Recognizing hazardous trees
Damaged or unhealthy trees that may fail (drop limbs or fall over) are hazardous if located where they are likely to injure people or damage property. Conversely, in more natural settings dead and declining trees provide benefits such as recycled nutrients and cavities for wildlife habitat.
Warning signs that trees may be hazardous include
- brackets, conks, mushrooms, or other fungal decay fruiting bodies on bark or around the tree base
- cankers or wounds in bark or wood
- cavities on the main trunk or at the tree base
- cracks in the main trunk or at crotches (where limbs fork)
- dead or dying limbs
- fissures in soil near the base of trees
- trunks that lean or tilt instead of growing upright
Some hazards are difficult to detect, such as internal decay or unhealthy roots. For example, a lightning strike can seriously damage roots or internal tissues of limbs and trunks even though damage is not visible externally or appears to be minor. Be aware that there are many causes of mechanical injury, and look-alike damage can be due to other maladies.
Some tree hazards can be avoided by preventing injuries to aboveground parts and roots. Take steps to protect trees during construction or they may decline, become hazardous, or die as a result of construction-related damage.
Examine trees regularly to ensure that they are receiving proper cultural care and being protected from injuries and are not hazardous. Look for signs that indicate a tree may be hazardous as listed above.
If trees are located where their failure could cause injury or damage property, have them regularly inspected by a competent expert, such as an arborist who is certified by the International Society of Arboriculture or registered with the American Society of Consulting Arborists. See Recognizing Tree Hazards: A Photographic Guide for Homeowners for more information.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).