Decay is one of those things that’s inevitable. Inspecting for decay should be an integral part of providing good tree care for customers, as the damage it can cause is pretty serious. Consider how decay can work together with other maladies or injuries to cause tree failure or classification as a hazard tree.

Responsible pathogens

There are dozens of pathogens responsible for causing tree decay on standing trees. An article published in an issue of the Journal of Arboriculture outlines a key to identification for 64 species, comprised of 36 genera within 17 families. However, unless you’re a mycologist, plant pathologist or at least a serious biologist, it’s pretty tough to tell the specific pathogens apart. Doing so can be compared to distinguishing between cultivars of bentgrass on golf greens; sure there are some subtle differences, but without a turfgrass plant breeder’s knowledge and experience, it’s really tough to identify specific cultivars by simple observation. Suffice it to say that there are many responsible pathogens, each capable of causing decay. The bottom line for the arborist is that some decompose heartwood faster than others and may interact differently with each species of tree.

Smashed cars from tree

The owners of these cars probably wish that a certified arborist had inspected their trees for decay. Photo: John Fech

Where to look

In addition to the inevitable nature of tree decay, another aspect that sets it apart from diseases that attack the foliage and flowers of woody plants is how fast it progresses. Compared with many foliage diseases, the time from initial entry and infection is commonly several years, as opposed to several months. Since decay is a disease that takes a few years to develop, older neighborhoods are likely candidates for infected trees. As you consider marketing efforts, endeavor to enlighten the residents of your town that decay could be lurking in their trees and a thorough inspection is a prudent activity.

Compounding injuries

Once decay has set in, it’s important to perform a thorough tree health inspection to fully consider overall viability. In many cases, other maladies exist in trees with decay, such as codominant leaders, included bark, cracks, girdling roots, basal plate defects and leaning trunks. Also, the overall growing conditions are often not ideal, with compacted soils, slopes and radiated heat adding insult to injury.

Cracks and included bark often are the cause of decay, in that an opening in the bark and cambium is required for disease organisms to enter the trunk. In almost every situation, the common thread that unites these different species is that they have penetrated the sapwood and heartwood via a break in the bark, usually due to a physical injury of some kind. Other factors such as borers, fungal cankers, animal damage or other biological agents can also compound the problem of decay.

Cracks and sunscald

Cracks and sunscald are two of the compounding factors that compromise the bark. Photo: John Fech

Regular inspections

As part of responsible tree care, regular inspections are essential. A customer of a tree service recently voiced her concerns to me related to an unfortunate situation in her landscape. In this case, a large branch broke off a tree and fell on her car. The branch that fell on the car had a large hole in it, and lots of soft heartwood near the point of detachment. I asked the customer what kinds of services were performed by her tree service over the past few years, and she replied, “All they do is spray for bugs and then send me a bill.” If accurate, this is a case of clear negligence on behalf of the tree service. Simple and quick inspections of the tree structure would have prevented the damage and provided quality service to the customer.


In addition to concerns of overall tree health and aesthetics, the considerations of safety, hazard and negligence are important parts of decay scenarios. In terms of negligence, there are several components that build on each other, starting with the duty to exercise reasonable care. This can be the client’s responsibility or the arborists, depending on the contract for work. Negligence arises when you have failed in that duty, the failure of duty caused injury, and the injury caused real harm to people and property.

Failure in duty occurs when it can be documented by expert inspection and follow-up testimony that customary practices were not followed. Generally, the old phrase “it was an act of God” is not a good defense for negligence. In court proceedings, the judge or jury commonly asks two basic questions: “Were the managers (arborist, superintendent, property owner) negligent?” and “Would the hazard have been recognized upon inspection prior to the failure of the tree/branch?”

To reduce the judgment of negligence, courts commonly recommend three action items: perform timely and systematic inspections; develop written documentation of risks and concerns; and use risk assessment results in current and future tree management. In many scenarios, a lack of systematic inspection protocol would be considered negligence. In addition to the tree defects that are noted in the inspection, the consideration of target, or “what could the tree or tree part fall on?” is crucial to the overall management plan.

Tree inspection

Regular inspection and reporting will help to lower the liability of trees on a property. Photo: Sharon Skipton, UNL

Inspection steps

Every arborist has his own methods to inspect for decay. The following methods have pros and cons, perhaps there’s one you haven’t considered.

Tap the tree with the butt of a hatchet. Somewhat like tapping a melon to see if it’s ripe, tapping the bark with a solid object can give a crude sense of how much soft wood is present. If a thud or hollow sound is heard after tapping, then it’s likely that some level of decay is present; what is unknown is the amount. The benefit of tapping is that the location of the pockets of major decay can usually be identified, while the down side is the distance that the decay has traveled since the original infection. A plastic or rubber mallet may also be used.

Use a resistograph. For several years now, a device known as a resistograph has been used to gain a better sense of the extent of the decay in a tree. It is used by placing the unit against the bark and engaging a fine, motorized drill through the sapwood and heartwood. A printout is produced showing the areas of softened or durable wood; indicated by measuring the resistance to the drill, hence the name, resistograph. The benefit of this device is that a more precise measurement is provided; the negative is the cost of the unit and the time required to operate it.

X-ray the tree. A noninvasive device, similar to an X-ray machine for humans is available. Called acoustic tomography, it provides a visual representation of the inside of a tree. The appearance of the printout is similar to the latest weather forecasting technology for TV broadcasts. The pros and cons are the same as for the resistograph, although price and availability may vary.

Use your experience. My father used to say that there was nothing as important as good old know-how. What he meant was that there is tremendous value in the lessons learned from years of experience. In this case, cutting down trees and making observations about the inner parts. Astute arborists have learned to make observations about trees before and after takedown, and correlating the outer appearance to the presence and extent of decay inside the tree. The best part of this method is that it’s free; the limiting factor is that external symptoms can be misleading relative to the internal condition of the wood.

Use a probe. A sharpened piece of rebar or headless golf club can also be effective. This method is appropriate only if the decay is visible from the outside of the bark, such as when a wound has failed to close after a limb removal. When pushed into the tree, a reasonable sense of the extent of the existing decay can be gained. This method is a good one to start with, particularly because it’s quick and easy to perform, and doesn’t call for expensive equipment. The major limitations are that it’s difficult to reach to the end of the decay stream/column, and that it only is applicable on outer tree wounds. Other measurement devices include compression meters, stress wave timers and electrical resistance meters.

All of these methods are designed to help calculate the extent or determine how much decay is present or, more importantly, how much punky soft wood exists compared to sound wood. Some combination is most likely the best course of action; figuring out the best approach is an individual choice based on the above factors.

Preventing decay

Unfortunately, at this time, fungicide products are not effective at stopping or reversing decay. As a result, decay prevention and management of existing decayed trees are the main methods of control. Preventative methods can be direct or indirect, but are all important. Direct methods of prevention include corrective pruning, eliminating codominant leaders early in the tree’s life and eliminating crossing, broken or diseased branches. Indirect techniques include proper siting of trees through considerations of right plant, right place, proper spacing, proper planting procedures to prevent damage to the bole from basal decay and root rot, as well as the promotion of a healthy root system and utilizing a coarse wood chip mulch to prevent mower blight and reduce grass competition. By starting the mulch application 3 inches away from the trunk and using a 2 to 3-inch depth, vigorous rooting is also likely to take place.