A healthy shade tree planted in the right place increases the value of the property, provides shade and beauty, improves the environment, lessens heat and cold and reduces noise and wind velocity.
Yet, a quick look at some of our urban trees will show that many are not getting good treatment, according to Dr. Alex L. Shigo, chief scientist, plant pathology, U.S. Forest Service, Northwestern Forest Experiment Station, Durham, N.H. Shigo has spent a major portion of his research career studying problems of trees.
We seem to see more and more urban trees in trouble, he says. Is it because we are becoming more aware of our trees? Or is it because the condition of our trees is declining? Probably both.
What can we do to help them? Here are some answers.
Trees, like people, are easily disturbed by changes in their surroundings. Increasing the amount of soil over their roots may make it more difficult for the roots to get oxygen. Trees so covered have trouble breathing and may die within 3 to 5 years. If possible, avoid adding excessive soil or impervious materials in the area under branches of a tree.
Prolonged flooding or inundation may kill trees, especially during the growing season. Flooding has an effect similar to that caused by covering roots with excess soil, asphalt, or concrete.
These trees are relatively tolerant of flooding and can be placed in low, wet places, Shigo says: Ash, cottonwood, elm, loblolly pine, overcup oak, red maple, river birch, silver maple, sweetgum, sycamore, white cedar and willows.
These trees are relatively intolerant of flooding and should not be planted in low, wet places: Hemlock, paper birch, ponderosa pine, red cedar, red pine, white pine and white spruce.
Soil compaction around trees is often caused by people, pets, bicycles and cars in parks and other recreation areas, heavily used areas surrounding public buildings, business centers, and multi-unit dwellings.
Compacted soil cuts off water and oxygen to tree roots. Dying leaves on mature trees and dying branches on young trees may indicate compaction injury.
The main gaseous air pollutants that injure trees are sulfur dioxide, fluorides, and oxidants. The most serious forms of air pollution are difficult to prevent without a community effort. Minimize the air pollution you produce by keeping your automobile tuned, limiting your use of internal combustion engines and obeying local openburning ordinances.
Lawnmowers, snowplows, and other lawn and garden equipment can severely injure trunks, branches and roots of trees. These injuries can be as serious as those caused by heavy construction equipment.
Be careful not to run into your trees with lawn and garden equipment. To prevent accidentally injuring young trees grass should be kept away from tree trunks. Don't use chemicals to kill the grass.
Herbicides (weed killers) can kill your trees. Even when properly applied, the chemicals may drift through the air and injure non-target plants. Avoid use of commercial fertilizer-herbicide (weed and feed) mixtures near trees because toxic amounts may be absorbed by tree roots. Recently planted trees are especially susceptible to such injury.
De-icing compounds used on highways, driveways and sidewalks in winter contain sodium chloride (table salt) and/or clacium chloride. These chemicals are toxic to trees.
Although tree wounds are often caused by the forces of nature, people are also responsible for many root and bark injuries and broken limbs. Trash fires and campfires built near the trunk of a tree may cause injury and kill the tree.
Improperly treated wounds are often followed by decay, Shigo says. Wounds should be treated by removing dead and torn bark tissue and by shaping the wound into a vertical oval. This will help the tree to quickly heal its wounds. Commercial tree dressings such as orange shellac or those with an asphalt base can be painted over the treated wound, for cosmetic purposes. However, no dressing will prevent decay.
Improper pruning is often followed by disease or decay. It is the worst thing a person can do for a tree. All of the text books state that all branches should be pruned flush with the stem. This is definitely not so. When branches are small, the recommendation is correct, because the branch collar on small branches is flush with the stem.
As the branch grows larger the branch collar increases in size. As the branch begins to wane and die, the branch collar increases in size rapidly. When pruning such branches, great care must be taken not to remove the branch collar. The cut should be as close to the collar as possible, but not into the collar.
A protective chemical barrier forms within the branch collar and if the collar is removed, then the chemical protective shield formed by the tree is destroyed.
Branch stubs permit invasion by decay-producing organisms. Remove injured or diseased branches before they die, says Shigo.