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Trunk wounds and decay

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Urban and suburban trees are more likely to have wounds and decay than trees in native stands because people cause most wounds. These wounds are usually unintentional, such as automobiles, construction equipment, or lawn mowers bumping the tree trunk or surface roots, or improper pruning. Naturally occurring events, such as storms, fires, or damage by birds or other animals, may also cause wounds.

Trunk wounds that penetrate the bark will damage the cambium layer, a thin layer of vascular tissue, which is vital to movement of water and nutrients. If less than 25% of the bark around the trunk has been damaged, the tree will probably recover. When fresh wounds occur on the trunk, the injured bark should be removed carefully, leaving healthy bark that is sound and tight to the wood. A wound dressing (tree paint) is not necessary. You will be able to observe the wound closing from the edges each year as the tree grows.

When an older wound is discovered, remove the dried and loose bark back to the area where the new wood can be seen along the edges of the wound. Trunk wounds that are not addressed could potentially be a hazard in the future.

Once a wound occurs, decay-causing fungi can enter the heartwood and the decay process begins. Trees have a unique defense. The wood around the wound begins to produce special compounds in the wood cells that set up a wall or barrier to isolate the infected area. This is called compartmentalization. In a vigorous tree, new growth continues to form and add to the sound wood Once compartmentalized, discoloration and decay will spread no further unless one of the barriers is broken.

Cleaning decayed wood from cavities is not recommended since the compartment wall might be breached and further decay of the trunk could result. Storm-damaged branches should be properly pruned to expedite the healing process. Avoid pruning directly against the trunk since flush cuts can lead to extensive decay. Prune hazardous branches immediately.


Years ago, filling cavities was an accepted practice. The wound would be cleaned and scraped down to sound wood and filled with cement, mortar, or bricks. These practices frequently penetrated the tree’s natural defensive barrier, allowing decay to spread. Fortunately, this practice has decreased, along with flush cuts and tree wound paints.

How to Treat a Tree Cavity

In most cases, it’s best to do nothing. A tree will seal over a small cavity eventually, and the tree’s new wood is stronger than anything put into the hole. Large cavities may never close, but as long as the tree does not sustain further damage, a basal cavity may not be a problem. A certified arborist can determine if the tree is safe or if it should be removed.

When is a tree a hazard?

  • When an inspection reveals a structural weakness, internal decay, or poor branching structure.
  • When there is a “target”(someone or something that could be hurt or damaged if the tree or limb falls).
  • If decay or structural problems are suspected, contact a professional arborist. Trees located in areas where people frequent should be inspected regularly.

Signs of Potential Hazards

  • Large dead or detached branches
  • Cavities or decayed wood
  • Signs of internal decay – mushrooms at the base of the tree or carpenter ants
  • Cracks or splits in the trunk where branches are attached
  • Many branches arising from one point on the trunk
  • Roots that have been cut or covered

What to do about Hazards

  • Remove the target
  • Prune the tree
  • Cable the weak branches
  • Remove the tree