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

Fall Color Changes in Leaves

September 11, 1997

Foliage of trees changing color one of the highlights of autumn in the midwest. This annual process is often misunderstood.

Leaves of deciduous trees and shrubs do not actually turn brilliant yellow, orange, or red. In reality, for most trees the color is actually there all season, but is covered by green pigment called chlorophyll. During most of the growing season, plants are producing chlorophyll at a rapid pace, and it masks any other color pigments in the leaves. As fall approaches, however, chlorophyll production slows as the days get shorter.

Chlorophyll is broken down, and other color pigments in the leaf then 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 already been turning color. 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.

Pines and other evergreens are also turning color now. Even though we refer to them as evergreens, the needles do not stay on the plant 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.

This is a normal function of the plant to shed older needles. 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.

 

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