Tree Body of Palos Park
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Don't think of frost as the cause of fall color in leaves. In fact, a severe frost kills the leaves, turns them brown, and causes them to drop. We don't understand all of the actions and interactions that bring on fall color; but, in general experts agree that the shortening length of day is the main trigger. Ed Hedborn, Director of Collections and fall color expert from the Morton Arboretum, says that the best fall color occurs during the shortening days of autumn when days are bright, sunny, and cool, when nights are cool but not below freezing, and when there has been ideal rainfall. In addition, the plant should be genetically disposed to pigment production, have proper nutrients, and be free of diseases and pests.

Essentially what happens is that the shortening length of day triggers a layer of special cork cells {abscission layer} to form at the base of each leaf. Water and mineral intake into each leaf is slowly cut off, reducing the leafs ability to manufacture chlorophyll. The green color of the chlorophyll usually masks any other color in the leaf, so leaves seem to be green. In Fall, when the chlorophyll fades, the orange and yellow pigments of the carotenoids that have been hidden begin to show through. The abscission layer also restricts the loss of sugar in the leaves, resulting in increased sugar concentration. Sugars plus light convert into anthocynanins, the pigment responsible for the reds, purples, and blends of the two. The brighter the light at this time, the more brilliant the color.

Sunshine is very important in the production of brilliant colors. The tops and south sides of a tree will color first. Trees along the edge of a woods and the tall trees in the canopy are the first to color. Small trees do not color until the taller trees have dropped their leaves and the sun can shine through to the shorter ones. We don't understand all the interactions; but, we understand that pigments, sunlight, moisture, chemicals, temperature, length of day, and the trees genetic traits, all make for a perfect autumn display. Fall color can shift as much as two weeks from year to year depending on the weather, sun exposure, site, and health of the plants. Like the sequence of color in the spring, one can also watch the sequence of fall color from September to November. It all begins in fall with the bright red color of at least three native woody plants: poison ivy; sumac; and Virginia creeper. Soon, the hickories appear, adding their rich mustard yellow to the autumn picture. This is followed by the white ash which begins the season a shade of light yellow and finishes it all in purple. It all ends too soon, for the ash drops its leaves, shortly after it turns purple. The sugar maple pushes in to show off her great display of yellow, red and orange. The Gingko tree follows with a pure yellow. The season ends with the deep reds, browns, and russets of the oaks.

Pines, spruces, cedars and firs don't lose their leaves, the needles, each year. However, there are several trees with needles that do-- the bald cypress, the European larch, and the American tamarack. In the fall, their needles turn a beautiful golden yellow before they fall. The Morton Arboretum in Lisle is a beautiful place in autumn, due to the variety of trees from all around the world with their variation in color. Peak color is hard to predict; but, in the Chicago area usually it is a period of a week before and a week after Columbus Day in October. Don't stay indoors on cloudy days, because fall color is frequently more vivid in cloudy weather. For information about fall color in our area, call the "Bloom and Color Hot Line" at 630-719-7955. Elsewhere in Northern Illinois, popular destinations for viewing fall color include Apple River Canyon State Park in Jo Daviess County, Mississippi Palisades State Park in Carroll County, Rock Cut State Park in Winnebago County, Big River State Forest in Henderson County, and Starved Rock State Park in LaSalle County. Information on scenic drives in Illinois is available by calling 800-226-6632 for the Illinois Bureau of Tourism