October 20, 1997
Only a few regions of the world are fortunate enough to have a display of color when the leaves of the trees change in the fall - Virginia is one of them. Some areas of the state are known for especially beautiful displays. These are the areas with just the right combination of tree varieties, temperature, and moisture.
Temperature and Daylight Make the Difference
The reason leaves of some deciduous, broadleaf trees change color is that variations in temperature and periods of daylight cause the leaves to stop their food-making process. The food-making process takes place in numerous cells of the leaves that contain the pigment, chlorophyll, which gives the leaf its green color. The chlorophyll absorbs energy from sunlight and uses it in transforming carbon dioxide and water into carbohydrates, such as sugar and starch.
Along with the green pigment, leaves also contain orange or yellow carotenoids which, for example, give carrots their color. During most of the year, these yellowish colors are masked by the greater amounts of green coloring. In the fall, however, when the food-making stops, the green color disappears. The yellowish colors become visible and make the leaves part of the fall splendor.
At the same time, other chemical changes may occur that cause the formation of additional pigments that vary from yellow to red to blue. Some of them give rise to the reddish and purplish fall colors of leaves on dogwoods and sumacs.
Sunny Days and Cool Nights Make Dramatic Red
The conditions that favor the most dramatic changes resulting in vibrant reds are warm, sunny days followed by cool nights when the temperature dips below 45F. Familiar trees that turn red are red and silver maples, flowering dogwood, sweetgum, black tupelo (black gum), northern red oak, scarlet oak, and sassafras. Leaves fall to the ground because a special kind of cell develops at the base of the leaf stalk where it is attached to the branch. Gradually, the tissues that support the leaf are severed.
Elements in Leaves Return To Soil
Leaves contain relatively large amounts of valuable elements, such as calcium and potassium. When they fall to the ground and decompose, they return to the soil part of the elements used to make the tree grow. At the same time, the layer of leaves forms a water-absorbing humus.
It is necessary to remove heavy accumulations of leaves. One of the easiest methods is to run over the lawn with a lawnmower with catcher attached. The shredded leaves will make an excellent mulch around trees and shrubs or can be mixed with other yard wastes in the compost pile where it will take up less space and decompose much faster than unshredded ones.
More gardening information is available in the Virginia Gardener Newsletter.
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