It seems that for much of the summer we were receiving just enough rain to keep the lawn mowers running, but not much extra. Lately, the rain has nearly turned off completely. Now that we are officially in the fall season, how can we expect trees to respond to these changes in terms of giving us a color show? Tree leaves change color each fall, but the timing and intensity of the changes vary from year to year based on several factors including: the availability of soil moisture and nutrients, temperature, sunlight, and length of day. The factor that would most concern me in looking forward to a lively color show this fall, would be a lack of soil moisture throughout August and September, which may indicate we won’t get as bright of a display as we usually do.
How trees change their color is a complicated process. Trees that change to yellow and orange, actually have those colors in their leaves all summer long, however they are masked by the presence of chlorophyll, which gives them their green color. Trees replenish their chlorophyll supply throughout the season; however, once the days grow shorter they will use chlorophyll faster than they can replace it. Once this happens, the green color fades and the other pigments show through.
Yellow, brown and orange colors come from pigments called carotenoids, the same pigment responsible for the color of carrots. Meanwhile, red and purple colors come from anthocyanin, which can also be found in cherries, grapes, apples and blueberries. These pigments are not present in the leaf all summer, rather are produced when the environment signals that the days are getting shorter and temperatures are growing cold.
Red colors tend to be most intense when days are warm and sunny, but nights are cool–below 45º F. The color intensifies because more sugars are produced during warm, sunny days; cool night temperatures cause the sugars to remain in the leaves. Pigments are formed from these sugars, so the more sugar in the leaf, the more pigment, and, thus, more intense colors. Warm, rainy fall weather decreases the amount of sugar and pigment production. Warm nights cause what sugars that are made to move out of the leaves, so that leaf colors are muted.
Leaf color can also vary from tree to tree and even from one side of a tree to another. Leaves that are more exposed to the sun tend to show more red coloration while those in the shade turn yellow. Stress such as drought, poor fertility, disease or insects may cause fall color to come on earlier, but usually results in less intense coloration, too. Stress or an abrupt hard freeze can cause leaves to drop before they have a chance to change color. If a tree in your landscape is acting abnormally in terms of color changes compared to other trees in the neighborhood of the same species, it may be a good indication that it is under stress.