Horticulture Forum: April 1999, Recognizing Tree Decline

Tree Decline is a difficult term to define because so many factors can cause it to occur. Some of the most common symptoms include stunted growth, premature leaf drop, late spring leaf development, sparse foliage, leaves that are light green or yellow, and twig and branch dieback.

Once in decline, trees often never recover – this is because the most visible signs of decline only become apparent after two or three years of stress. By this time, the tree has used up much of its reserves trying to survive.

Helping trees avoid the perils of decline requires the landscape manager to recognize what environmental conditions, both man-made and natural, cause the stress responses that lead to decline.

Poor Growing Conditions. Many species of trees have specific site requirements, such as soil texture and pH, nutrient levels and drainage that must exist for the tree to thrive and overcome other stresses. Urban landscapes create many other problems, such as poor soil quality, proximity of buildings, sidewalks, streets, utility pipelines and other trees.

Diseases and Insects. Most trees are able to withstand some leaf loss, but several years of it due to insects and diseases can lead to decline.

Low Temperature Injury. There are three primary ways low temperature can affect trees. Winter desiccation, especially in evergreens, is caused by dry, freezing winter winds as the frozen ground prevents the tree from taking up water from the soil.

Freeze damage can occur when late or early hard freezes hit before a tree has gone dormant or after a tree has come out of dormancy. Trees that are marginally hardy for colder zones are especially susceptible.

Sunscald is common in thin-barked trees. Injury occurs one of two ways – either the living bark tissues suffer dehydration following exposure to winter sunlight or tissues in the bark are killed due to rapid temperature changes after sundown in winter.

Chemical Injury. Herbicides used in the lawn may have an adverse affect on trees, as can the improper use of insecticides and fungicides.

De-icing salts can also be harmful to trees, especially sensitive species such as crabapples and white pines.

Physical Injury. Adding or removing soil around trees can also cause stress to the root system, as well as the compacting effect of heavy equipment operated on top of the root zone. Other equipment that removes bark, such as a string trimmer or mower, may allow for insect and disease infestation.

Girdling Roots. Occasionally, a root will grow around the circumference of a tree at or slightly below the surface of the soil, restricting water flow.

What To Do? Here are some guidelines for avoiding a decline situation:

• Start out right. Plant trees that match the site, and consider the limitations of existing hardscapes and other vegetation on the new trees.

• Avoid damage. Mechanical damage can be avoided by mulching a 4-foot diameter circle around the trunk 2 to 6 inches thick. The mulch also mimics the natural nutrient recycling process that occurs in wooded areas.

• Water well. During drought periods, anticipate stress responses by watering either above or below ground. Once-a-week waterings are usually adequate, but it makes sense to check the soil moisture level with a soil probe to ensure this is working.

• Fertilization. Proper fertilization helps the tree get the nutrients it needs, especially in problem soil. A soil test is the best way to determine what the tree does and doesn’t have available. Fertilizer should be applied at the drip line at the proper rates and is most effective when applied in spring just before growth occurs.

Information for this article was provided by the department of horticulture, Iowa State University, Ames, Iowa.

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