These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.
Fall Colors Highlight Season
September 21, 2000
Now that fall has arrived and cool weather is back, trees will be turning color. Trees turning brilliant color is one of the highlights of the fall season. How this occurs is often misunderstood, however.
While actively growing, leaves on trees are green because chlorophyll is abundant inside them. Other color pigments are actually present, but are hidden by the 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.
Predicting the annual fall color show of trees 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.
Some 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.
Pines and other evergreens will also show color change of needles in fall. 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. 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.