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 About to Begin

September 20, 2001

Many people consider fall to be their favorite time of year. Certainly trees turning brilliant colors is a highlight of the season. While often associated with frost, how this color change actually occurs is often misunderstood.

During the season, leaves on trees are green because of the chlorophyll inside them. Other color pigments are actually present, but are hidden by the abundant chlorophyll. As the season winds down and days get shorter, chlorophyll production in leaves slows down as trees and shrubs prepare for dormancy.

Once chlorophyll breaks down in the leaf, other color pigments in the leaf become visible. For example, carotene and xanthophyll pigments give leaves orange and yellow colors. 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.

Sunny days, and cool nights generally means good color. Frost can actually be detrimental to the process by causing early drop with poor color. Some trees under stress have been turning color since August because they have slowed in the production of chlorophyll.

Pines and other evergreens may also show color change in fall, as needles do not stay on the evergreens forever. All the inner, older needles may turn brilliant yellow or brown at once, and then drop. Green growth will remain on the ends of branches.

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 before you know it trees will be bare and the cold and snows of winter will be back!


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