Technical Services Topic – Hazard Tree
Management On Utility Corridors

By Geoff Kempter
Technical Services Manager

To be effective, a hazard tree evaluation program must systematically rank defects in trees. The presence of fungal fruiting bodies is an indicator of internal decay, but the relative risk posed by the tree may depend on other factors, including line priority, size of the tree, and the condition of other trees on the system.

Trees add immeasurable beauty to urban and rural landscapes, in parks and along roadways. However, along with the desire for beautiful shade trees also goes an inherent risk posed by trees in congested urban areas. What level of risk is acceptable?

We cannot predict when any tree or tree part will fail, but we know that every tree will eventually come down. The object of a hazard or danger tree program is to remove the trees or tree parts posing the greatest risk to people or structures before they fail unexpectedly.

Even on individual properties, with limited numbers of trees, assessing the risk posed by each tree can be a complicated and imprecise task. For example, imagine the care the National Park Service must take in assessing the risk posed by trees growing on the grounds of the White House! Then consider a large electric utility that may have hundreds of thousands of trees on or near its rights-of-way, all of which pose some degree of risk. Assessing the relative risk posed by these trees is part of comprehensive vegetation management.

In the example of the White House, each tree would be thoroughly inspected for defects on a regular basis, often using specialized equipment and time-consuming techniques. It is not practical for utilities to provide this level of care with thousands of trees to inspect.

However, risk can be reduced by systematically looking for significant, easily detectable defects. By targeting these higher-risk trees, utilities can reduce the overall threat of service interruptions due to tree failure.

A close examination of this tree showed a split in the trunk reaching almost down to the ground. In addition, the lack of root flare indicated that topsoil had probably been added to the base of the trunk, increasing the risk of failure.

The exact methods used by individual utilities in designing a hazard tree management program will vary depending on many factors, including past maintenance practices, species mix, line location (street side vs. backyard) and line priority. At minimum, a program should attempt to locate and rank the risk posed by trees on the utility right-of-way. Certain defects are quickly and easily assessed, and can be predictors of imminent failure.

Obvious signs include deadwood, hanging limbs, major structural cracks, and large cavities or other visible areas of decay. More subtle signs might be decayed areas in the crown around old pruning wounds, hidden cavities, conks or mushrooms on the bark, and weak branch attachments within the crown. Especially difficult to detect can be hidden pockets of decay in the stem or roots, or root damage caused by construction, later covered by landscaping.

Nobody can accurately predict when any tree will fail, but with a well-designed hazard tree management program, utilities can significantly reduce the risk posed by trees on their rights-of-ways. For more information about hazard tree assessment, a number of publications are available through the ISA (toll free 888-ISA-TREE or www.isa-arbor.com). You may also contact Asplundh Technical Services in Willow Grove at ext. 1362.

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