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Seven Signs of Hazardous Trees


Healthy trees are beautiful, ecologically important components of local habitats, but unhealthy trees represent a significant safety hazard. While it is impossible to determine exactly when or if a given tree will fail, several clues can indicate that a tree is at increased risk of dropping branches or falling over completely. 

Learn to recognize these seven signs associated with hazardous trees, and have an ISA-certified arborist perform a tree evaluation for all that display such problems.  

Codominant Leaders

Strong branch unions feature wide angles of attachment. This means that healthy junctions should look more like the letter “U??? than the letter “V???. Arborists call V-shaped junctions codominant leaders or stems. Codominant leaders fail more commonly than wider branch junctions do, especially when they form as part of the main trunk. 

Significant Lean

Leaning trees are frequent victims of blowdown. Trees with a slight lean are often reasonably safe, as trees often compensate by producing something called reaction wood, which helps offset the stress caused by the tree’s tilted posture. However, trees cannot always produce enough reaction wood – or produce it quickly enough – to offset the pull of gravity. This is especially true when a tree shifts suddenly; mounding soil is commonly seen at the base of extremely hazardous trees. 

Large Dead Branches

Trees shed dead or decaying branches as part of a process called compartmentalization. In this way, the tree can protect its trunk, roots and the rest of the crown by dropping branches before the decay spreads. While this is an effective survival strategy for the tree, it is very dangerous for those people or items that are underneath the tree. Additionally, if high winds cause blowdown, the dead branches may become projectiles as they snap off the trunk. 

Mushrooms on the Trunk or Roots

Mushrooms are the fruiting bodies of fungi. When you see a mushroom growing on a tree or its roots, it means that a fungus has colonized the wood underneath the bark. Not all fungi are pathogenic – some may coexist peacefully with the tree, but many digest the tree’s cells at an alarming rate, which compromises its structural integrity. This makes it an extremely hazardous tree and subject to blowdown. Arborists can identify some mushrooms visually during the course of a tree evaluation, but others require laboratory assessment. 

Horizontal or Vertical Cracks 

Trees with large cracks require immediate assessment. This is especially important for cracks that form in the horizontal plane. Cracks indicate that some of the nearby wood has already failed, and it is only a matter of time before the damage spreads. Eventually, the crack will extend far enough to cause the branch or trunk to fall. 

Cavities in the Trunk

While they may indicate significant internal decay, tree cavities are not always harmful. If a tree effectively compartmentalizes a wound, it may persist for decades before some other malady eventually causes it to die. Conversely, some voids may be caused by very aggressive and destructive fungi, thereby representing an immediate hazard.  Cavities whose edges appear “rolled??? are almost always associated with serious internal decay. 

Cavity Nesting Wildlife

Sometimes, cavities may go unnoticed, especially if the openings are not visible from the ground. Accordingly, it is wise to watch for animals that make their homes in trees, as they may indicate that a tree has an internal void. Contact an arborist if you notice bees, ants, birds, raccoons or squirrels, spending time on trees that do not produce food. 


While it is important to keep an eye out for these seven signs, it is important to understand that these signs do not always necessitate removal of the tree. Some trees may be stable enough “as-is???, while others may be saved with the help of corrective measures, such as corrective pruning or attaching supportive cables to weak branch junctions. 

Only an ISA-certified arborist can assess the tree and determine its likelihood of failure; so, Bay Area residents and proprietors should err on the side of caution, and contact Arborist Now to schedule a tree evaluation.  



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