We just don’t have weather that is ‘fall enough’ for fall leaves

Published 4:55 pm Thursday, October 17, 2019

Keeping things simple, many of us have stuck with the premise that the fall color of tree leaves comes about because green leaf chlorophyll fades away, leaving other color pigments that have been in the leaves all along. And that is mostly true.

It is true in leaves that turn shades of yellow, orange and brown. But the autumn reds and purples are not there during the entire growing season just waiting for the green to get out of the way.

Red and purple and their hues come about because their pigments are produced at the end of the growing season when leaf phosphate levels drop. Phosphate prevents those pigments from forming in spring and summer.

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As with the yellow and orange colors, the weather determines the intensity of red and purple in leaves. 

Of course, genetics determines which tree species have which fall pigments to show when the weather cooperates.

Around here, sycamore, hickory, ash, beech and some white oak species are known for yellow leaves in fall. Among the red leaf species are numerous oaks, red maple, dogwood, persimmon and sassafras.

There are four things necessary for the photosynthesis that keeps leaves green; sunlight, water, carbon dioxide and the natural chemical compound chlorophyll.

All four are temporary in plants, meaning they are not present year-round.

As days grow shorter and nights longer, plants prepare for the upcoming non-growing dormant season.

One thing that occurs is the development of a layer of cells right where leaves are attached to stems. This abscission layer slows and eventually cuts off water and all it carries to leaves, as well as the flow of created plant food from leaves. Thus, green chlorophyll becomes history for the year.

Our local fall color is not brilliant enough nor predictable enough to get in on autumn forest tourism, whereby folks go by car or tour bus just to have a look-see at leaves.

Like all things having to do with plants, weather impacts fall color and, thus, leaf tours. For the best fall color, ample spring rain is the starting point and we usually have that. But then comes summer and we stay too hot for too long for the best of leaf color later.

Ideally, late summer weather would be warm days and moderate nighttime temperature nights. Here, August and September come with very hot days and warm nights. Based on recent 90-plus days lasting even longer than normal, I’ll go out on a limb so to speak and speculate fall tree color here will not approach the best ever.

Areas of the United States with the best fall color are mostly to the north or at higher elevations, both providing warm days and cool nights late in the growing season.

The New England states are known for good fall color as are the mountainous areas of the South. Lots of people go to the Smoky Mountains to see fall leaves. Nobody drives to Blue Mountain, Miss. just for the leaf color.


Terry Rector writes for the Warren County Soil and Water Conservation District.