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Recognizing Hazardous Defects In Trees

trunk wound with large opening in barkYard trees should be checked annually, or after severe storms, for hazardous defects. A tree is considered hazardous when two criteria are met: (a) a defect which will likely result in failure, and (b) existence of a target. Failure refers to the fact that the tree could split, crack, fall or break off at the point of the defect. The target is an object that could be damaged, destroyed or killed if the tree fails. This could be a structure, people, power lines, or property. Defects can be caused directly or indirectly by storm damage, construction or maintenance equipment, improper pruning or decay. During storms, trees with defects are particularly susceptible to failure or damage at the point of the defect.

Safety be the top priority when assessing trees for defects and/or when performing the work to eliminate such defects or removing the trees. Professional arborists or a professional tree-care service should be consulted when removing or considering the removal of a tree or branches too high to reach from the ground. In addition, special training is needed to safely prune branches over or adjacent to power lines. For these situations, homeowners should contact an arborist trained in power line clearance or their local power company.

Remember, if there is no target within striking distance, as when a tree is in a forest, it is not considered a hazard. In some cases, simply removing the target eliminates the hazardous situation. For instance, if a picnic table is located under a tree with significant decay, move the table to another location.

broken branch hanging in tree topDefects can be broken down into a few categories:

  • Broken or loose branches
  • Trunks or branches with missing bark
  • Leaning trees and other root problems
  • Weak branch connections
  • Cracks and splits
  • Dead wood
  • Evidence of significant decay

Broken or Loose Branches

Broken or loose branches should be removed as soon as possible. They could fall at any time and cause injury to people or property. They also pose a substantial liability to the landowner.

an RV is the target of a hazardous treeTrunks or Branches with Missing Bark and Wood

Trunks or branches with missing bark and wood can be caused by branch failures, wind, snow or other injuries and decay. Two rules can determine if the tree or branch is at high risk of failing: If the shell, bark, is less than 2" for each 6" of stem diameter and the opening in the shell is greater than 30% of the stem circumference, or if more than 40% of the tree is missing. If either of these situations exists the tree or branch should be removed.

Leaning Trees

If a tree is leaning, even only slightly, and the soil around the base of the tree is somewhat raised on the far side of the lean, a root failure has most likely occurred. Urban environments often restrict structural root growth because of restrict growing space. Root systems don't have room to grow down and/or out due to underground utilities, soil compaction or structures, such as storm sewer pipes, subways, concrete barriers, etc. This situation leads to smaller anchoring systems trying to support large tree canopies. Particularly during storms, this unequal support system fails and the trees uproot themselves. Root failures in established trees are not reparable; the tree will not reestablish the structural anchoring necessary to be stable. The tree should be removed.

Weak Branch Attachments

Branches with strong attachments create a ridge of raised bark at the intersection of the branch and the trunk. In weak attachments, the ridge is absent and the bark grows into the branch attachment. The upper side of the branch is unable to secure itself to the trunk. Epicormic branches (also called water sprouts or suckers) are formed as a response to bad pruning, topping, other injury or environmental stress. Epicormic branches always have weak attachments and grow very quickly.

branch unions
branch unions
epicormic branching
epicormic branching

Co-dominant stems are approximately the same size and arise from the same point. Co-dominant stems and narrow "V" shaped attachments often are weakly attached. These are the points at which failure will occur when the tree is under stress from high winds, snow or ice. The best time to correct a weak attachment is while the tree is young.

codominant stems caused a vertical crack in trunk
This is a perfect example of
co-dominant stems with a weak
attachment resulting in a
vertical crack.

Cracks and Splits

Trees with split trunks will likely fail completely in a later storm and should be removed. Vertical cracks can be caused by a variety of reasons, may be associated with decay and will eventually fail. Horizontal cracks indicate that the tree is already failing and should be removed as soon as possible.

Dead Wood

A dead tree, top or branch could fail at any time. A dead tree top might indicate disease or insect damage. Dead wood should be removed back to live wood and dead trees should be removed.


Decay causing fungi can attack a tree through a pruning cut, a broken branch, a lawn mower injury or other break in a tree's bark. Rotted wood is structurally weak. Again, if the shell is less than 2" for each 6" of stem diameter and the opening in the shell is greater than 30% of the stem circumference, or if more than 40% of the tree is missing the tree is likely to fail at the point of decay.

What NOT to Do

  • Don't do any corrective pruning that cannot be done from the ground. Trees too large for this should be pruned only by professional arborists.
  • Don't try to support a damaged tree with rope, cable, wire, bolts, or similar materials. The effort will probably NOT increase the safety of the tree. If cabling and bracing are necessary, they should be done only by a trained arborist.
  • Don't try to save a tree that was pushed over by a storm unless it was recently planted. The tree's roots will likely never develop well enough to adequately support the tree again.
  • NEVER top trees! Over the years, this will make trees even more of a hazard.
  • Don't use paint or wound dressing to cover wounds. These actually interfere with the tree's natural wound sealing process.
  • Don't fertilize damaged trees. The use of nitrogen can make a stressed tree even more susceptible to insect pests and diseases and reduce the ability of the tree to deal with the damage that has occurred.

For more information contact your local DEC office.

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