Kelly Jackson
Christian County Extension Office
Tree Cavities
It is not uncommon for older trees to develop hollow centers due to wood decay. Fortunately, most of a tree’s center consists of non-living tissue. So long as a healthy cylinder of wood and bark three to four inches in thickness exists around the hollow the tree could live for hundreds of years. This does not mean a hollow tree poses no risk to your property or people. Hazards most often go unnoticed until high winds or heavy snowfall hastens limb or trunk breakage. Where large hollow trees pose an immediate risk to your home, car, or people consult an arborist to determine ways to lessen your risks or possible removal of the tree. However, if the tree is located a safe distance away from your home or other personal property, consider leaving it. As you will see later in this article, a hollow tree may prove very useful.
A cavity in a tree often begins in a seemingly innocent way. A branch is removed a little too close to the trunk, the lawnmower makes a solid “wack” as it bumps off a section of bark, or strong winds break a weak-angled limb out of the top of the tree. These and any type of injury that damages the bark and exposes the sapwood or heartwood can serve as entry points for disease-causing organisms.
The decay organisms that cause hollow trunks often progress undetected for decades. But panic is still the most common reaction to a homeowner’s discovery of a cavity in a beloved, old tree. Often compounding the problem is the presence of carpenter ants. The two questions that follow are “What should I use to kill these bugs?” and “What should I use to fill the hole?”
The answer to the first question is nothing. Carpenter ants cause no damage because they do not feed on or excavate living wood tissue. They will tunnel in decaying wood to create nests but wood removed will not contribute much to the loss of structural integrity. The answer to the second question is also nothing. In the past, everything including cement, asphalt, masonry, polyurethane foam, rocks, and gravel has been used to fill tree cavities. This is no longer the practice for several reasons:
- these materials are very abrasive, natural swaying of the tree leads to rubbing between the materials and the inside surface of the living tissue, allowing decay to expand,
- these materials do not bend, making a tree more susceptible to storm damage,
- and these materials do not bond to wood and gaps soon occur that trap water and hasten fungal development.
For these reasons, a reasonable course of action is simply to do nothing.
Another old practice is to bore holes at the lower end of a trunk cavity to drain water. This practice guarantees exposure of healthy, living tissue to decay-causing organisms and is not recommended. A water-filled cavity does not speed decay and may actually slow down the process.
Rather than focusing on the cavity, take steps to improve the overall health of the tree. A vigorous growing tree will be able to slow the spread of decay on its own. Moderate fertilization, irrigation during drought, mulching, and aerification by means of holes augered into the soil around the tree may be all a tree needs to halt the progress of decay in its trunk.
You may also take comfort in knowing that a variety of wildlife depends on tree cavities for nesting and denning. A recent study in Maine found fifty-eight different species of birds and mammals use tree cavities. Leaving trees with cavities or even dead trees, commonly called snags, where they do not pose a safety hazard, is necessary to provide habitat. Species you may see visit your hollow tree include downy woodpecker, yellow-bellied sapsucker, pileated woodpecker, northern flicker, black-capped chickadee, barred owl, Eastern screech owl, white-breasted nuthatch, tufted titmouse, gray squirrels, and flying squirrels. Let’s make sure they have a home.