Defects in Trees
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Contents
Trees add to our enjoyment of outdoor experiences whether in forests, parks, or urban landscapes. Too often, we are unaware of the risks associated with defective trees, which can cause personal injury and property damage. Interest in hazard tree management has increased in recent years due to safety and liability concerns resulting from preventable accidents. Recognizing hazardous trees and taking proper corrective actions can protect property and save lives.
A “hazard tree” is a tree with structural defects likely to cause failure of all or part of the tree, which could strike a “target.” A target can be a vehicle, building, or a place where people gather such as a park bench, picnic table, street, or backyard.
This brochure was created to help home owners and land managers in recognizing hazardous defects in trees and to suggest possible corrective actions. We recommend that corrective actions be undertaken by professional arborists.
Because of the natural variability of trees, the severity of their defects, and the different sites upon which they grow, evaluating trees for hazardous defects can be a complex process. This publication presents guidelines, not absolute rules for recognizing and correcting hazardous defects. When in doubt, consult an arborist.
Inspect trees is your responsibility every year. Tree inspections can be done at any time of year, leaf-on or leaf-off. To be thorough, inspect trees after leaf drop in fall, after leaf-out in spring, and routinely after severe storms.
Inspect trees carefully and systematically. Examine all parts of the tree, including the roots, root or trunk flare, main stem, branches, and branch unions. Be sure to examine all sides of the tree. Use a pair of binoculars to see branches high off the ground.
Consider the following factors when inspecting trees:
Tree Condition: Trees in poor condition may have many dead twigs, dead branches, or small, off-color leaves. Trees in good condition will have full crowns, vigorous branches, and healthy, full-sized leaves; however, green foliage in the crown does not ensure that a tree is safe. Tree trunks and branches can be quite defective and still support a lush green crown.
Tree Species: Certain tree species are prone to specific types of defects. For example, some species of maple and ash in the Northeast often form weak branch unions, and aspen is prone to breakage at a young age (50-70 years) due to a variety of factors, including decay and cankers.
Tree Age and Size: Trees are living organisms subject to constant stress. Pay particular attention to older trees, which may have accumulated multiple defects and extensive decay.
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.
Dead wood
Dead wood is “not negotiable”-- dead trees and large dead
branches must be removed immediately! Dead trees and branches are
unpredictable and can break and fall at any time. 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...
Cracks
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.
Take action if...
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. The included bark may also 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, 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...
Decay
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. 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.
Take action if...
Cankers
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. 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...
Root Problems
Trees 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; raising or lowering the soil
grade near the tree; parking or driving vehicles over the roots; or
extensive root decay.
Soil mounding, 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, above ground symptoms may serve as the best warning.
Take action if...
Poor Tree Architecture
Poor architecture is a growth pattern that indicates weakness or
structural imbalance. Trees with strange shapes are interesting to look
at, but may be structurally defective. Poor architecture often arises
after many years of damage from storms, unusual growing conditions,
improper pruning, topping, and other damage.
A leaning tree may be a hazard. Because not all leaning trees are dangerous, any leaning tree of concern should be examined by a professional arborist.
Take action if...
The recognition of multiple defects in a tree is critical when evaluating the tree’s potential to fail. Multiple defects that are touching or are close to one another should be carefully examined. If more than one defect occurs on the tree’s main stem, you should assume that the tree is extremely hazardous.
Corrective actions begin with a thorough evaluation. If a hazardous situation exists, there are three recommended options for correcting the problem: move the target, prune the tree, or remove the tree.
Move the Target
Removing the target is often an inexpensive and effective treatment for correcting a hazard tree. Easily moved items like play sets and swings, RV's, and picnic tables can be placed out of the reach of the hazardous tree with little effort and expense.
If the target cannot be moved and a serious hazard exists, consider blocking access to the target area until the hazard can be properly eliminated.
Prune the Tree
A hazardous situation may be caused by a defective branch or branches, even though the rest of the tree is sound. In this case, pruning the branch solves the problem.
Prune when...
Pruning a tree properly early in its life is a good way to effectively avoid many potential problems when the tree is older and larger. When done correctly, routine pruning of trees does not promote future defects. If done improperly, immediate problems may be removed, but cracks, decay, cankers, or poor architecture will be the ultimate result, creating future hazards.
We recommend that the "natural target" pruning method be used. This pruning method is fully described in How to Prune Trees (Bedker, O’Brien & Mielke, 1995).
Remove the Tree
Before cutting a tree down, carefully consider the alternatives. The effects of removing a tree are often pronounced in landscape situations and may result in reduced property values. Tree removal should be considered as the final option and used only when the other two corrective actions will not work. Tree removal is inherently dangerous and is even more serious when homes and other targets are involved. Removal of hazardous trees is usually a job for a professional arborist.
Cabling and bracing does not repair a hazard tree, but when done correctly by a trained arborist, it can extend the time a tree or its parts are safe. Done incorrectly, it creates a more serious hazard. We do not recommend cabling or bracing as treatment for a hazard tree unless the tree has significant historic or landscape value.
Topping and Tipping--Poor Pruning Practices
Topping is the practice of pruning large upright branches at right angles to the direction of growth, sometimes used to reduce the height of the crown. Tipping is the cutting of lateral branches at right angles to the direction of growth to reduce crown width. Both of these practices are harmful and should never be used. The inevitable result of such pruning wounds is decay in the remaining stub, which then serves as a very poor support to any branches that subsequently form. Trees that are pruned in this manner are also misshapen and esthetically unappealing.
Evaluating and treating hazard trees is complicated, requiring a certain knowledge and expertise. This publication outlines some of the basic problems that may alert you to a hazardous situation. Never hesitate if you think a tree might be hazardous. If you are not sure, have it evaluated by a professional. Consult your phone book under "Arborists" or "Tree Service."
Remember that trees do not live forever. Design and follow a landscape plan that includes a cycle of maintenance and replacement. This is the best way to preserve the health of our trees and ensure a safe and enjoyable outdoor experience.
Minnesota DNR
U.S. Forest Service
Please cite this publication as follows:
Minnesota DNR
Minnesota Department of Natural Resources
500 Lafayette Road
St. Paul, MN 55155-4040
info@dnr.state.mn.us
U.S. Forest Service
U.S. Forest Service
1400 Independence Ave., SW
Washington, D.C.
20250-0003
(800) 832-1355
The United States Department of Agriculture (USDA) Forest Service is a diverse organization committed to equal opportunity in employment and program delivery. USDA prohibits discrimination on the basis or race, color, national origin, sex, religion, age, disability, political affiliation and familial status. Persons believing they have been discriminated against should contact the Secretary, U.S. Department of Agriculture, Washington, DC 20250, or call 202-720-7327 (voice), or 202-720-1127 (TTY).
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