Continuing Education and
Communication Services
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
8/27/01
Contacts:
Chris Feeley, Extension Forestry, (515) 294-1458, e-mail cfeeley@iastate.edu
Del Marks, Continuing Education and Communication Services, (515) 294-9807,
delmarks@iastate.edu
Yard and Garden Column for week beginning Aug. 31
Identifying Hazardous Trees
By Chris Feeley
Extension Forestry
Iowa State University
Think of all the benefits that trees add to our lives. Can you imagine going to your local park to set up your picnic in late summer with no shade? Or going on a nature hike through the local convenience store's asphalt parking lot? Of course not. Trees add to our enjoyment of the outdoors whether we are in a forest, park, or urban landscape.
Despite our appreciation of trees, we must keep in mind that trees have a life span and that some day they may become hazardous. Interest in hazard tree management has greatly increased in recent years due to safety and liability concerns. Recognizing hazard trees and taking proper action can protect property and save lives. The ability to recognize hazard trees takes years of experience, but this article will cover some of the basics that may help homeowners recognize possible hazard trees.
A hazard tree is loosely defined as a tree with structural defects likely to cause failure of all or part of the tree, which could strike a target. The key words of that definition are "could strike a target." For a tree to be hazardous it must have a potential target. A dead tree with large branches falling would not be of concern in one of Iowa's forests. On the other hand, that same tree along a city street or in a playground has many potential targets. A target can be a vehicle, building, or simply a place where people gather such as sidewalks, parks, streets, and yards.
Trees should be inspected once a year for hazardous defects such as dead wood, cracks, weak branch angles, included bark, decay, cankers, and root problems. Certain tree species are prone to specific types of defects. For example, hackberry trees tend to form weak branch angles with included bark making them prone to splitting. Trees known to be prone to defects, or trees in poor condition, should be watched carefully and managed. To identify potential hazard trees, look for the following:
Dead wood: Dead trees or large dead branches must be removed immediately. Dead wood is often dry and brittle and cannot bend in the wind or with the weight of snow or ice. Take immediate action if a tree is dead, a broken branch is lodged in a tree, or if you find a dead branch of sufficient size to cause damage or injury.
Cracks: Cracks are deep splits through the bark that extend into the wood of the tree. Cracks are considered very dangerous since they already indicate that the tree is failing. The tree should be removed if a crack extends deeply into or through the trunk of the tree, two or more cracks are present, or the cracks are in contact with other defects mentioned in this list.
Weak branch angles: In general, the wider the angle of the branch the stronger it is. Likewise, the narrower branch angles are weaker. However, some trees, such as little leafed lindens, have very narrow branch angles and are not prone to breaking. Once again, it may take a professional to detect weak branch angles. Proper pruning of young trees can prevent weak branch angles. To learn more about proper pruning of young trees, contact your local ISU county extension office for publications SUL 5, Pruning Trees and Shrubs ($1), and PM 1591, Community Tree Planting and Care Guide (single copy free, also available on the World Wide Web at www.extension.iastate.edu/pubs/).
Included bark: Included bark usually forms where two or more branches
grow closely together (weak branch angles) and the bark grows between
the branches inside the branch union. Bark does not have the same strength
of wood, and the union is much weaker than one without included bark.
Action should be taken if a weak branch angle with included bark occurs
on the main stem, an area with included bark is cracked, or the included
bark area is associated with any other defect. Once again, proper pruning
can help prevent included bark.
Decay: Decaying trees have an increased risk of structural failure, but
the presence of decay by itself does not make the tree a hazard tree.
Advanced decay where the wood is hollowed out, soft, crumbly, has a foul
odor, or has the presence of mushrooms/conks can create serious hazards.
Take immediate action if advanced decay is present or associated with
weak branches or included bark.
Cankers: A canker is defined as a localized sunken area on the trunk or branch. Wounding or disease can cause cankers, and the presence of cankers increases the chances of the trunk or branch breaking. The tree/branch should be removed if the canker affects more than half of the trunk's/branch's circumference.
Root problems: Root problems are the hardest to detect. Trees with root problems are more likely to blow over in windstorms and may fall without the presence of wind. There are numerous kinds of root problems that should be considered. The most common type of root damage in the urban landscape is severing the roots for streets, curbs, building foundations, sidewalks, and pipelines (e.g., sewer and gas). Severing the roots decreases the support of the tree, which increases the chance of failure or death of the tree. In addition, burying the roots deeper by adding backfill around the tree greatly increases the chance for root mortality, which reduces the support of the tree. Likewise, root decay will increase the chance of failure. This condition is very difficult to diagnose and professional opinion should be sought. Indications or root failure may include tree death, leaning, and mounding soil around the base of the tree.
Hazard tree related accidents can be minimized by carefully examining trees annually for the defects listed above. Contact your local arborist or tree service company for advice on managing trees that you have identified as being potentially hazardous. Remember that proper plant selection and pruning can eliminate many potential hazardous defects. A good landscape plan always includes tree maintenance and replacement when needed, which preserves the health of our trees and ensures a safe outdoor experience.
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