September ushers in hues of autumn

By Michael McVey

Mid-September is awfully early for the leaves to be turning, isn’t it?

Not really, according to botanist Paul Sorenson, professor of biology. Sorenson said it is normal for trees to start changing color before the end of September in northern Illinois. While the early frost and cool temperatures could have hastened the fall colors somewhat, even a warm, moist September and October would delay the color changes by no more than two weeks.

Sorenson attributed the perennial consistency of the date of peak color to the dependence of leaf coloring on the length of day and night. From early September to early October the nights gain about an hour and a half on the days.

When days shorten and nights lengthen, Sorenson said, trees form an abscission layer of cells at the bases of the leaf stalks. This blocks moisture and nutrients from circulating into the leaves, preventing a continuation of photosynthesis.

The lack of nourishment kills the green pigments first, leaving either yellow or red pigments already in the leaf. During the summer the green pigment obscures the pigments of other colors. The fall colors result from a process of selectively washing out the green dyes rather than an actual change in the color of the leaves.

Eventually the abscission layer starves the red or yellow pigments. Then the leaves turn brown, wither and die. When this happens, often in November, a breeze can remove the leaves from the trees. Since November is quite windy, leaves at this stage “fall” in short order.

Sorenson also attributed regional differences in the date and intensity of peak fall coloring to the types of trees which dominate a region, which is only indirectly related to latitude or climate. While trees in northern areas turn in response to a more rapid loss of daylight than occurs in the south, regions with abundant maple trees have longer and more spectacular fall seasons than regions dominated by oak trees.

Oaks are plentiful in New England, but also dominate southern Indiana.

Sorenson said the average start of fall colors is late September, and the colors normally peak in mid-October. He also noted the local shrubs and woodlands that fail to change color, or that turn unusually late in the year, are imported from Europe. Most native American trees other than needleleaf evergreens change color in the autumn months, except along the Gulf Coast.

It would be advisable to make the best of the red, orange and yellow now. In two months the prevailing colors will be gray and white.