Gardening

Chemicals, Not Frost, Create Fall Colors

by Dr. William M. Johnson, Galveston County Extension Agent - Horticulture

November 9, 2005

The orange, yellow and red hues of fall foliage on the lowly Chinese tallow tree can provide a relatively spectacular autumn display along roadways. Photo Credit: Dr. William M. Johnson

Each fall a glorious spectrum of colors blankets the hardwood forests of the United States. In Colorado, it's the gold of aspen trees that catches the eye. In New England, it's the brilliant oranges and yellows of the sugar maples. And in the South, it's the deep scarlet of the red oaks, the reddish-orange of sumac and the multi-colors of sweetgum.

Despite appearances, Mother Nature doesn't paint with broad brush strokes. Paint-by-numbers might be the more accurate analogy, because each tree has its own fall color bound up in the chemical composition of the sap, which provides the "instructions" on what color to turn.

According to the American Forest Council, trees change colors according to complex chemical formulas. Depending on how much iron, magnesium, phosphorus or sodium is in the tree, and the acidity of the chemicals in the leaves, trees might turn amber, gold, red, orange or just fade from green to brown. Scarlet oaks, red maples and sumacs, for instance, have a slightly acidic sap which causes the leaves to turn bright red. The leaves of some varieties of ash, growing in areas where limestone is present, will turn a regal purplish-blue.

What prompts the change? Although many people believe that a mischievous Jack Frost is responsible for the color change, the weather has only a small part to play. As the days grow shorter and the nights longer, a chemical clock inside the trees starts up, releasing a hormone which restricts the flow of sap to each leaf.

As autumn progresses, the sap flow slows and chlorophyll, the chemical that gives the leaves their green color in the spring and summer, starts to disappear. The residual sap becomes more concentrated as it dries, creating the colors of fall. In other words, the colors are always there, but as the predominant green dissipates the others become enhanced and begin to show through. Sunlight, nutrients and moisture enhance the process and cool weather seems to slow things down to bring out the full effect.

A long, cool, sunny, and moderately moist fall seems to provide the best color show in a given year. Well, we thus far have missed out on two of the four parameters for ideal color as the fall season thus far has been downright warm at times and we’re definitely on the low end of the moisture meter.

Obviously, this area is not a hot spot for fall color along the roadways as we don't have the aspens of Colorado nor the sugar maples of New England. Along the highways in Galveston County–well, it's basically the orange, yellow and red hues of the lowly Chinese tallow tree. However, in some years even it can provide a relatively spectacular display, especially among massive plantings. Even the dreaded poison ivy can put on a respectable show of fall colors.

This year's autumn color receives only a "one-star rating" from me, but in some years it has been quite spectacular in some areas. So, unless you have a Chinese tallow that showed off or a Drummond red maple or similar tree that grows well here or even a patch of poison ivy, you may miss our fall season of color!

Gardeners Q&A

Question: A few weeks ago, we were covered up with mosquitoes and flies every time we tried to eat outside. Will a bug zapper control these insects if we put one outside?

Answer: Studies have shown that bug zappers tend to kill mostly beneficial insects and others of no consequence. Mosquitoes and other biting insects are not strongly attracted to these 'zappers'. One of the few instances that they can be beneficial is for those residents who live near ponds or lakes. It will help with the midges.

On a more interesting note, it is not recommended to have bug zappers near picnic tables and grills. A recent study shows that when bugs get zapped they spew bacteria and viruses as far as 8 feet and could get into your food. When houseflies hit the zapper they literally explode.

If your bug zapper is near your pool and it’s maintained properly, don't worry. The chlorine in the pool will take care of any bacteria and virus that may land there. Share all this information at the next cookout you are invited to. You will be the center of attention but may not be invited back.

Question: The figs on our fig bush are all falling off. This seems to happen every year and what looks like a great crop of figs never ripens. What is wrong with this plant?

Answer: It is most likely due to the lack of rainfall. Figs will drop off very quickly if the soil is not kept moist. The figs we grow here originated in northern Italy. There they do not encounter hot or very dry summers.

The roots of figs are just under the surface of the soil and will suffer when temperatures turn hot and rainfall is infrequent. Their root system must be kept moist and cool through the use of mulch.

I would maintain at least a 4-to-6 inch layer of mulch and water at least twice a week or as needed to maintain an adequate soil moisture level. Be sure to mulch the entire area under the limbs and not just around the trunk. The roots of figs go well beyond the limbs.

Dr. Johnson is a horticulturist with the Galveston County
Extension Office of Texas Cooperative Extension, Texas A&M University. Visit his web site at http://aggie-horticulture.tamu.edu/galveston/index.htm

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