HOW to

NA-FR-01-96

Recognize

Hazardous

Defects in

Trees

United States Forest Service

Department of

Agriculture

Northeastern Area

State & Private 

Forestry
How to

Recognize Hazardous Tree Defects

Contents Page

 

Introduction 1

Inspecting Trees 2

What to Look For 3

Dead Wood 3

Cracks 4

Weak Branch Unions 5

Decay 7

Cankers 8

Root Problems 9

Poor Tree Architecture 11

Multiple Defects 12

Corrective Actions 12

Move the Target 13

Prune the Tree 13

Remove the Tree 14

Cabling and Bracing 14

Topping and Tipping--Poor

 Pruning Practices 15

Suggested Reading 16

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).
1

Introduction

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.
2

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 (page 7) and

cankers (page 8).
3

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

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).
4

Figure 1. Dead branches can break and fall at

any time.

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.
5

Figure 2. A serious crack like this one indicates

that the tree is already failing!

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
6

Figure 3. This weak branch union has failed,

creating a highly hazardous situation.

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.
7

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.

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 (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.
8

Figure 4. This seriously decayed tree should

have been evaluated and removed before it

failed.

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.
9

Figure 5. The large canker on this tree has

seriously weakened the stem.

A canker is physically connected to a

crack, weak branch union, a cavity, or

other defect.

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 (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
10

Figure 6. Severing roots decreases support and

increases the chance of failure or death of the

tree.

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.
11

Figure 7. The mound (arrow) at the base of this

tree indicates that the tree has recently begun to

lean, and may soon fail.

Advanced decay is present in the root

flares or “buttress” roots.

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 (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.
12

Figure 8. This tree is decayed and badly out of

balance because of poor maintenance. It is

dangerous, and extremely unattractive!

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,
13

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.
14

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

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. 
15

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.
16

Suggested Reading

Albers, J.; Hayes, E. 1993. How to detect,

assess and correct hazard trees in

recreational areas, revised edition. St.

Paul, MN: Minnesota DNR. 63 p.

Bedker, P.J.; O’Brien, J.G.; Mielke, M.E.

1995. How to Prune Trees. NA-FR-01-

95. Radnor, PA: USDA Forest Service,

Northeastern Area State and Private

Forestry. 30 pp. Also available on the

Internet via FTP or the World Wide Web

at: http://willow.ncfes.umn.edu.

Fazio, J. 1989. How to Hire an Arborist.

Tree City USA Bulletin No. 6. Nebraska

City, NE: National Arbor Day

Foundation; 8 p.

Fazio, J. 1989. How to Recognize and

Prevent Hazard Trees. Tree City USA

Bulletin No. 15. Nebraska City, NE:

National Arbor Day Foundation; 8 pp.

Robbins, K. 1986. How to Recognize and

Reduce Tree Hazards in Recreation

Sites. NA-FR-31. Radnor, PA: USDA

Forest Service, Northeastern Area;. 28

p.

Shigo, A. L. 1986. A New Tree Biology.

Durham, NH: Shigo and Trees,

Associates; 595 p.
17

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.

Please cite this publication as follows:

Minnesota Department of Natural

Resources, and USDA Forest Service.

1996. How to Recognize Hazardous

Defects in Trees. USDA Forest Service

NA-FR-01-96. 20 pp.
18

Minnesota DNR

Minnesota Department of Natural Resources

500 Lafayette Road

St. Paul, MN 55155-4044

USDA Forest Service

Northeastern Area State & Private

Forestry

Headquarters Office

USDA Forest Service

Northeastern Area State and Private Forestry

5 Radnor Corporate Center, Suite 200

Radnor, PA 19087-4585

Durham Field Office

USDA Forest Service

Northeastern Area State and Private Forestry

Louis C. Wyman Forest Sciences Laboratory

P.O. Box 640

Durham, NH 03824-9799

Morgantown Field Office

USDA Forest Service

Northeastern Area State and Private Forestry

180 Canfield Street

Morgantown, WV 26505-3101

St. Paul Field Office

USDA Forest Service

Northeastern Area State & Private Forestry

1992 Folwell Ave.

St. Paul, MN 55108-1099
19

Notes

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.
How to Recognize Hazardous

Defects in Trees

For further information, contact: