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Inspecting Trees For Hazards

A tree presents a hazard if a defective part poses a high risk, upon its structural failure, to cause personal injury or property damage.

A tree is also a hazard if it obstructs the routine activities of people such as blocking motorists' vision, raising sidewalks, or interfering with utilities or signs. It is imperative that arborists, landscapers, and grounds maintenance personnel are able to recognize that a particular tree presents a real hazard.

This fact sheet is a very brief and incomplete summary of some of the information that should be considered when evaluating a tree. The references given at the end of this sheet provide more thorough information

When doing an evaluation, priority must be given to the threat to humans and to property. Record the...

  1. human activity, noting how often and how many people are near tree
  2. value of property near tree

Always document your work in writing. Note...

  1. inspection procedures used
  2. observations made and any measurements taken

It is wise to photograph the tree from several different angles and distances to document your observations.

  1. your conclusions including the specific hazard posed and the magnitude to the threat posed
  2. recommendations

Keep good records of conversations (face-to-face and telephone) including the dates on which they took place.

First, inspect all trees that could possibly pose a hazard. Then, identify those that actually pose a hazard.

All trees, regardless of previous health, should be inspected after a severe storm.

Structural defects are best detected in the late fall, winter and early spring when there are no leaves on the trees. Defects caused by diseases are best detected in the late summer and early fall when dead branches are obvious and fungal fruiting structures are most apparent. The following are just some of the factors, related to the tree itself, to consider during your inspection.

Site conditions Biotic\abiotic factors Human Activity
depth of bedrock pathogens or diseases present neaby tree removal
soil type insect activity and severity excavation removal of roots
direction prevailing winds drought or flood effects paving obstructing roots
erosion near the tree tree community makeup nearby past pruning practices

wind damage injuries from mowers, etc.

Evaluate the importance of any defects that are observed on the tree.

Potential Failure
High Moderate
Cracks with decay Callus closing crack
Cracks with callus forming V-Shaped branch union with upturned bark
In-rolled bark widening the crack Any branch with up to 66% of side branches dead
4" diameter or larger branch with crack Branch with sharp bend
2 or more cracks in the same trunk or branch Lopsided or unbalanced crown, especially if nearby trees were pruned or removed in last 10 yrs.
Crack that allows independent movement on either side of the crack
V-shaped branch union with in-rolled bark
Cankers and rot with conks of shelf fungi
Cankers affecting more than 50% circumference
Canker connected to a crack
Dead tree
Dead branch
More than 50% of root system severed within the drip line
Tree leaning > 45%
Learning tree with any defect on the trunk below the first branch above the ground

References

  • Metheny, N. P. and Clark, J. R. 1994. A photographic guide to the evaluation of hazard trees in urban areas. Second Ed. International Society of Arboriculture, Savoy, IL. 85 pp.
  • Albers, J. and Hayes, E. (principal authors). 1993. How to detect, assess and correct hazard trees in recreational areas. Minnesota Department of Natural Resources, St. Paul, MN. 63 pp.
  • Also helpful in examining trees in general: Guide for establishing values of trees an other plants. International Society of Arboriculture, Savoy, IL.

Help for the homeowner

In order to have a tree fully evaluated, it is suggested that a Certified Arborist (International Society of Arboriculture, ISA Certified) be contacted to examine the tree. Consult the yellow pages of a telephone book or the electronic equivalent to locate a certified arborist. Or, contact your local county Cooperative Extension office for assistance in locating a certified arborist.

Prepared by Gary W. Moorman, Professor of Plant Pathology

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Inspecting Trees For Hazards

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