Fall color in tree leaves is determined by many factors. Any tree grown from a seed, such as an acorn, will have genetic variability in developing fall color. This is observed by watching our native “Spanish Oaks” as the leaves begin to turn in fall. Many will have beautiful red fall color, but we also observe some trees with leaves that simply turn brown and hang on the tree most of the winter…. What if I want to be SURE that a tree that I plant will have fall color? Trees grown from seed, such as Chinese Pistache, Cedar Elm, Big Tooth Maple, and Shumard Red Oak can be variable in their ability to form fall color. The only way to be certain it has the potential to have fall color is to BUY IT IN THE FALL WHEN IT HAS FALL COLOR!
If a tree is “grafted”, such as the Bradford Pear, the genetics will be identical on all Bradford Pears, as they all come from one common ancestor. Crape Myrtle varieties which are propagated from cuttings will also be genetically identical. This means that when conditions are optimum for fall color to appear, these trees will be very likely to have the expected fall color.
Why do trees turn color in the fall? What are the “optimum” conditions for fall color to appear?
Color changes are the result of transformations in leaf pigment. The green pigment in leaves is the chlorophyll, which absorbs red and blue light from sunlight. The light reflected from the leaf is diminished in red and blue and the leaf appears green. Chlorophyll is not a very stable compound, and degrades in low temperatures and bright sunshine. This is what begins the process of fall color changes in trees.
Carotene is a compound found in many leaves. It absorbs blue-green and blue light, and the light reflected from carotene appears yellow.
Anthocyanin absorbs blue, blue-green and green light. Therefore the light reflected from leaves containing anthocyanin appear red.
Shortening days and cooling nights of autumn trigger other changes in the tree. An abscission layer forms between the leaf stem and the branch, interrupting the nutrient flow and the production of chlorophyll. If the leaf contains Carotene, the leaf will change from green to bright yellow as the chlorophyll disappears. In some trees, the concentration of sugar in the leaf increases, which reacts to form Anthocyanins. These pigments cause the leaf to turn red if the sap is acidic or purple if the sap is less acidic.
The range and intensity of fall color is greatly influenced by the weather. Low temperatures destroy chlorophyll, and if they stay above freezing, promote the formation of anthocyanin. Bright sunshine also destroys chlorophyll and enhances anthocyanin production. Dry weather, by increasing sugar content in sap, also increases the amount of anthocyanin. So the brightest autumn colors will be produced when dry, sunny days are followed by cool, dry nights!