Why do leaves change color?

Autumn has come gently this year with hot weather continuing to the end of September. This is rather unusual because our average first frost date is usually Sept. 15. The leaves have barely begun to turn color.

At University of Nevada Cooperative Extension, Master Gardeners are often asked why leaves change color. Is the cold responsible? This seems like a logical reason, but it is not the case. Many leaves start to change long before the cold weather hits. What actually affects the colors is the change in the daylight and day length.

At this time of year, leaves slow down their food-making processes preparing for winter shut-down. Chlorophyll begins breaking down. Chlorophyll gives plants their green color through spring and summer. When the green color breaks down, other pigments, such as the carotenoids, show through. Carotenoids give leaves their yellow or orange colors. Although temperature does influence the process, day length is the controlling factor.

Other plant chemical changes also occur, allowing the red colors to come out in such plants as sumac, Virginia creeper, burning bush and others. The fall weather conditions that favor brilliant red color are warm sunny days with nights of approximately 45 degrees. The cooler nights trap sugars that were produced in the leaves during the sunny day. This forms the pigment anthocyanin for the red color tones.

Color intensity varies with location, from sunny exposures to shady ones. Color can even vary on individual trees. Rainy, cloudy weather can reduce fall color because sugar production is reduced.

Prepare your plants for winter now. Harden them off. Do this by reducing your watering. Water deeply but less often. Plants need to quit growing, so they won't be damaged when the freeze hits. Lawns need about 1 inch of water per week now that the weather has cooled. Plants that are a little drought-stressed often respond better to freezing temperatures than those that are still producing young tender growth. Wait until after a hard freeze or two to fertilize your lawn, trees and shrubs. Don't be fooled by the lovely weather we were having. A plant that is properly hardened off will be hardier next spring.

Watch the weather reports - if a freeze is predicted at night or if the night is calm and crystal clear, cover your tomatoes, peppers and other tender plants.

Call 887-2252 or e-mail 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.