There is much speculation and some folklore associated with the color changes we generally see in the autumn in our hardwood trees.

One “theory” has to do with warm, sunny days and cool nights.

Another is based on the changes in the length of daylight.

Some say it is related to soil acidity; still others say it is related to frost.

All these ideas are true to some extent.

A common misconception is that the leaves are dying.

The development of fall colors is an active process, and trees must be alive to undergo the change in color and drop their leaves.

Chlorophyll, the compound most responsible for the manufacture of foods (carbohydrates) by plants, gives them their green color.

Chlorophyll breaks down under bright sunlight and is constantly being replaced.

If plants are actively growing, enough chlorophyll is produced to keep them green.

Other pigments are also present in green plants.

They serve as energy absorbers and help chlorophyll do its job.

They appear mostly yellow and orange in color, but they are not seen much if chlorophyll is being manufactured in the leaves.

The changing of leaf color is associated with winter dormancy in all perennial plants.

A chemical called phytochrome triggers plants to go into the dormant period when nights get long enough.

Therefore, day length does trigger leaf color change.

One of the things that happens when dormancy begins is the development of a corky layer of cells between the leaf stalk (petiole) and the woody part of the tree.

This “abscission layer” slows the transport of water and carbohydrates.

As this happens, the manufacture of chlorophyll is slowed and the green color in the leaves begins to fade, allowing the other pigments to show through.

Since the transport of water is slowed down, food manufactured by the remaining chlorophyll builds up in the sap of the leaf and other pigments are formed, which cause the leaves to turn red or purple in color, depending on the acidity of the sap.

For example, sumac almost always turns red because red pigments are present and its leaf sap is acidic, while many of the oaks will get a purplish color because the sap is less acidic.

The brown color of hickories can be attributed to a buildup of tannin, which is a waste product in the leaves.

There are endless variations of pigmentation in trees, which change as fall progresses.

Weather throughout the fall season has much to do with the development of fall color.

Cool night temperatures destroy chlorophyll quickly, but below-freezing temperatures inhibit production of red pigments.

The brightest displays of color occur when we have an early fall of bright, sunny days and cool nights.

These are excellent conditions for the development of red pigments.

Cloudy days and warm nights will produce fewer brilliant colors because chlorophyll breaks down slowly and the red pigments are not formed quickly enough.

Dry weather causes a greater buildup of sugars in the leaves, enhancing the production of the red pigments.

Windy, rainy weather causes many leaves to fall prematurely, lessening the intensity of the display.

A killing frost will destroy the leaf cells, and the coloration process will not function effectively.

Perhaps the most important thing about fall color is to enjoy it!

“Growing Green” is contributed by Lehigh County Extension Office Staff and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613