HOW to

NA-FR-01-96

RecognizeHazardousDefects inTrees

United States Forest ServiceDepartment ofAgriculture

Northeastern AreaState & Private Forestry

How to Recognize Hazardous Tree Defects

Contents Page Introduction 1Inspecting Trees 2What to Look For 3

Dead Wood 3Cracks 4Weak Branch Unions 5Decay 7Cankers 8Root Problems 9Poor Tree Architecture 11

Multiple Defects 12Corrective Actions 12

Move the Target 13Prune the Tree 13Remove the Tree 14

Cabling and Bracing 14Topping and Tipping--Poor Pruning Practices 15Suggested Reading 16

The United States Department of Agriculture (USDA)Forest Service is a diverse organization committed to equalopportunity in employment and program delivery. USDAprohibits discrimination on the basis or race, color, nationalorigin, sex, religion, age, disability, political affiliation andfamilial status. Persons believing they have beendiscriminated against should contact the Secretary, U.S.Department of Agriculture, Washington, DC 20250, or call202-720-7327 (voice), or 202-720-1127 (TTY).

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Introduction

Trees add to our enjoyment of outdoorexperiences whether in forests, parks, orurban landscapes. Too often, we are unawareof the risks associated with defective trees,which can cause personal injury andproperty damage. Interest in hazard treemanagement has increased in recent yearsdue to safety and liability concerns resultingfrom preventable accidents. Recognizinghazardous trees and taking proper correctiveactions can protect property and save lives.

A “hazard tree” is a tree with structuraldefects likely to cause failure of all or part ofthe tree, which could strike a “target.” Atarget can be a vehicle, building, or a placewhere people gather such as a park bench,picnic table, street, or backyard.

This brochure was created to help homeowners and land managers in recognizinghazardous defects in trees and to suggestpossible corrective actions. We recommendthat corrective actions be undertaken byprofessional arborists.

Because of the natural variability of trees,the severity of their defects, and thedifferent sites upon which they grow,evaluating trees for hazardous defects can bea complex process. This publication presentsguidelines, not absolute rules forrecognizing and correcting hazardousdefects. When in doubt, consult an arborist.

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Inspecting Trees

Inspect trees under your responsibility everyyear. Tree inspections can be done at anytime of year, leaf-on or leaf-off. To bethorough, inspect trees after leaf drop in fall,after leaf-out in spring, and routinely aftersevere storms.

Inspect trees carefully and systematically.Examine all parts of the tree, including theroots, root or trunk flare, main stem,branches, and branch unions. Be sure toexamine all sides of the tree. Use a pair ofbinoculars to see branches high off theground.

Consider the following factors wheninspecting trees:

Tree Condition: Trees in poor conditionmay have many dead twigs, dead branches,or small, off-color leaves. Trees in goodcondition will have full crowns, vigorousbranches, and healthy, full-sized leaves;however, green foliage in the crown does notensure that a tree is safe. Tree trunks andbranches can be quite defective and stillsupport a lush green crown.

Tree Species: Certain tree species are proneto specific types of defects. For example,some species of maple and ash in theNortheast often form weak branch unions(page 5 ), and aspen is prone to breakage at ayoung age (50-70 years) due to a variety offactors, including decay (page 7) andcankers (page 8).

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Tree Age and Size: Trees are livingorganisms subject to constant stress. Payparticular attention to older trees, which mayhave accumulated multiple defects andextensive decay.

What to Look For

Hazardous defects are visible signs that thetree is failing. We recognize seven maintypes of tree defects: dead wood, cracks,weak branch unions, decay, cankers, rootproblems, and poor tree architecture. A treewith defects is not hazardous, however,unless some portion of it is within strikingdistance of a target.

Dead wood

Dead wood is “not negotiable”-- dead treesand large dead branches must be removedimmediately! Dead trees and branches areunpredictable and can break and fall at anytime (Fig. 1). Dead wood is often dry andbrittle and cannot bend in the wind like aliving tree or branch. Dead branches and treetops that are already broken off (“hangers”or “widow makers”) are especiallydangerous!

Take immediate action if...

A broken branch or top is lodged in atree.A tree is dead.A branch is dead and of sufficient size tocause injury (this will vary with heightand size of branch).

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Figure 1. Dead branches can break and fall atany time.

Cracks

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

Take action if...

A crack extends deeply into, orcompletely through the stem.Two or more cracks occur in the samegeneral area of the stem.

A crack is in contact with another defect.A branch of sufficient size to causeinjury is cracked.

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Figure 2. A serious crack like this one indicatesthat the tree is already failing!

Weak Branch Unions

Weak branch unions are places wherebranches are not strongly attached to thetree. A weak union occurs when two or moresimilarly-sized, usually upright branchesgrow so closely together that bark growsbetween the branches, inside the union. Thisingrown bark does not have the structuralstrength of wood, and the union is muchweaker than one that does not have includedbark (Fig. 3). The included bark may also

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Figure 3. This weak branch union has failed,creating a highly hazardous situation.

act as a wedge and force the branch union tosplit apart. Trees with a tendency to formupright branches, such as elm and maple,often produce weak branch unions.

Weak branch unions also form after a tree orbranch is tipped or topped (page 15), i.e.,when the main stem or a large branch is cutat a right angle to the direction of growthleaving a large branch stub. The stubinevitably decays, providing very poorsupport for new branches (“epicormic”branches) that usually develop along the cutbranch.

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Take action if...

A weak branch union occurs on the mainstem.A weak branch union is cracked.A weak branch union is associated witha crack, cavity, or other defect.

Decay

Decaying trees can be prone to failure, butthe presence of decay, by itself, does notindicate that the tree is hazardous. Advanceddecay, i.e., wood that is soft, punky, orcrumbly, or a cavity where the wood ismissing can create a serious hazard (coverphoto). Evidence of fungal activity including mushrooms, conks, and brackets growing onroot flares, stems, or branches are indicatorsof advanced decay.

A tree usually decays from the inside out,eventually forming a cavity, but sound woodis also added to the outside of the tree as itgrows. Trees with sound outer wood shellsmay be relatively safe, but this depends uponthe ratio of sound to decayed wood, andother defects that might be present.Evaluating the safety of a decaying tree isusually best left to trained arborists (Fig. 4).

Take action if...

Advanced decay is associated withcracks, weak branch unions, or otherdefects.A branch of sufficient size to causeinjury is decayed.

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Figure 4. This seriously decayed tree shouldhave been evaluated and removed before itfailed.

The thickness of sound wood is less than1" for every 6" of diameter at any pointon the stem.

Cankers

A canker is a localized area on the stem orbranch of a tree, where the bark is sunken ormissing. Cankers are caused by wounding ordisease. The presence of a canker increasesthe chance of the stem breaking near thecanker (Fig. 5). A tree with a canker thatencompasses more than half of the tree'scircumference may be hazardous even ifexposed wood appears sound.

Take action if...

A canker or multiple cankers affect morethan half of the tree's circumference.

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Figure 5. The large canker on this tree hasseriously weakened the stem.

A canker is physically connected to acrack, weak branch union, a cavity, orother defect.

Root Problems

Trees with root problems may blow over inwind storms. They may even fall withoutwarning in summer when burdened with theweight of the tree’s leaves. There are manykinds of root problems to consider, e.g.,severing or paving-over roots (Fig. 6);raising or lowering the soil grade near thetree; parking or driving vehicles over theroots; or extensive root decay.

Soil mounding (Fig. 7), twig dieback, deadwood in the crown, and off-color or smaller

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Figure 6. Severing roots decreases support andincreases the chance of failure or death of thetree.

than normal leaves are symptoms oftenassociated with root problems. Because mostdefective roots are underground and out ofsight, aboveground symptoms may serve asthe best warning.

Take action if...

A tree is leaning with recent rootexposure, soil movement, or soil mounding near the base of the tree.

.More than half of the roots under thetree’s crown have been cut or crushed.These trees are dangerous because theydo not have adequate structural supportfrom the root system.

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

Advanced decay is present in the rootflares or “buttress” roots.

Poor Tree Architecture

Poor architecture is a growth pattern that indicates weakness or structural imbalance.Trees with strange shapes are interesting tolook at, but may be structurally defective.Poor architecture often arises after manyyears of damage from storms, unusualgrowing conditions, improper pruning, topping, and other damage (Fig. 8).

A leaning tree may be a hazard. Because notall leaning trees are dangerous, any leaningtree of concern should be examined by aprofessional arborist.

Take action if...

A tree leans excessively.A large branch is out of proportion withthe rest of the crown.

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Figure 8. This tree is decayed and badly out ofbalance because of poor maintenance. It isdangerous, and extremely unattractive!

Multiple Defects

The recognition of multiple defects in a treeis critical when evaluating the tree’spotential to fail. Multiple defects that aretouching or are close to one another shouldbe carefully examined. If more than onedefect occurs on the tree’s main stem, youshould assume that the tree is extremelyhazardous.

Corrective Actions

Corrective actions begin with a thoroughevaluation. If a hazardous situation exists,

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there are three recommended options forcorrecting the problem: move the target,prune the tree, or remove the tree.

Move the Target

Removing the target is often an inexpensiveand effective treatment for correcting ahazard tree. Easily moved items like playsets and swings, RV's, and picnic tables canbe placed out of the reach of the hazardoustree with little effort and expense.

If the target cannot be moved and a serioushazard exists, consider blocking access tothe target area until the hazard can beproperly eliminated.

Prune the Tree

A hazardous situation may be caused by adefective branch or branches, even thoughthe 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 causeinjury is cracked or decayed.A weak branch union exists and one ofthe branches can be removed.Branches form a sharp angle, twist, orbend.A branch is lopsided or unbalanced withrespect to the rest of the tree.A broken branch is lodged in the crown.Remove the branch and prune the stub.

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Pruning a tree properly early in its life is agood way to effectively avoid manypotential problems when the tree is older andlarger. When done correctly, routine pruningof trees does not promote future defects. Ifdone improperly, immediate problems maybe removed, but cracks, decay, cankers, orpoor architecture will be the ultimate result,creating future hazards.

We recommend that the "natural target"pruning method be used. This pruningmethod is fully described in How to PruneTrees (Bedker, O’Brien & Mielke, 1995).

Remove the Tree

Before cutting a tree down, carefullyconsider the alternatives. The effects ofremoving a tree are often pronounced inlandscape situations and may result in reduced property values. Tree removalshould be considered as the final option andused only when the other two correctiveactions will not work. Tree removal isinherently dangerous and is even moreserious when homes and other targets areinvolved. Removal of hazardous trees isusually a job for a professional arborist.

Cabling and Bracing

Cabling and bracing does not repair a hazardtree, but when done correctly by a trainedarborist, it can extend the time a tree or itsparts are safe. Done incorrectly, it creates a more serious hazard. We do not recommendcabling or bracing as treatment for a hazardtree unless the tree has significant historic orlandscape value.

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Topping and Tipping--PoorPruning Practices

Topping is the practice of pruning largeupright branches at right angles to thedirection of growth, sometimes used toreduce the height of the crown. Tipping isthe cutting of lateral branches at right anglesto the direction of growth to reduce crownwidth. Both of these practices are harmfuland should never be used. The inevitableresult of such pruning wounds is decay inthe remaining stub, which then serves as avery poor support to any branches thatsubsequently form. Trees that are pruned inthis manner are also misshapen andesthetically unappealing (see Fig. 8).

Conclusions

Evaluating and treating hazard trees iscomplicated, requiring a certain knowledgeand expertise. This publication outlinessome of the basic problems that may alertyou to a hazardous situation. Never hesitateif you think a tree might be hazardous. If youare not sure, have it evaluated by aprofessional. Consult your phone bookunder "Arborists" or "Tree Service."

Remember that trees do not live forever.Design and follow a landscape plan thatincludes a cycle of maintenance andreplacement. This is the best way to preservethe health of our trees and ensure a safe andenjoyable outdoor experience.

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Suggested Reading

Albers, J.; Hayes, E. 1993. How to detect,assess and correct hazard trees inrecreational 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 PrivateForestry. 30 pp. Also available on theInternet via FTP or the World Wide Webat: http://willow.ncfes.umn.edu.

Fazio, J. 1989. How to Hire an Arborist.Tree City USA Bulletin No. 6. NebraskaCity, NE: National Arbor DayFoundation; 8 p.

Fazio, J. 1989. How to Recognize andPrevent Hazard Trees. Tree City USABulletin No. 15. Nebraska City, NE:National Arbor Day Foundation; 8 pp.

Robbins, K. 1986. How to Recognize andReduce Tree Hazards in RecreationSites. NA-FR-31. Radnor, PA: USDAForest Service, Northeastern Area;. 28p.

Shigo, A. L. 1986. A New Tree Biology.Durham, NH: Shigo and Trees,Associates; 595 p.

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The Authors:

Minnesota DNR

Jana Albers, Plant Pathologist, GrandRapids, 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 NaturalResources, and USDA Forest Service.1996. How to Recognize HazardousDefects in Trees. USDA Forest ServiceNA-FR-01-96. 20 pp.

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Minnesota DNR

Minnesota Department of Natural Resources500 Lafayette RoadSt. Paul, MN 55155-4044

USDA Forest ServiceNortheastern Area State & PrivateForestry

Headquarters OfficeUSDA Forest ServiceNortheastern Area State and Private Forestry5 Radnor Corporate Center, Suite 200Radnor, PA 19087-4585

Durham Field OfficeUSDA Forest ServiceNortheastern Area State and Private ForestryLouis C. Wyman Forest Sciences LaboratoryP.O. Box 640Durham, NH 03824-9799

Morgantown Field OfficeUSDA Forest ServiceNortheastern Area State and Private Forestry180 Canfield StreetMorgantown, WV 26505-3101

St. Paul Field OfficeUSDA Forest ServiceNortheastern Area State & Private Forestry1992 Folwell Ave.St. Paul, MN 55108-1099

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Notes

How to Recognize Hazardous Defects inTrees was written to help people identifypotential problems with trees. Trees withserious defects can pose an extreme hazardand should be treated with caution. The bestway to correct a hazardous tree is to hire aprofessional arborist. Information in thispublication can help to identify trees thatrequire attention.

How to Recognize HazardousDefects in Trees

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