University of Illinois Extension
Bruce Spangeberg

These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.

Stateline Yard & Garden

Annual Fall Color Show to Begin

October 1, 1998

October is the first full month of fall. And when thinking fall, brilliant foliage colors of trees comes to the mind of many. How this occurs is often misunderstood, so here’s my annual column on trees changing color in fall.

While we say the trees are turning color, the color in most cases has actually been there all season. However, when actively growing, green chlorophyll is abundant in leaves and tends to mask the other color pigment present. Thus the leaf looks green. As fall approaches and days get shorter, however, chlorophyll production slows.

Chlorophyll breaks down, allowing other color pigments in the leaf to become visible. For example, orange and yellow color is due to pigments called carotenes and xanthophyll. Red color is due to production of anthocyanins, which is favored by warm, sunny days and cool nights in fall. Eventually the leaves will dry up and drop from trees as a wall-like layer is formed where the leaf stalk (petiole) joins the twig.

Predicting the fall color show can be difficult. Sunny days, and cool nights generally means good color. Contrary to popular belief, "jack frost" does not help, but in fact can be detrimental to the process by causing early drop with poor color.

Trees under stress have been turning color since August. This is because they have slowed in the production of chlorophyll. Check trunks and root zones of shade trees that have been turning early. Injury to these areas may be the source of the problem.

Over the past few weeks, many pines and other evergreens have been having needles turn yellow or brown. Needles do not stay on the evergreens forever. It is normal for pines, arborvitae, and others to lose the inner needles. All the inner, older needles may turn brilliant yellow or brown at once, and then drop.

Keep in mind the older needles should be affected only, and there is usually an abrupt break between the yellow or brown needles and the newer, green growth as you advance outward on the branch. This is a normal function of the plant.

Get out and enjoy the upcoming fall colors, as the cold and snows of winter are not too far behind!


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