JoAnne Skelly: Why do leaves change color?

Autumn has come gently this year with hot weather continuing into October. The leaves are finally turning color. At University of Nevada Cooperative Extension, Master Gardeners are often asked why leaves change color.

Is the cold responsible? This seems logical, but it is not the complete answer. Many leaves start to change long before the cold weather hits. The most important factors affecting the colors are the change in daylight and day length.

Four leaf pigments are responsible for the colors in leaves. These are chlorophylls, carotenoids, anthocyanins and tannins. At this time of year, leaves slow their food-making process (photosynthesis) preparing for winter shutdown. Chlorophyll begins breaking down and disappears from leaves. Chlorophyll gives plants their green color through spring and summer. When the green color fades, other pigments show through, such as the carotenoids, which give leaves their yellow and orange colors. Previously, these colors were hidden by the green chlorophyll. Although temperature does influence the process, day length and the reduction of chlorophyll production are the controlling factors.

Other plant chemical changes also occur during autumn producing the red and purple colors we see in plants such as sumac, Virginia creeper, burning bush and others. The fall weather conditions that favor brilliant red color are warm sunny days with crisp nights without freezes. The cooler nights trap sugars that were produced in the leaves during the sunny day. This allows the anthocyanin pigments to produce red color tones. Anthocyanins are usually only produced in the fall. Tannins produce the brown tones in fall leaves. They accumulate in dead tissue. Brown leaves are simply waste materials leftover in leaves.

Fall color also is affected by what happened to the plant during the growing season, particularly in regards to moisture. Rainy cloudy weather can reduce fall color because sugar production is reduced when there is less light intensity. Summer drought can play a role by delaying the onset of fall color by a few weeks. Since soil moisture varies from year to year, no two autumns are ever alike. Color intensity varies with location, from sunny exposures to shady ones. Color can even vary on individual trees or by tree types. Early frosts weaken the strong colors because leaves are injured by the cold.

Call 775-887-2252, or email skellyj@unce.unr.edu for a free copy of “Preventing Winter Injury to Landscape Plants.”

JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu or 887-2252.