Inspecting for Tree Decay

A tree that has fallen due to decay

Odd-shaped growths on a tree trunk are good indicators of internal decay. Tree decay is one of those inevitable things in life. By itself, decay can be a major concern, especially if found in a soft-wooded tree species such as silver maple or poplar. Fortunately, some species are quite resistant and if other stressors aren’t present in a significant capacity, it may not be as worrisome as other problems such as poor location, planting errors, over fertilization or drought.

Decay is Risky

When considering overall risk analysis of a mature tree, a basic formula can be used to determine how much risk is posed by each specimen. Risk equals the probability or likelihood that a particular tree will fail times the consequences of the occurrence. The presence and extent of the decay is just one of many factors that lead to tree failure along with the tree’s ability to compartmentalize and slow its spread. The capacity to predict tree failure is limited, however, when many defects (root girdling, compacted soils, leaning, codominant leaders, cracks, etc.) are involved, the potential is greater.


The second part of the equation, the consequences of failure, can be estimated by observing the specific site details such as nearby parking lots, homes, commercial buildings, bike paths, driveways, school activity and churches. Simply put, any human presence or property of significant value creates a “target.” The consequences of failure are largely tied to potential targets, in that trees or tree parts that could cause their loss are directly dependent on their nature. Trees near important targets such as those mentioned have a high degree of potential loss, whereas trees growing in the middle of pastures are concerning only if failure occurs at the same time a high-value cow is paying more attention to the forages than to a falling limb.

Hazards Versus Negligence

All trees carry some degree of risk. A hazard exists when a reasonable level of injury threshold has been surpassed. In this case, we’re talking about tree decay as a hazard. If a small amount of decay is present, which is common for smaller trees or trees with a slow rate of spread potential, then it’s a low hazard and classified as something to monitor. If extensive decay is present and notable in rot pockets, in cracks, etc., then the level of threshold has been surpassed and a distinct hazard exists.

You have a duty to provide reasonable and proper care for trees. Negligence occurs when you have failed in that duty, and the failure has caused injury that caused real harm to people and/or property. In the specific defect of tree decay, ignoring or delaying action when a substantial amount of decay is present could be considered negligence.

Regular and Thorough Inspection

In order to prevent damage to people and/or property, it’s important to perform regular and thorough inspections. How often is regular? The answer is – to a certain extent – it depends. If your property has targets (most of them do), then inspections should be made more frequently than if no targets exist. The frequency also depends on the tree species. Decay-prone tree species should be inspected more frequently than those that naturally resist decay such as walnut, Osage orange and black locust.

Another part of the answer is frequency of use, somewhat involving the existing targets. For example, if the grounds of a cemetery contain a large number of mature trees, a target-rich environment is probable. While the target is high, in this case the frequency is low, as 90 percent of visitors pay their respects to the departed on Memorial Day and Veterans’ Day. The opposite is true for shopping malls and campus grounds, where lots of people are present during the day, and there are also lots of high-value property targets. On these sites, the desired frequency is perhaps not weekly, but monthly wouldn’t be out of the question. Bottom line: On high-value sites with significant targets all mature trees should be inspected for the presence of decay at least seasonally.

Inspection Technique

Inspectors who have studied trees for many years have most likely have had the opportunity to work with hundreds of professionals and scientists and would find that each one has a slightly different methodology to determine whether or not decay is present. Though it’s hard to say if one is more effective than another, short of cutting a tree down and slicing it into wood cookies, a step-by-step approach works best when inspecting trees for decay.

  1. Use your eyes. Look for rot pockets, oozing, weeping, conks, and different colors on the bark and branches.
  2. Walk the property extensively and identify possible targets. Interview the property owner and neighbors to get a handle on the frequency of use on the site.
  3. Use your experience. Certain tree species in certain locations are likely to develop decay. Locate tree parts that could fall on a target.
  4. Look closer using probing tools: golf club, rebar, and irrigation flag. Use a rubber mallet or the butt of a hatchet to tap the tree trunk where you suspect decay is present.
  5. If necessary, use invasive tools such as a resistograph or core sampler. Reserve these for important tree specimens. For example, inspecting an oak at the entrance to the “Harvard Oaks” subdivision or a memorial tree. If the property owner has plenty of money to spend on inspection, consider the use of a sonic tomograph, a device that can illustrate the inside of the tree without cutting into it.
  6. Consider the potential for each tree defect to cause failure in conjunction with the proximity of an important target.
  7. Put it all together in the form of a relative hazard assessment, combining the presence and extent of the decay with other defects.