Chapter  3

  Appendix B

{Tree City USA} {Bulletin 15}

How to Recognize - and Prevent - Hazard Trees

Worrying about hazards has resulted in the unnecessary removal of many trees. Although the problem of hazard trees needs to be addressed by every landowner and land manager, removal should be an act of last resort. Instead, some technical knowledge and a lot of common sense are the keys to preventing injuries, property damage and lawsuits due to unsafe trees.

   There once was a young arborist who was placed in charge of the trees on a beautiful college campus. Most of the trees under his care had been planted decades before, then carefully nurtured over the years to provide shade and lend grace to the academic setting.

   Not long after the new arborist arrived, trees began being felled and a "hit list" of others was presented to the faculty committee that oversaw such matters. Soon, the arborist was known as The Grim Reaper, a title out of character in a profession dedicated to prolonging the lives of trees!

   After some investigation, it was learned that the arborist had been to a training session about hazard trees. The course had done such a good job in some respects that the man's sharpened eye saw potential trouble in nearly every tree. The course had frightened him so badly about potential injuries, property damage and, above all, liability suits, that he viewed tree removals as the only course of action. Old poplars in a park, a pear tree by a dorm and dead snags in the arboretum were viewed the same-and all were scheduled for removal.

   In this issue of the Bulletin, some of the signs that warn about dangerous trees are presented, as well as a reminder that those of us who own or manage trees are, indeed, responsible for the safety of people and property in the vicinity of our trees. But you will find no photos of dented cars or smashed houses. Scare tactics will backfire. The better approach is to learn to analyze the setting, consider the risks and benefits, and carefully plan for actions that prevent or correct hazards whenever possible. It is toward that end that this issue is dedicated.


Recognizing Tree Hazards

When damage, injury or death occurs because of a defective tree, the law usually holds the tree's owner responsible. (In a public place such as a park, this responsibility shifts to the managers of the tree.) Under the law, it is your duty to exercise care, good judgment, caution, and foresight by inspecting your trees regularly and recognizing situations that may cause them to break or fall.

What is a Hazard Tree?

What is Not a Hazard?

{Hazard tree} {Non-hazard tree}
A hazard tree has a structural defect that may cause the tree or a portion of the tree to fall on someone or something else of value. This is a legal grey area, but for a tree to be a hazard, a "target" must be within the falling distance of the tree or its part that fails. A "target" means people, vehicles and structures. Therefore, a defective tree in the woods or an open field or away from paths in an arboretum need not necessarily be considered a hazard. See Bulletin No. 13 for a case to be made on behalf of leaving old or dead trees for wildlife.

To look for hazardous conditions, inspect each tree systematically. Start by scanning the top, using binoculars if necessary. After reviewing the crown, look downward along the trunk, then carefully examine the root zone. On the following pages are some important signs to watch for in your visual inspection.

1. Examine the Top and Crown

Differences In Species
Some species are simply more brittle than others. This is one reason why city ordinances sometimes prohibit or discourage trees such as willows, box elders and silver maple. Plant these trees only in open areas. If they already exist on your property, a minimum precaution would be to avoid locating play areas or patios beneath these trees.
{Differences in species} Are There Dead Branches?
Loggers call these "widowmakers" and treat them with great respect. Homeowners should do likewise. Dead limbs are an accident waiting to happen. They can fall in the slightest breeze, when a mower bumps the tree, or a child climbs in it. They sometimes give way even on a calm day. Dead limbs are a red-flag signal for prompt action.
{Dead branches}

{History} What's the Tree's History?
Sometimes past events warn of potential trouble. For example, previous topping will almost invariably result in weakly attached regrowth, Similarly, broken branches with stubs un-pruned, or sprout-like regrowth after storm damage, set the stage for breakage. Recent, seemingly unexplained loss of large limbs may also be a sign of internal problems.
{Cross or rub} Do Some Branches Cross or Rub?
Branches that cross or rub invariably lead to weak spots. These should be pruned off as soon as they are spotted, and the smaller the better.

Is the Tree Dead or Dying?
With the exception of trees for wildlife where structures or human traffic are absent or rare, dead and dying trees should be promptly removed. Felling a large tree is extremely dangerous. Call an expert to do the job.
{Dead or dying} How Vigorous is the Tree?
Evaluating a tree's vigor is somewhat subjective. However, experts say it is the surest early warning that there is a serious health problem in a tree. Vigor is reflected in the amount of leaf cover, and leaf size, color and condition. By comparing your tree with others of like size, you will be able to detect a less vigorous crown.

Why Branches Break

{Large limbs}
{Branch collar} {Trunk tissue}
Large limbs can be weakened by rubbing, unrepaired storm damage, or poor pruning of side branches. The limb responds by forming barrier zones around each wound. These are weak spots that sometimes snap under the pressure of wind or ice. A break at the branch collar is part of normal self-pruning, often caused by decay. Regular inspections for decay at branch junctions, followed by pruning, can prevent unexpected breakage. Supporting trunk tissue sometimes gives way under stress. More research is needed to determine exactly why, but allowing large, horizontal limbs to develop may put unreasonable demands on the tree.

2. Check the Trunk

  • Watch for Forked Trunks

    Forked trunks are signals of potential weakness, especially if one side of the fork grows outward instead of upward like the other. Narrow-angled forks are also prone to infection, often indicated by sap or pitch being exuded. Early pruning of one side of the fork can prevent these problems; cables or braces are corrective actions taken by arborists to strengthen the fork in trees of higher value. {Forked trunk}

  • Look for Signs of Decay

    Clues to internal decay of the trunk or large branches are cavities, disfiguration (cankers) and the fruiting bodies of fungi (conks). Sometimes there are no outward indications. Arborists then use one of the methods shown below to check for decay.

    According to the USDA Forest Service, internal decay does not automatically render a tree unsafe. Working with pines, the Forest Service determined that if the amount of sound wood surrounding internal rot exceeds that established by the line on the graph, the tree can be considered relatively safe from failure.


  • What About Balance?

    {Balance} Leaning or lopsided trees present more of a hazard than those growing vertically, but if a tree has always grown off center, it generally is not an undue risk. However, any sudden lean indicates breakage or weakening of support roots and should be cause for alarm and immediate action.

  • Examine Wounds and Cracks

    {Wounds and cracks} Any trunk wound is an opening for decay. Wounds extending into the ground, including lightning scars, should be of particular concern and examined regularly. Some cracks, such as frost cracks, have little effect on the strength of a trunk. However, if two vertical cracks appear on opposite sides of the tree, it can be a sign of root injury or breakage. It is usually associated with a circumferential separation of wood internally and is extremely dangerous.

Tools Used by Arborists to Check for Internal Decay

{Mallet} {Increment borer} {Drill and bit} {Shigometer}
This method is harmless to the tree but relies on differences in sound as the tree is struck. A small core of the tree about 1/4" thick is removed and examined. This causes some wounding. An electric drill and 1/8" bit can reveal rot through changes in drilling speed or ease, and condition of the wood chips extracted. This instrument uses a pulsed electrical current to measure the resistance of wood, and thereby the presence of decay. Drilling is necessary.

3. Don't Forget the Roots

Roots are an important cause of trees becoming hazardous. Tree scientist Pascal Pirone reported that in his 30 years of examining tree problems more than half were traced to root disease or injuries.

  • Any Signs of Root Decay?

    {Root decay} Root decay is often insidious and difficult to detect. Noted tree expert Dr. Alex L. Shigo calls the organisms that cause root problems "the sneaky fungi." Sometimes their work in weakening support roots goes completely unnoticed because the smaller feeder roots may go right on absorbing water and lawn fertilizer. Then, suddenly, one day the tree falls over! To detect root decay, look carefully for "mushrooms" on or near the base of the tree. If found, or if root trouble is suspected, have an arborist dig up some roots to sample for decay organisms.

  • Are Any Roots Severed?

    Trenching or construction within the root zone is a major cause of hazard trees. The problem is two-pronged. First, severed roots lose their ability to support the trunk and crown, especially if located on the windward side of the tree. Second, severed roots are open wounds that invite decay organisms. See Bulletin No. 7 for ideas about saving trees during construction.
    Note: Allowing roots to be cut, then watering and fertilizing to aid recovery is not a guarantee against decay. The reason is that decay organisms thrive on this treatment, too!

A Checklist for Preventing Hazard Trees

  • Inspect your trees carefully several times each year and in all seasons. Annually, have a qualified arborist (See Bulletin No. 6) inspect your trees and provide you with a written report.
  • Avoid planting brittle species where falling limbs could injure people or property. Some examples:
    • Silver Maple
    • Lombardy Poplar
    • Box Elder
    • Willows
  • Prune trees when they are young (Bulletin No. 1) and regularly thereafter.
  • Use correct pruning methods, always making the pruning cut outside the branch collar.
  • Don't allow trees to be topped!
  • Always plant the right tree in the right place. For example, avoid planting large-growing trees under power lines, or too close to your house, and make sure the species selected matches the soil and other site characteristics. See Bulletin No. 4 for other ideas.
  • Water deeply during dry periods, slowly applying at least 1" of water.
  • Erect barriers around or slightly beyond the dripline of trees during construction. Insist that these root protection zones be honored by construction workers.
  • Consider cabling or bracing weak forks or branches in older trees of high value. This is work for a professional arborist.
  • Do not plant trees with narrowly-forked stems.
  • Where a high value tree may be suspected of developing into a hazard, use landscaping to keep people at a safe distance. This may require techniques such as re-routing walks, moving patio furniture, or planting shrubs and hedges as barriers to foot traffic.

Remember: A healthy, vigorous tree that receives regular care is less likely to become a hazard than one that is ignored. Prevention is the best solution to the tree hazard problem.

A Case for Old Trees: Indiana University

Like the misguided arborist described on page one, it is easy to believe that any tree with a defect must be removed. Fortunately, allowance for a more prudent approach can also be found in the law. Besides a "reasonable" standard of care being expected, such as regular safety inspections, corrective actions may be weighed against benefits. Clearly, despite the difficulty in assigning monetary value to trees that all parties will accept, a no-trees or no-older-or-larger-trees approach to urban forest management would forego too many community benefits to be expected under the law.

Wise tree managers strike a balance between providing proper inspections, proper care, removal of un-correctable, dangerous trees and the retention of as many large, old-age trees as possible. Where safe, some dead trees are even allowed to stand for the benefit of wildlife and people who enjoy seeing wildlife.

In contrast to the college campus that harbored The Grim Reaper, Indiana University stands as a model of urban forest protection.

Indiana University's current site in Bloomington was once Dunn's Woods, a place referred to in 1884 as "unsurpassed in the State for its natural beauty." The founders were determined to keep it that way. Evidence that they succeeded is found in a commencement speech delivered 34 years later by Theodore Roosevelt when he noted the great maples and beeches and said that the scene was the most beautiful he had ever observed on a college campus.

{Indiana University}

Today the tradition lives on. Old giants that witnessed the arrival of the university's first students may still be found along the pathways and in the islands of green that dot the campus. Trees with broken tops or cavities that house the abundant squirrel population do not face the damnation of worried arborists. To the contrary, Superintendent of Grounds Michael J. Crowe says he would catch the wrath of top-level administrators if he removed anything but dead or seriously defective trees that clearly pose a threat to safety. Whether it is grounds keeping or the construction of a new building, the protection and proper replacement of trees to provide a complete, natural community of all ages are among the highest priorities on campus.

In an age of lawsuits, super-safety standards and artificiality, the words of former Chancellor Herman B. Wells bring perspective and guidance. In his last presidential address to the university's alumni, Wells said, "I hope our alumni will always insist upon retention of our precious islands of green and serenity -our most important physical asset, transcending even classrooms, libraries, and laboratories, in their ability to inspire students to dream long dreams of future usefulness and achievement -dreams that are an important part of the undergraduate college experience."

Tree Hazards and the Community Forestry Program

Lawsuits and countersuits are becoming part of the American way of life. It is said that in a single year as many as one suit per fifteen citizens may be filed. To the frustration of many, these actions are -shaping the way some communities provide services to their residents. Diving boards disappear from public pools, campgrounds close, fences go up and swings come down.

To prevent the threat of litigation dooming trees in public places, the first step is to understand the law well enough to keep your municipality, campus or institution from placing itself in an indefensible position. For this purpose, we recommend the following publication for an excellent overview of the legal questions and how they have been resolved in court: Legal Liabilities for Defective Trees in the United States by L.M. Anderson (Landscape and Urban Planning, 15 (1988) 173-184; Elsevier Science Publishers B.V., Amsterdam).

There are, of course, many variations in conditions, and in how state and local laws are written. Therefore, each case is unique in some way, and there is no intent here to dispense the kind of legal advice that can be given only by a qualified attorney. However, from Anderson's research there are at least seven keys to staying out of legal trouble:

  1. As research reveals new knowledge and urban forestry becomes more sophisticated, a higher level of duty to protect people and property from defective trees is expected. In short, up-to-date knowledge and the practice of good tree care is required.
  2. Managers of trees in an urban setting must carry out frequent, close inspections of every tree within striking distance of a road or street. According to one court case, this means walking inspections, not drive-bys, and it means that inspections must be made more often than once a year.
  3. Documentation of the inspection process is important, both to help plan and manage inspections and follow-up, and to provide supportive evidence in case of litigation. Computerized tree inventories can be a significant help in meeting these needs.
  4. Trained inspectors must be used, because valid liability claims have extended to trees in which defects would not be recognized by untrained observers.
  5. The warning signs of defective trees can not be ignored, nor is ignorance of urban forestry practices accepted as an excuse for damage.
  6. Not hiring or contracting with a professional, or choosing not to develop an urban forestry program, does not excuse a community from exercising proper management of its trees.
  7. The best defense against litigation is a sound, comprehensive urban forestry program. This must include not only systematic inspection of trees, but also responsible selection of species to plant, regular pruning, and general maintenance of tree health through watering and protection from pests.

{Indiana University}

Other Sources of Information

Tree City USA Bulletin will inform readers of helpful, up-to-date publications which provide more depth, serve as good models, or are readily available for community distribution. The editor welcomes sample copies to consider for inclusion in future editions.

Helpful Materials from Shigo

Probably no other single individual has done as much research and public education about understanding tree decay and hazards as Dr. Alex L. Shigo, former chief scientist and project leader with the USDA Forest Service. Some pertinent materials about hazard trees are listed below and are available from Dr. Shigo's consulting firm. Inquire about prices.

Shigo and Trees, Associates
4 Denbow Rd.
Durham, NH 03824
(Phone: 603/868-7459)

Tree Hazards -13 Questions That Could Save a Life (Leaflet)
A New Tree Biology (Book)
Tree Biology and Tree Care (Book)
Targets for Proper Tree Care (Article published in the Journal of Arboriculture, 9(11): Nov. 1983)



Tree Health Management: Evaluating Trees for Hazard

Written and produced by E. Michael Sharon and Dave Steinke of the USDA Forest Service, this 39-minute VHS cassette presents trees as both friend and foe, explains the nature of tree defects, and presents a "reasonable" approach to evaluating trees for hazard. Available for $25 ppd. from:

International Society of Arboriculture Research Fund P.O. Box 908, Urbana, IL 61801


Detection and Correction of Hazard Trees in Washington's Recreation Areas: A How-To Guide for Recreation Site Managers by Lynn J. Mills and Kenelm Russell

Washington Dept. of Natural Resources
Forest Pest Management, MQ11, Olympia, WA 98504

This 37-page booklet is well illustrated and contains much information that is useful beyond the State of Washington. Single copies are free.


Oak Tree Hazard Evaluation by Gary W. Hickman, Janet Caprile and Ed Perry. Journal of Arboriculture 15(8): August, 1989.

Although this article focuses on an evaluation method for only two species of oaks native to California, it also presents information about other evaluation systems and demonstrates the kind of information that needs to be considered when trying to develop an objective method of rating trees for hazard.

To order additional Bulletin copies ... Friends of Tree City USA members may obtain a single copy of any Tree City USA Bulletin free of cost. Quantities of any issue are available at 25 for $6.25 or 500 for $100. To order, specify the issue number and quantity, and make your check payable to: The National Arbor Day Foundation, 100 Arbor Avenue, Nebraska City, NE 68410.

To join the Friends of Tree City USA ... to receive a subscription to the Tree City USA Bulletin ... and to become more involved in the community forestry movement in your town and throughout America, send a $10 dues-donation to Friends of Tree City USA, National Arbor Day Foundation, 100 Arbor Avenue, Nebraska City, NE 68410. Make your check payable to: National Arbor Day Foundation.

To Help Publicize Trees ... ask for a free catalog of publicity and gift items available to help promote tree planting, tree care, and Tree City USA. Send your name and address to Arbor Day Catalog, National Arbor Day Foundation, 100 Arbor Avenue, Nebraska City, NE 68410.

Tree City USA Bulletin ©1990 National Arbor Day Foundation. John E. Rosenow, publisher; James R. Fazio, editor; Gerreld L. Pulsipher, graphic design; Gene W. Grey, William P. Kruidenier, James J. Nighswonger, Steve Sandfort, technical review committee. Although copyright is vested with the Foundation, permission is hereby granted for the contents of this bulletin to be reproduced for noncommercial educational or public-service purposes provided the source is acknowledged.

Published for the
Friends of Tree City USA

{National Arbor Day Foundation}

{Tree City USA}
   The Tree City USA program is sponsored by The National Arbor Day Foundation in cooperation with the U.S. Forest Service and National Association of State Foresters. To be named as a Tree City USA, a town or city must meet four standards:

Standard 1: A Tree Board or Department
Standard 2: A City Tree Ordinance
Standard 3: An Annual Community Forestry Program
Standard 4: An Arbor Day Observance and Proclamation

   Each winning community receives a Tree City USA flag, plaque, and community entrance signs. Towns and cities of every size can qualify. Tree City USA application forms are available from your state forester or The National Arbor Day Foundation.

Return to: Chapter 3 - Preparing for Natural Disasters