Trying to track our weather patterns this year has been a bit of a roller coaster.

Now that we are officially in the fall season, how can we expect trees to respond to these drastic 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, temperate, sunlight, length of day, and nighttime temperatures.

The factor that would most concern me in looking forward to a lively color show this fall is a lack of soil moisture throughout the months of July, August and most of 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 environmental signals (short days, cool weather) occur.

Red colors tend to be most intense when days are warm and sunny, but nights are cool – below 45 degrees Fahrenheit. 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 also can 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.

Andrew Westfall is director of Purdue Extension White County (Ind.).

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