To be sure, the appeal of autumn’s foliage changing all around us is it’s plethora of subtle and intense colors. There are shrubs and trees that turn various hues of red, purple, yellow, orange and brown.
The causes for this array of colors are plant pigments found within the leaves. The normal green color that leaves exhibit during summertime comes from the familiar pigment chlorophyll, the compound that uses the energy of the sun along with water and CO2, to produce sugars and O2. The color green that we see is actually the color being reflected by the chlorophyll, as the other colors in the visible light spectrum given by sunlight are being absorbed.
Other pigments reflecting hues of red, yellow, orange, etc., are sometimes seen in certain plants all season long like purple leaf plums, or golden euonymus’ as examples. But the real showcase is in autumn when one sees shrubs and trees exhibiting various colors all around town.
It is interesting to know, however, the red and purple colors are primarily caused by a pigment called anthocyanins (or Betalains). The colors of yellow are caused by the xanthophylls pigments, where the oranges are by a combination of carotenes (of which there are 6 kinds) and xanthophylls. Browns are the result of tannins present in the leaf. Most of these compounds are present throughout the growing season, but are drowned-out by the green color reflected by the dominating chlorophyll pigment. Anthocyanins (red and purple) are the exception and are produced after the chlorophyll is destroyed, and phasing out in the fall. (Note: blueish colors are usually the result of refracted light rather than a pigment, or a combination of certain pigments.)
If you have ever seen pictures of the north eastern US in the fall, you might have wondered why trees in Kansas do not look the same way in the fall. This difference is in part due to the tree species mostly found in New England. Certain oaks and maple varieties just naturally produce great fall color. These colors can also be determined by the weather.
Warm, sunny days and cool nights are requisites for good fall color. Sunny days encourage photosynthesis causing sugars to be built-up in the leaves. As fall progresses, the leaves develop a callus (abscission) layer at the base of the petiole (leaf stem) preventing these sugars from being transported down to the roots for winter storage. This high sugar content in the leaves produces those intense colors. Likewise, cloudy days and warm nights prevent some of the sugar build-up, resulting in less vibrant colors being reflected from the leaves.
The weather at other parts of the growing season can also have an effect. Both heavy rains in the early spring or hot, dry weather during the summer months can have a diminishing effect on fall color. Furthermore, the length of time trees or shrubs keep their fall color also depends on the weather. The pigments that cause red, yellow and orange hues are short-lived when and if trees undergo frosts and freezes.
Rip Winkel is the Horticulture agent with K-State Research and Extension – Cottonwood District. Contact him by email at firstname.lastname@example.org or call either 785-682-9430, or 620-793-1910.