Gardening

Don't Be Your Plant's Worst Enemy

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

July 13, 2005

Many sickly, stunted trees that were planted years ago but haven't grown well have been damaged by improper use of string trimmers. Look at the base of their trunks, and you will often see scars and callus growth from repeated injury done to the base of the tree. Photo credit: William M. Johnson

You can be one of your plants' worst enemies-unless you're cautious with mowers and string trimmers, avoid damaging roots and exercise care when using pesticides and fertilizers.

Avoid String Trimmer and Mower Damage

String trimmers that use a monofilament line for cutting down weeds and grass can be very damaging to young trees. String trimmers that use two monofilament lines typically are more powerful and can inflict greater damage.

Young trees have relatively thin bark. If the line is allowed to hit the trunk, part of the bark will be removed with each contact of the line. If you are not careful, you might even remove an entire ring of bark all the way around the trunk-thus girdling the tree.

Even more, mowers pushed hard or dragged around the base of young trees can be almost as damaging. To explain why this damage is such a problem, the part of a tree's "circulatory system" that carries food manufactured by the leaves downward to the roots (which can make no food for themselves) lies just under the bark.

Damage that occurs when string trimmers or mowers remove patches of bark interferes with the tree's ability to send food to its roots. As the roots are deprived of food, they become stunted and function poorly, and this leads to a stunted, unhealthy tree. If you remove a complete ring of bark, you may cut off food to the roots altogether, leading to the death of the plant.

In addition to interfering with food movement, the open wounds created by mowers and trimmers can provide entry points for disease organisms that can cause decay.

Many sickly, stunted trees that were planted years ago but haven't grown well have been damaged in this way. Look at the base of their trunks, and you will often see scars and callus growth from repeated injury done to the base of the tree.

To prevent these problems, do not allow grass to grow close to the base of young trees for the first three to five years after planting. Keep an area at least a foot out from the trunk free of grass. A mulch 2- or 3-inches thick spread evenly over the area, but pulled back slightly from the trunk, will help a lot.

Shrubs are generally planted in beds, so they are less at risk. But I have seen this problem occasionally when ground covers, such as Asiatic jasmine, are trimmed away from the base of shrubs with string trimmers.

Whether you maintain your landscape yourself or pay someone to do it for you, don't let this kind of needless damage happen to your trees and shrubs.

Don't Damage Roots

Trees also are vulnerable to root damage from construction and/or filling. If you plan on doing construction- whether building a new home, adding on to an existing one or even putting in a patio or repairing driveways or sidewalks-tree roots will likely be an issue.

Tree roots extend well beyond the reach of the branches, and most feeder roots (those that absorb the water and nutrients from the soil) are located in the upper 8 inches to 12 inches of the soil. This makes them much more vulnerable to damage than most people realize.

If you will be doing construction or filling around valuable existing trees, consider consulting with a licensed arborist before the work is done to make sure the trees are damaged as little as possible.

Be Careful With Pesticides and Fertilizers

Another way gardeners damage landscape plants is to use pesticides and fertilizers improperly. These products are useful and sometimes necessary in maintaining a healthy attractive landscape. But if they're misused, they can do more harm than good.

Pesticides commonly used in the landscape include insecticides (to control bugs and other insects), fungicides (to control diseases caused by fungal organisms) and herbicides (to control weeds). Landscape plants can be damaged by all three, but most damage occurs from insecticides, because we use them more often than other types, and herbicides, because they are designed to kill plants.

You can avoid damaging plants with pesticides by reading and following label directions carefully. I know the print can be very fine, but do like me and get out the magnifying glass if you need to. Without complete information on how to use a product, your efforts may be wasted, because applying the pesticide incorrectly does not control the pest, and you may even injure the plants you were trying to help.

Insecticides will list on their label which plants may be damaged by them and any temperature limitations (some insecticides will damage plants if applied during hot weather). Many insecticides also will burn or damage plants if you mix them too strong. You can see how important it is to know of these potential problems and avoid them by following label directions.

Since herbicides are designed to kill plants, we must be particularly careful when using them around desirable ornamentals. Again, read the label to make sure the herbicide will do the job you need it to do and to understand how to use it properly.

Many people also damage plants with fertilizers or plant food. Gardeners often think if a little is good, more is even better but fertilizers should never be applied stronger than label recommendations. You may apply less than is recommended, but mixing fertilizer stronger or applying more than is recommended on the label can lead to serious damage to your plants.

Practice being an ally to your landscape trees and shrubs-you will be rewarded with plants that are healthy and less of a burden on your pocketbook.

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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