cool nights follow, these sugars are unable to move out of the leaves into the vascular system of the tree, and essentially become trapped in the leaves. When they are trapped, the leaf responds by manufacturing anthocyanin, the red pigment.

If ideal weather conditions are not present, fall color is not as brilliant. When fall days are cloudy or drought conditions exist, less sugar is manufactured, and there will be fewer of the striking color contrasts of red/green, orange/green or red/purple. Strong autumn winds or heavy thunderstorms can also adversely affect fall color by pulling down leaves prematurely before they have had a chance to fully color up.

At the Garden, look for the following trees and shrubs that demonstrate beautiful fall color: serviceberry, certain maple and ash varieties, buckeye and horsechestnut, callery pear, burning bush, sumac, dogwood, certain viburnum and azalea varieties, sweet gum, red oak, blueberry bush, smoke bush, fothergilla, cotoneaster and American hornbeam.