Common Urban Tree Problems
Georgia Forestry Commission Forest Health Monitoring Factsheets
Written by Terry Price, Georgia Forestry Commission
Adapted for the web by the Bugwood Network
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Urban shrubs and trees are left in a development or planted to beautify the surroundings of which they are a part. The plants must have a healthy vigorous appearance to be attractive. For these conditions to exist the various plants must be suitable to the site characteristics. A shade loving tree or shrub would not be planted in full sunlight, nor would a plant requiring full or partial sun be planted in deep shade. The plant must be suited to the temperature extremes that normally can be expected to occur from year to year. The moisture and nutrients required by the various plants must be within reasonable limits to that offered by the site, and to the amount of available growing space in which the tree or shrub will be planted.
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When homes, apartments, shopping centers or office complexes are under construction, trees and shrubs remaining on the site may be subjected to many abuses. More common causes of plant mortality around construction sites are destruction of roots by trenching, mechanical bruising and wounding, soil compaction of the roots by heavy equipment and grade changes resulting in lowering of the water table or excess soil moved over the root area of the plant (Figure 1).
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Figure 1
photo by USDA Forest Service Archives |
Extensive grade changes and cuts within the drip line of trees may upset the water balance so, that trees die from symptoms similar to that of drought even though weather conditions are normal. Excessive cuts or earth removal several hundred feet away may so drastically alter the water table that site conditions become too wet or too dry and the tree is lost. It would probably be best to clear the area around major construction sites and replant with two to three inch caliper trees of acceptable species after the site is completed and final grading is accomplished.
Some tree species will die when suddenly released from all surrounding vegetation as in a closed stand. Dogwood trees suddenly released in this fashion often decline in health and eventually die.
Any effort to save trees around a proposed construction site should begin prior to actual construction. The trees that are to be left standing should be selected and marked. All phases of construction should be kept in mind during this selection phase. It would be rather futile to attempt to save a tree where a large cut is required immediately next to it, or where excessive grade changes are required. The
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contractor should be given explicit instructions as to procedures and care to be taken to prevent injury. Boardwalks may be constructed under the drip line to prevent excessive soil compaction and heavy machinery should not be allowed within this area. No large piles of dirt or debris should be piled around the trunk or within the drip line. Trees to be left should be prominently marked with tape, ribbon or other means that do not damage or wound the bark (Figure 2).
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Figure 2
photo by USDA Forest Service Archives |
If excessive soil is removed from around the tree a temporary wall should be immediately constructed and backfilled and water should be applied to drench the soil. Roots are very sensitive to air and dry out quickly, which results in the death of part or all of them. It is best to install the final retaining wall whenever possible. In all cases, soil
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disturbance, by removal, addition, or compaction should be kept to a minimum. As little as three inches of additional soil (heavy clay) placed over the root area may be sufficient to reduce the aeration of the soil and cause the death of a tree. Tree wells, properly constructed, may allow a fine specimen tree to survive and grace the site for many years. However, the amount of time and effort to properly construct a tree well is seldom expended. If a tree well is to be constructed, proper methods as shown in various texts on tree care should be closely followed (Figure 3).
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Figure 3
photo by Terry Price |
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The root system of a tree needs the proper amount of air and water in order to maintain satisfactory growth. Trees growing near streets, driveways, and sidewalks or in parking lots can be described as growing in an asphalt jungle. We do everything possible to keep our trees from getting adequate amounts of air and water to the root system (Figure 4). Yet we seem surprised if the tree appears sickly. Trees with limited root space should be periodically watered during droughts, aerated or vertically mulched and fertilized.
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Figure 4
photo by Bill Godfrey |
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It is not uncommon in new urban developments to find large old hardwood trees with butt scars caused by previous fires or other types of injury. These wounds are often unsightly but there is little one can do to correct the damage. However, the wound should be cleaned out to sound wood by removing the decayed wood to facilitate wound closure (Figure 5). If the owner wants the cavity filled it should be done with a pliable asphalt material that will not harden and will give when the tree is swayed by the wind. Never fill cavities with a rigid material that may separate from the wood, creating a moist chamber ideal for additional decay development.
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Figure 5
photo by Georgia Forestry Commission Archives |
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After high winds, heavy snow or ice storms, many limbs are broken from trees. These broken branches need to be removed for safety and aesthetic purposes. When removing such damage, the trees should be climbed with ropes and never with spurs. Spur wounds on pine trees are attractive to bark beetles and those on hardwoods may be suitable for entrance of heart rot fungi.
Lightning often strikes urban trees. Trees most likely to be struck include oak, pine, poplar, and maple. Lightning that spirals down the tree is more likely to kill the tree than a single streak (Figure 6). Lightning struck trees are often infested with ambrosia beetles and other wood boring insects within days of the strike. Oftentimes if insects are kept out of lightning struck trees, the trees have a good chance of surviving.
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Figure 6
photo by Robert L. Anderson |
Lightning scars should have the loose bark and splinters removed and the edges smoothed. As most lightning struck trees are tall, wounds will usually have to be treated by a competent tree climber. Television and radio antennas should never be installed in trees since they often attract lightning.
Homeowners ordinarily do not consider fertilizing their shade trees; however, trees have nutritional requirements for growth and health. A tree may be fertilized by applying a granular fertilizer into pre-drilled holes in the soil, or with liquid fertilizers using hydraulic injectors or as foliar sprays. The most frequently used and easiest method is broadcasting a granular fertilizer on top of the soil. Broadcasting is not
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recommended when trees are located on slopes where runoff can occur or where heavy sod (grass) is present. These situations would warrant drilling holes in the soil as shown in figure 7. Start four feet from the tree base, punch or bore holes about two feet apart in concentric circles out to the drip line. The holes should be about two inches in diameter and 6 to 12 inches deep. The amount of fertilizer to be placed in each hole will depend on the size and age of the tree. Homeowners should have the soil tested by their county extension agent before applying a fertilizer. A soil analysis will provide specific recommendations to follow.
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Figure 7
photo by Tom Hall |
It is necessary to inspect trees regularly to keep them beautiful and in good growing condition. They occasionally need to be pruned. Pruning will improve the appearance, guard its health and make it stronger. If pruning is done when the need arises, it can easily correct defects that, if delayed, would require major surgery. Pruning should remove (1) dead or dying branches; (2) branches that grow toward the center of the tree; (3) branches that rub each other; (4) V-crotches on young trees, (5) more than one leader; and (6) nuisance growth, such as branches that
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interfere with wires, streetlights, create traffic hazards, cut off cooling breezes or interfere with mowing. Care should be taken when branches are removed. Small branches up to 2 inches in diameter can be easily removed by sawing. Branches more than 2 inches in diameter should have an undercut to prevent bark stripping below the cut. Stubs should not be left on the tree trunk. The pruning cut should be made just beyond the collar ridge to promote maximum wound closure (Figure 8).
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Figure 8
photo by Tom Hall |
Pruning may be done at any time of the year, but for faster wound closure, it is recommended late winter or early spring. Pines should be pruned in winter if possible to prevent pine bark beetles. For safety sake, dead or broken branches should be removed before they fall and injure someone.
Arborists in recent years have been told by professionals in forest research not to use wound dressings any longer when pruning trees. Although research indicates wound dressings do very little to prevent wood decay from pruning wounds, indications are they can discourage attacks by elm bark beetles, southern pine bark beetles and insects that transmit the oak wilt fungus. Wound dressings may prevent infection from diseases like pitch canker. They have also been recommended to protect wounded roots from soil contact. Severed roots should be trimmed to remove splintered wood and then treated with an asphalt base wound dressing. Homeowners should check with their local forestry expert before pruning to see if a wound dressing is warranted.
Another common type of injury in urban areas is that caused by air pollution. Densely populated areas are more subject to pollution damage because of industrial complexes and the concentration of automobiles. Four common types of air pollutants are; sulfur dioxide, fluoride, chlorine and oxidants. Sulfur dioxide forms as a result of burning coal and fluorides by the manufacture of brick, pottery, steel and phosphate fertilizers. Ozone is a naturally occurring oxidant present in the upper atmosphere. Ozone is the major ingredient in urban smog and may be formed as a result of hydrocarbons and other materials contained in auto exhausts. Air pollutants may cause damage in very low concentrations to the foliage (Figure 9 & 10).
Figure 9 - Chlorine damage to loblolly pine
photo by Robert L. Anderson |
Figure 10 - Ozone damage on maple
photo by Robert L. Anderson |
Oftentimes homeowners desire to plant large caliper trees. Trees larger than 4 inches in diameter should not be transplanted. Although larger trees can be successfully planted, the failure rate can be very high. Balled trees should be planted to the original depth. Too often, landscape companies will elevate the ball several inches above ground
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and then apply a 12” or greater mulch layer (Figure 11). This is not a wise practice and should be avoided. The mulch layer should be no more than 3 to 4 inches deep. When it is necessary to elevate the ball above the ground a few inches, the ball should be covered with soil and then the 3-4 inch layer of mulch. The planting hole should be about twice the size of the ball to cradle the roots properly. The cord or wire tied at the top of the ball should be removed before the tree is planted. Surprisingly, this is an often-overlooked problem in urban plantings. Some balled trees are cradled in wire baskets. These baskets should be removed prior to planting. Native tree species should be favored over exotic species when practical.
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Figure 11
photo by Bill Godfrey |
The same insects and diseases that attack forest trees attack shade trees. Some insects and diseases most likely to be found on shade trees are southern pine beetle, black turpentine beetle, Ips engraver beetles, eastern tent caterpillar, mimosa webworm, bagworm, fusiform rust, annosus root rot, needle cast, spot anthracnose of dogwood and Dutch elm disease.
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