I love driving around and observing the fall colors. In most years the yellows, reds and oranges are spectacular. However, has anyone else noticed this is a subpar fall color year locally? Perhaps we should analyze the factors contributing to fall color and maybe it will give us a clue.
In general, warm wet springs mean dense tree canopies. More leaves provide a greater chance of a colorful fall. That’s the first part of the equation. If summers are neither too hot nor too dry those leaves tend to stay on the trees.
Fall weather is critical. Favorable color conditions are warm sunny days and cool but not extremely cold nights. In fall, the day length shortens and overall temperature falls, triggering the color changing process.
Chlorophyll is the substance in leaves that makes them green. It also is essential for photosynthesis, the most important chemical reaction in the world. Photosynthesis provides plants with sugar so they can grow and provide everything else with food.
Chlorophyll is not a very stable compound. Sun breaks it down and it must be replaced continually. In warm weather that’s not a problem. When day length shortens and temperatures fall chlorophyll doesn’t get replaced and leaves begin to lose their green color.
Green gets replaced by yellows oranges, reds and purples. In short, the xanthophylls, caraotenoids and anthocyanins start showing their true colors. How strong they develop and how long they persist depends largely on fall weather. Freezing temperature can halt the process and some pigments fail to emerge fully.
We often associate fall leaf color with frost, but cold temperatures don’t cause leaves to change color. In fact, hard frosts and freezes stop the process and leaves turn black. Locally, many leaves show signs from the frosts we had back a few weeks when temperatures fluctuated drastically for a few days.
In fall when day length shortens, lower sap levels trigger a process that creates a layer at the point where leaves meet stems. It’s called an abscission layer. Cells thicken and vascular tissue narrows, reducing the flow of liquids. Eventually the leaves fall.
Stress from disease can cause leaves to abscise prematurely too. I’ve noticed this on crape myrtles especially this year. Color change starts but within a few days all foliage is gone.
Various sucking insects were abundant this year. They led to a bad sooty mold problem in many locales. Upon closer inspection other foliage diseases developed as well. I found Cercospora leaf spot on most of the fallen leaves. Leaves are covered with brown speckles. This recent period of cloudy rainy weather probably contributed to it.
Some pockets still provide good color. Certain species such as southern red oak have yet to develop their color yet, so we still have some potential left. Nearly all sycamore leaves are on the ground and yellow poplars are getting there quickly. It seems this fall that many leaves fell without changing much and the rest are still green. Let’s hold out hope for some more clear mild days with some night temperatures in the low 40s.
Ted Manzer teaches agriculture at Northeastern High School. (firstname.lastname@example.org)