Risk. It’s part of everyday life.

You face risk when you wake up and shower, drive to work, get gasoline in your truck, hire a new employee, apply a pesticide and eat lunch.

These otherwise non-obtrusive, mundane activities can turn south if the water gets turned off and you’re left with soap in your eyes, get T-boned by a drunk driver, spill gas on your pants because the hose wasn’t maintained, get stuck with a low performer who lied on his job application, see phytotoxicity because the weather suddenly turned hot after an insect control measure and get sick because the dude who cooks at your favorite diner didn’t wash his hands after using the restroom.

As such, we’ve all gotten used to taking steps to prevent negative outcomes such as these. Inspecting the gas hose and looking both ways before pulling into an intersection are simple, common-sense prevention actions that should be a part of everyday life.

Turning our attention to the trees in a park, cemetery or a customer’s property, risk is present there as well. In fact, all trees that are older than a couple of years carry some level of risk. Caring for them is akin to all other tree care business course operations — the accounting, maintaining tools, climbing with ropes and operating chain saws. Of course, some of these are inherently riskier enterprises, but you don’t have to stretch your imagination or memory too far to consider possible hazards.

What’s a hazard? Simply put, it designates a likely source of harm, and that some level of injury threshold has been surpassed or is likely to be surpassed.

The rationale for tree risk assessments is four-fold. First, safety and accident prevention for property owners and tree care workers. Second, overall tree health; healthy trees are an asset and lead to many positive features, including shade benefits, wildlife enhancement and lower maintenance costs. Third, overall tree aesthetics, especially for those property owners who place beautiful surroundings high on the list of why they live where they do. And fourth, potential lawsuits, which are perhaps the greatest justification, because one mistake can run a city or shopping mall into bankruptcy.


Nobody wants to be negligent. It means that you didn’t do what you were supposed to do. Like most things in life, there are consequences. Negligence is a sequential cause-and-effect phenomenon. It goes like this — the property owner has a duty to exercise reasonable care; he or she failed in that duty, also called a breach; the result was an injury; the injury caused real harm to people and property.

Failure in duty is sometimes defended by claiming that an injury was caused by “an act of God.” Unfortunately, this is a problematic justification. The only real way for this defense to be plausible is for the tree to be (1) planted by nature and (2) never have been significantly influenced by humans… which pretty well excludes all trees in a well-cared-for landscape. Perhaps the trees in the back alley might qualify…but why would you want to take the risk? Thus, “an act of God” doesn’t really apply here.

It’s common for specimens to have more than one defect, which increases the degree of hazard.

Court questions and recommendations

When a catastrophic tree failure occurs, the injured party may turn to the courts for satisfaction or to recover perceived losses. Generally speaking, judges and juries are most interested in determining if the property owner was negligent, and if a hazard would have been recognizable before the failure. Both the plaintiff and defendant are likely to seek evidence and documentation by expert testimony that customary practices were not followed. Often, written documents and photos are useful in proving negligence or establishing that quality tree care was, in fact, provided.

In either case, it’s common for the court to recommend that, going forward, regular inspections be made by certified or registered arborists to document existing defects and prioritize the trees on a given property in terms of risk. As well, these inspections and monitoring plans are encouraged to be the basis for the development of future maintenance plans for the property. In many scenarios, a lack of a systematic inspection protocol would be considered negligence.

Co-dominant leaders spell trouble for long-term stability.

The heart of the matter

Broadly defined, a tree risk is the likelihood that a particular tree will fail, multiplied by the consequences of the occurrence. Whenever risk is discussed, the term “hazard” also is commonly brought to the conversation. Whether a tree and its surroundings are considered a risky or hazardous situation, there are two foundations to be evaluated: a particular defect or point of weakness in the tree, and the “target,” or something of value that a tree or tree part could fall on.

First, the target

Putting compassion for people first, a target can be looked at in several ways. It’s reasonable to assume that a person in a hazardous landscape is unaware of his or her surroundings and that the property owner has done due diligence and provided proper tree care. That being said, there are targets and then there are targets; some are more likely to be problematic than others. Generally, the frequency of usage and exposure and the value of the object are the key elements.

Categorizing targets as low, medium and high can be helpful in making these situations tangible. For example, a low target would be one where older trees were growing in a fence line alongside a pasture with cows. The grass that the cows eat and the cows themselves are the target. Unless the density of cows is unusually high or the owner is an obsessive carnivore, the target is low.

Leaning should be noted and monitored over time for change in angle.

A medium target might be an occasionally used park, where joggers exercise each day on running trails that pass by mature trees. In this scenario, the salient points are that a person or persons are present each day as well as the number of times each day that a person is present. Another medium-target situation would be where a privacy fence or storage shed is in place on the edge of a seldom visited residential property.

A high target is best described as a site where both human activity is either constant or frequent in close proximity to mature trees. Golf course practice greens, benches along sidewalks on a college campus and driveways and sidewalks in residential neighborhoods are high targets.


There are seven significant defects that cause concern if a tree is in a target-rich environment. There are others, usually variations of the seven big ones, but the primary defects are:

Regular, thorough inspections to document existing defects is of key importance in reducing risk.

  • Cracks: Cracks are a physical separation of bark, cambium, softwood and sometimes heartwood tissue. They cause a loss of structural support as well as an opening for decay, other diseases and insects to invade.
  • Root plate issues: Caused by mower blight, deep planting and root rot diseases, the softening and degradation of the root flare is crucial. The connection to the lateral root system is often lost in these cases, which leads to instability.
  • Stem-girdling roots: Roots that grow around the tree trunk instead of horizontally away from the stem establish themselves in such a way that they grow to compete with the trunk itself. As both the trunk and the root add layers of new growth and expand in girth, they begin to impinge on each other, creating points of stress and contention.
  • Leaning: A tree develops a lean, or change of orientation from 90 degrees vertical, due to loss of attachment of roots to soil particles. It’s common to see a raised mound of soil on the opposite side of the lean in these situations where the roots have come loose. The most important consideration in leaning trees is to monitor the degree of lean over time; if it increases noticeably from one season to the next, the tree should be removed.
  • Decay: Softening of the inner heartwood tissues usually is due to heartwood decay, an infection that is the result of entrance through a wound, such as a removed limb or hail damage. Some tree species, such as walnut and Osage orange, are highly resistant to decay, while others, such as silver maple and cottonwood, are susceptible.
  • Co-dominant leaders: A failure to remove one of two equally prevailing terminal stems is the cause of a tree with a co-dominant leader. If both are left to grow, the crushing effects similar to stem girdling roots are realized, often resulting in the failure of one of the leaders.
  • Growing conditions: Though not a true defect, poor growing conditions (compacted soils, slopes, excessively frequent irrigation, competition with turfgrass, etc.) often lead to defects. Trees growing in poor conditions often have underdeveloped root systems, at risk for basal defects, decay and leaning.
  • Non-defects: Often confused by clients as defects, occurrences of peeling bark and yellowing needles can be natural features of a given tree. Species such as sycamore and hickory often exhibit exfoliating bark as designed by Mother Nature, while a third to half of the inner foliage of evergreens usually is shed each fall. These incidents are a good opportunity to demonstrate an arborists’ education and experience to the customer.

Inspection tools

The most important inspection tools that can be used are eyes. Arborists can spot defects, assess growing conditions and determine the level of target on a property. In addition, tree care workers can get help from noninvasive tools like probes, screwdrivers, garden cultivators, chisels (my personal favorite), pocket knives and sonic tomographs. Invasive tools include shovels, air spades and resistographs. Each has limitations and great potential for finding out more information about the status of a tree and how much of an overall hazard exists.

Risk calculations

At some point in the assessment process, a calculation needs to be made. There are three main parts of the risk assessment calculation:

  • Probability of failure: Defects, tree species, growing conditions;
  • Size of tree part: One branch, several branches or the whole tree;
  • Target: High, medium or low

A point system can be used to rate each part, based on a thorough inspection of each tree on a property. There are many ways to assign points; for example, between 1 and 5 points for the probability of failure, 1 to 3 points for the size of the tree part and 1 to 4 for target, for a total of 12. Creating the possible number of points allowable for each category is a bit like Congress marking up changes the tax code, in that these numbers place higher or lower value on each component, much like tax deductions and credits. The most important consideration when assigning points is to be consistent as an assessor so that trees are evaluated fairly across time and space.