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Decline in Trees and Shrubs

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When shade trees and shrubs gradually lose vigor and display pale green or yellow color, small leaves, poor growth, early leaf drop, early fall color, and dieback of twigs and branches, it is referred to as tree “decline.” There are some “declines” caused by specific diseases and environmental stresses, but here we discuss a general decline in which no specific cause has been identified. Symptoms observed in the above-ground parts of the plant usually indicate problems in the root system.

Over a period of years, trees and shrubs may be subject to insect attack, disease, adverse weather conditions, and other stressful environmental factors. Soil compaction, changes in drainage, soil fill over the root system, mechanical damage to roots from construction, and excess salt accumulation are examples of environmental stress factors affecting root systems. These stress factors alone, or collectively, reduce growth and may cause dieback. Also, sidewalks, roadways, and building foundations may restrict root growth and subject the tree to further stress. Shade and ornamental trees and shrubs are especially vulnerable to “people pressure” problems and other adverse environmental conditions because they are often planted in locations unfavorable for optimum growth. Weakened plants are subject to attack by so called “secondary” agents. Trees weakened by drought, for example, are much more susceptible to attack by canker-causing fungi and borer-type insects than healthy, vigorous plants.

Once the symptoms of tree decline begin, it is difficult to stop or reverse the progression. PREVENTION is the key to control, and there are three important steps to follow:

Match the tree or shrub to the site.

A common mistake is the selection of trees or shrubs that will grow to a size too large for confining sites, such as between the sidewalk and street or next to a building. Also, many tree species have very specific site requirements and grow poorly in certain locations. Common examples include planting pin oak in alkaline soils (pH 7.0+) or white pine in poorly drained soils. In both cases, trees eventually show typical symptoms of decline.

Maintain tree health.

The single most important step in maintaining good vigor in trees and shrubs is to supply adequate water. With normal rainfall and a favorable site, adapted trees and shrubs require only occasional additional watering. However, during periods of drought or on dry sites, regular watering is important.

Avoid changes in the growing site.

Any change in the site conditions of a tree or shrub may cause decline. A delicate balance exists between the plant root system and its soil environment. Damage to roots from trenching or construction, change in grade by filling over or cutting away the root zone soil, a change in drainage, or other site changes, often result in root damage and tree decline. As indicated, such decline is usually irreversible, so prevention is the key to control.

Trees in the early stages of decline can sometimes be stabilized through proper management. First, follow the recommendations outlined above for routine watering. Next, a competent arborist should check the tree for problems such as girdling roots, unfavorable soil pH, damage by borer-type insects and diseases, and treating the trees if necessary. Pruning may be necessary to remove dead wood and reduce the crown size. With proper care and management, the rate of decline may be reduced and further problems prevented. Some information contained in this brochure was adapted from Purdue Cooperative Extension Service publication.