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Fall colors provide a beautiful palette of spectacular hues. The leaves change from green to brilliant shades of red, gold, orange, purple and amber. As they lose their luster, they fall to the ground and decompose to nurture future plants. This is not death, but only a mechanism for the tree to survive the upcoming winter.
As fall approaches, leaves stop growing and a corky layer of cells forms at the petiole or base of the stem. This layer of cells, called the abscission layer, separates the leaf from the tree. This layer holds the leaf in place until wind or frost breaks it free. As this layer forms, food production in the leaf slows down. The veins become clogged and because no more chlorophyll is produced, the green color gradually disappears.
The fall colors that begin to show were not created by Jack Frost, but were only hidden by the stronger green colors. As the green becomes less dominant, other pigments show off their splendor. Yellow and orange pigments are carotenoids and red and purplish colors are anthocyanins. Yellow pigments form in the protoplasm of leaf cells. The reds and purples from in cell sap that is rich in sugar. For this reason, maples, especially sugar maple, sumac and some oaks show the most brilliant red colors. They develop these colors when exposed to high sunlight. Trees in the shade never develop brilliant colors.
Jack Frost usually gets credit for autumn color, but frost has less of an effect than many other factors. Red pigments form in the sunlight in leaves high in sugar. Cloudy, rainy weather or hot, dry weather prevents pigment development. During warm weather, sugars drain out of the leaves, so the best colors develop with bright, sunny days and cool nights.
Frost is the chief reason that leaves drop. On cold, frosty nights, ice crystals form in the abscission layer and break the woody fibers that hold the leaf in place. As the ice melts in the morning, the leaves flutter to the ground in a shower of color. Abscission layers do not form on leaves that have died prematurely from diseases or other causes. Dead branches are easily recognized because the leaves have not dropped. Some trees (certain oaks and others) never form abscission layers, and the leaves often remain throughout the winter.
Maples show some of the most spectacular fall colors. Norway maple, our most common street tree, shows bright yellow autumn color. Hedge and silver maple and box elder are primarily yellow, although all these maples show some red under the right conditions. Sugar maple shows the famous fall yellow, orange and red colors in the Midwest and New England. Although it is not commonly grown in Utah it is a satisfactory tree for well-drained, loamy soils. Japanese maples are often red, but the trees are much smaller and lack the dramatic effect of larger trees. Vine maples turn red if the temperature drops suddenly.
Red maples are becoming more popular in our area. Many named varieties have spectacular autumn color. "Autumn Radiance," "October Glory" and "Red Sunset" are a few excellent selections. The outstanding performers in our mountains are the Bigtooth and Rocky Mountain maple. Bigtooth maple has larger leaves and develops yellow, orange and red hues, while the Rocky Mountain maple has smaller leaves and turns bright orange or red.
Birch has a golden yellow fall color. Tree form and bark color are often more spectacular than the leaves. Elm, locust, linden, zelkova, hornbeam, catalpa, hackberry, redbud, cottonwood, poplar and aspen all turn yellow, although some aspens turn orange and red.
White ash develops beautiful purple to wine-red autumn color. Green ash is less highly colored, but still turns a beautiful gold. Ginko develops the same color.
Honeylocust turns a less brilliant, attractive yellow. Unfortunately, the leaves do not remain on the tree for very long.
Liquid amber or sweetgum trees turn crimson, orange or yellow. Crabapples get most of their fall color from their fruits although a few varieties have yellows and reddish tones in the fall. American hawthorns usually turn red in the fall, while European species turn yellow.
Fall colors of plum, cherry, nectarine and peach range from yellow to rusty yellow. Flowering pears, by contrast, often develop a blazing red to maroon leaf color. Sumacs show orange or red fall color. Oaks show variable fall color. Scarlet oak and red oak develop orange to bright scarlet color. Burr oak turns yellow, as does our native gambel oak.
The colors of autumn are wonderful landscape assets. Spend some time with the trees as their leaves turn and drop. Note the beautiful specimens in our mountains and landscapes so you can utilize them in creating similar beauty in your garden.