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Tree Damage and Defects

Tree Damage and Defects – What to Look For

Hazardous defects are visible signs that the tree is failing. We recognize seven main types of tree defects: dead wood, cracks, weak branch unions, decay, cankers, root problems, and poor tree architecture. A tree with defects is not hazardous, however, unless some portion of it is within striking distance of a target.

red dotDead wood

Dead wood is “not negotiable”– dead trees and large dead branches must be removed immediately! Dead trees and branches are unpredictable and Figure 1 - Dead branches can break and fall at any time.can break and fall at any time (Fig. 1). Dead wood is often dry and brittle and cannot bend in the wind like a living tree or branch. Dead branches and tree tops that are already broken off (“hangers” or “widow makers”) are especially dangerous!

Take immediate action if…

  • A broken branch or top is lodged in a tree.
  • A tree is dead.
  • A branch is dead and of sufficient size to cause injury (this will vary with height and size of branch).

Figure 1. Dead branches can break and fall at any time. (photo right)

Fig 2 - A serious crack like this one indicates that the tree is already failing!

red dotCracks

A crack is a deep split through the bark, extending into the wood of the tree. Cracks are extremely dangerous because they indicate that the tree is already failing (Fig. 2).

Take action if…

  • A crack extends deeply into, or completely through the stem.
  • Two or more cracks occur in the same general area of the stem.
  • A crack is in contact with another defect.
  • A branch of sufficient size to cause injury is cracked.

Figure 2. A serious crack like this one indicates that the tree is already failing! (photo right) red dotWeak Branch Unions

Weak branch unions are places where branches are not strongly attached to the tree. A weak union occurs when two or more similarly-sized, usually upright branches grow so closely together that bark grows between the branches, inside the union. This ingrown bark does not have the structural strength of wood, and the union is much weaker than one that does not have included bark (Fig. 3). The included bark mayalso act as a wedge and force the branch union to split apart. Trees with a tendency to form upright branches, such as elm and maple, often produce weak branch unions.Weak branch unions also form after a tree or branch is tipped or topped (page 15), i.e., when the main stem or a large branch is cut at a right angle to the direction of growth leaving a large branch stub. The stub inevitably decays, providing very poor support for new branches (“epicormic” branches) that usually develop along the cut branch.

Take action if…

  • A weak branch union occurs on the main stem.
  • A weak branch union is cracked.
  • A weak branch union is associated with a crack, cavity, or other defect.
Figure 3
Figure 3. This weak branch union has failed, creating a highly hazardous situation.

red dotDecay

Figure 4

Decaying trees can be prone to failure, but the presence of decay, by itself, does not indicate that the tree is hazardous. Advanced decay, i.e., wood that is soft, punky, or crumbly, or a cavity where the wood is missing can create a serious hazard (cover photo). Evidence of fungal activity including mushrooms, conks, and brackets growing on root flares, stems, or branches are indicators of advanced decay.

A tree usually decays from the inside out, eventually forming a cavity, but sound wood is also added to the outside of the tree as it grows. Trees with sound outer wood shells may be relatively safe, but this depends upon the ratio of sound to decayed wood, and other defects that might be present. Evaluating the safety of a decaying tree is usually best left to trained arborists (Fig. 4).

Take action if…

  • Advanced decay is associated with cracks, weak branch unions, or other defects.
  • A branch of sufficient size to cause injury is decayed.
  • The thickness of sound wood is less than 1″ for every 6″ of diameter at any point on the stem.

Figure 4. This seriously decayed tree should have been evaluated and removed before it failed. (photo right)

red dotCankers

A canker is a localized area on the stem or branch of a tree, where the bark is sunken or missing. Cankers are caused by wounding or disease. The presence of a canker increases the chance of the stem breaking near the canker (Fig. 5). A tree with a canker that encompasses more than half of the tree’s circumference may be hazardous even if exposed wood appears sound.

    Take action if…

  • A canker or multiple cankers affect more than half of the tree’s circumference.
  • A canker is physically connected to a crack, weak branch union, a cavity, or other defect.
Figure 5
Figure 5. The large canker on this tree has seriously weakened the stem.

red dotRoot Problems

Figure 6Trees with root problems may blow over in wind storms. They may even fall without warning in summer when burdened with the weight of the tree’s leaves. There are many kinds of root problems to consider, e.g., severing or paving-over roots (Fig. 6); raising or lowering the soil grade near the tree; parking or driving vehicles over the roots; or extensive root decay.

Soil mounding (Fig. 7), twig dieback, dead wood in the crown, and off-color or smaller than normal leaves are symptoms often associated with root problems. Because most defective roots are underground and out of sight, aboveground symptoms may serve as the best warning.

Take action if…

  • A tree is leaning with recent root exposure, soil movement, or soil mounding near the base of the tree.
  • More than half of the roots under the tree’s crown have been cut or crushed. These trees are dangerous because they do not have adequate structural support from the root system.
  • Advanced decay is present in the root flares or “buttress” roots.

Figure 6. Severing roots decreases support and increases the chance of failure or death of the tree. (photo right)

Figure 7
Figure 7. The mound (arrow) at the base of this tree indicates that the tree has recently begun to lean, and may soon fail.

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