As the days get cooler, deciduous trees like bald cypress, blackgum, Bradford pear, Chinese pistache, crape myrtle, dogwood ginkgo, maples, oaks, sassafras and sweetgum put on an autumn show in all shades of yellow, orange, red and purple. In many parts of the country, autumn leaves are an important factor in tourism. I think they should be here, as well, since most of Texas has little fall color to speak of.
Many think that cool weather or frosts causes the leaves to change color. While temperature may dictate the color and its intensity, it is only one of the environmental factors that play a part in coloring up deciduous trees. To understand the whole process, it is important to understand the growth cycle of deciduous trees. During the growing season, trees store up carbohydrates to support next year’s growth.
The process that starts the cascade of events that result in fall color begins in early autumn when the days begin to get shorter and the nights longer. When nights reach a certain length, the cells at the base of the leaf stalk form an abscission layer and slowly begin to block the transport of carbohydrates from the leaf to the branch.
During the growing season, chlorophyll, which makes leaves green, is constantly replaced. Chlorophyll breaks down with exposure to light, so the leaves have to constantly manufacture new chlorophyll to replace that loss. In autumn, when the connection between the leaf and the rest of the plant begins to be blocked off, the production of chlorophyll slows and then stops. In a fairly short time period, the green chlorophyll disappears entirely.
This is when our classic fall colors are revealed. Chlorophyll normally masks the yellow pigments known as xanthophylls and the orange pigments called carotenoids. Both then become visible when the chlorophyll is gone. These colors are present in the leaf throughout the growing season. Red and purple pigments come from anthocyanins. In the fall, anthocyanins are manufactured from the sugars that are trapped in the leaf. Some years more are produced and in some years less. The more, the merrier!
Temperature, sunlight and soil moisture greatly influence the quality of the fall foliage display. A growing season with ample moisture followed by a rather dry, cool, sunny autumn marked by mild days and cool, frostless nights provides the best conditions for development of the brightest fall colors. Early hard freezes, heavy rain and high wind can put an end to the show. Naturally, there’s no guarantee to what we will get. But the unexpected, fleeting beauty of it all is part of its allure.
Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service. He is author of “Texas Fruit and Vegetable Gardening,” “Heirloom Gardening in the South” and “The Rose Rustlers.” You can read his “Greg’s Ramblings” blog at arborgate.com, his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com) or follow him on Facebook at “Greg Grant Gardens.” More science- and research-based lawn and gardening information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu and aggie-horticulture.tamu.edu.