10/25/2025
Why are the leaves turning colors?
By Joyce Block
Fall officially started on September 22. Fall leaf tours in the New England are wildly popular. In the northern part of the country, watching for peak color of the fall leaves is a daily event. Jumping into the car and driving along riverbanks, is a popular activity and a great time to take pictures.
In Texas, it seems like Fall takes its time and doesn’t arrive until after Halloween. That is when people notice that the leaves are changing and there is a slight briskness to the air.
Let’s start with a basic difference between trees, there are Deciduous trees and Evergreen trees. Deciduous trees, Maples, most Oaks, and Pecans, will have leaves that change color and the leaves will drop in the fall of the year. An exception to the Oaks is the Live Oak tree, they will drop their old leaves as the new leaves emerge in the spring. Then there are evergreen trees, firs, spruces, pines, yews, and cedars that keep their leaves all year long. Evergreen trees can be divided down to broadleaf evergreens, like Live Oak and narrow leaf, like the pines and spruces. Magnolias will also keep most of their leaves in the fall.
According to the USDA, as the nights become longer, and cooler the biochemical processes in the leaves start the change of the colors. The leaves of trees respond to the shortening days and less sunlight by closing of a layer of cells at the base of each leaf. The clogged veins trap sugars in the leaves, and once the separation is completed and totally sealed off, the leaf will fall.
Frost on the trees will affect the leaves by rupturing the cells and causing the leaves to die without changing color. To have spectacular fall color we need to have warm, sunny days and cool
nights. Drought stress will cause leaves to fall prematurely, which we have seen this year.
Leaves have various pigments in the leaves that cause the leaves to turn the correct color. Yellow, orange, and brown colors are produced by Carotenoids. Carotenoids are found in corn and
carrots. Burgundy, red and purple are water soluble colors that are called Anthocyanin. Chlorophyll is the basic green of all leaves that let the plants use sunlight to create sugars for food.
Oak trees, except for the Live Oaks, will turn red or brown, Maples will turn red or orange, and Chinese Pistache will turn yellow. This is due to chemical reactions in the leaves.
During the growing season most leaves contain chlorophyll and carotenoids. The anthocyanins are produced in the fall while there is bright light and excessive plant sugars. As the length of darkness increase, chlorophyll in the leaves is destroyed and the carotenoids and anthocyanin become present creating the fall colors.
At work, I have noticed the Maples and the Japanese Maples have started to change color. A small Chinese Pistachio in my yard is starting to change color, also. Though at the ranch in Grandview,
most of the pecans have dropped their leaves. It will take some time for the leaves to change color, but peak fall color usually occurs in the middle or late November.
Joyce Block has been a Master Gardener 2007 in Johnson County, Texas.
