Inspecting Trees
Inspect trees under 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 (page 5 ), 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.
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.
Dead
wood
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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 (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).
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Figure 1. Dead branches can
break and fall at any time.(photo right
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
(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! (see Fig 3 photo below)
Weak
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 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...
- 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.
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Figure 3. This weak branch
union has
failed, creating a highly hazardous situation. |
Decay
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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...
Figure 4. This
seriously decayed tree should have been evaluated and removed before
it failed. (photo right) |
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- 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.
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 (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.
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Figure 5. The large canker on
this tree
has seriously weakened the stem. |
Root
Problems
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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
(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...
Figure 6. Severing
roots decreases support and increases the chance of failure or death
of the tree. (photo right) |
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- 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.
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Figure 7. The mound (arrow)
at the base of this
tree indicates that the tree has recently begun to
lean, and may soon fail. |
Poor
Tree Architecture
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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 (Fig. 8).
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...
- A tree leans
excessively.
- A large
branch is out of proportion with the rest of the crown.
Figure 8. This tree is
decayed and badly out of balance because of poor maintenance. It is
dangerous, and extremely unattractive! (photo right)
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Multiple
Defects
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
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...
- A branch is dead.
- A branch of sufficient size
to cause injury is cracked or decayed.
- A weak branch union exists
and one of the branches can be removed.
- Branches form a sharp angle,
twist, or bend.
- A branch is lopsided or
unbalanced with respect to the rest of the tree.
- A broken branch is lodged in
the crown. Remove the branch and prune the stub.
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.
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
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 (see Fig. 8).
Conclusions
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.
The Authors:
Minnesota DNR Jana Albers, Plant
Pathologist, Grand Rapids, MN. -Tom Eiber, Entomologist, St. Paul, MN. - Ed
Hayes, Plant Pathologist, Rochester, MN
USDA Forest Service Peter Bedker,
Plant Pathologist, St. Paul, MN. - Martin MacKenzie, Plant Pathologist,
Morgantown, WV. Joseph O’Brien, Plant Pathologist, St. Paul, MN. -
Jill Pokorny, Plant Pathologist, St. Paul,
MN. Mary Torsello,
Plant Pathologist, Durham, NH.
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How to Recognize Hazardous
Defects in Trees was written to help people identify potential problems
with trees. Trees with serious defects can pose an extreme hazard and
should be treated with caution. The best way to correct a hazardous tree
is to hire a professional arborist. Information in this publication can
help to identify trees that require attention. |
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).
