Inspecting Trees For Hazards
Gary W. Moorman, Professor of Plant Pathology
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...
- human activity near tree noting how often and how many people are near tree
- value of property near tree
ALWAYS document your work in writing. Note...
- inspection procedures used
- observations made and any measurements taken
It is wise to photograph the tree from several different angles and distances to document your observations.
- your conclusions including the specific hazard posed and the magnitude to the threat posed
- 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 breanch 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.
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