Tree Hazard Inspection: Identifying Structural Defects and Disease Risks

Defining a Hazard

A tree presents a hazard if a defective part poses a high risk of structural failure causing personal injury or property damage. Also a hazard if it obstructs routine activities (blocking vision, raising sidewalks, interfering with utilities/signs). Arborists, landscapers, and grounds personnel must recognize real hazards.

This fact sheet is a brief, incomplete summary. References at end provide thorough information.

Evaluation Priorities and Documentation

Priority: threat to humans and property. Record:

1. Human activity (frequency, number of people near tree)
2. Property value near tree

Always document in writing:

1. Inspection procedures used
2. Observations and measurements

Photograph tree from multiple angles/distances. Record conclusions (specific hazard, threat magnitude) and recommendations. Keep records of conversations (dates, face-to-face/phone).

Inspection Process

1. Inspect all trees that could pose a hazard.
2. Identify those that actually pose a hazard.

All trees, regardless of previous health, should be inspected after severe storms. Structural defects best detected in late fall, winter, early spring (no leaves). Disease defects best detected late summer/early fall (dead branches obvious, fungal fruiting structures apparent).

Factors to Consider

Site Conditions	Biotic/Abiotic Factors	Human Activity
Depth of bedrock	Pathogens/diseases present	Nearby tree removal
Soil type	Insect activity/severity	Excavation
Prevailing winds direction	Drought/flood effects	Root removal
Erosion near tree	Tree community makeup	Paving obstructing roots
		Past pruning practices
		Wind damage
		Mower injuries, etc.

Evaluating Defects: 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 crack	Any branch with up to 66% side branches dead
4"+ diameter branch with crack	Branch with sharp bend
2+ cracks in same trunk/branch	Lopsided/unbalanced crown (especially if nearby trees pruned/removed last 10 yrs)
Crack allowing independent movement on either side
V-shaped branch union with in-rolled bark
Cankers and rot with conks/shelf fungi
Cankers affecting >50% circumference
Canker connected to a crack
Dead tree
Dead branch
>50% root system severed within drip line
Tree leaning >45°
Leaning tree with any defect on trunk below first branch above ground

References

• Metheny, N.P. and Clark, J.R. 1994. A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas. 2nd 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 Dept. of Natural Resources, St. Paul, MN. 63 pp.
• Also helpful: Guide for Establishing Values of Trees and Other Plants. International Society of Arboriculture, Savoy, IL.

Help for Homeowners

For full evaluation, contact a Certified Arborist (ISA Certified). Check yellow pages or electronic equivalent. Or contact local county Cooperative Extension office for assistance locating a certified arborist.

Prepared by Gary W. Moorman, Professor of Plant Pathology.