From Larry Caplan, Extension Horticulture Educator
Purdue Extension Service, Vanderburgh County
One of my earliest horticultural memories involves walking waist deep (well, waist deep for a four-year old) in crunchy dry leaves in the fall. And while some homeowners may curse the annual dropping of the leaves, many gardeners look forward to the free soil amendment dropping out of their trees. A large number of people plan their vacations around viewing fall leaf color. And of course, children love collecting pretty colored leaves!
It's often hard to predict how colorful the fall season will be, or when will the peak time for viewing fall leaves be. The timing and intensity of fall colors varies depending on factors such as availability of soil moisture and plant nutrients, as well as environmental signals such as temperature, sunlight, and length of day. The droughty conditions experienced over much of the summer are likely to have decreased the amount of fall color pigment.
Growing conditions throughout the season affect fall color as does current weather. Colors such as orange and yellow, which we see in the fall, are actually present in the leaf all summer. However, those colors are masked by the presence of chlorophyll, the substance responsible for green color in plants during the summer. Chlorophyll uses sunlight and carbon dioxide from the air to produce carbohydrates (sugars and starch), which the tree uses for food. Trees continually replenish their supply of chlorophyll, which is used up in making food during the growing season.
As the days grow shorter and temperatures cooler, the trees use chlorophyll faster than they can replace it. The green color fades as the level of chlorophyll decreases, allowing the other colored pigments to show through. Plants that are under stress--from conditions like prolonged dry spells--often will display early fall color because they are unable to produce as much chlorophyll.
Yellow, brown and orange colors, common to such trees as birch, some maples, hickory and aspen, come from pigments called carotenoids, the same pigments that are responsible for the color of carrots, corn and bananas.
Red and purple colors common to oaks, sweet gum, dogwoods and some maples are produced by another type of pigment called anthocyanin, the pigment responsible for the color of cherries, grapes, apples and blueberries. Unlike chlorophyll and carotenoids, anthocyanins are not always present in the leaf but are produced in late summer when environmental signals occur. Anthocyanins also combine with carotenoids to produce the fiery red, orange, and bronze colors found in sumac, oaks, and dogwoods.
Red colors tend to be most intense when days are warm and sunny, but nights are cool--below 45 degrees. The color intensifies because more sugars are produced during warm, sunny days; cool night temperatures cause the sugars to remain in the leaves. Pigments are formed from these sugars, so the more sugar in the leaf, the more pigment, and, thus, more intense colors. Warm, rainy fall weather decreases the amount of sugar and pigment production. Warm nights cause what sugars that are made to move out of the leaves, so that leaf colors are muted.
Leaf color also can vary from tree to tree and even from one side of a tree to another. Leaves that are more exposed to the sun tend to show more red coloration while those in the shade turn yellow. Stress such as drought, poor fertility, root damage, disease or insects may cause fall color to come on earlier, but usually results in less intense coloration, too.
One of the most important lessons to teach children about collecting leaves is to identify the plant they get the leaves from. Poison ivy has beautiful red leaves in the fall, but the oils that cause many of us to get an itchy rash are present year-round. Remember: leaves of three, let it be!