It seems many of the mature trees around town have started to show signs of drought stress. After inspecting several of the distressed trees, it is apparent that there is another, more serious problem, arising. Many of the mature trees that I have looked at lately, are hollow — a condition that can create a hazardous situation.

Here is information to help you identify and care for these unsafe hazards in the landscape:

A hollow tree is one that has lost some or all of the inner wood, called heartwood. Physically, heartwood is not alive. It does not conduct water or nutrients and is not used for energy storage. It is a “warehouse” for tannins, acids and other chemical compounds not needed by the tree. Although it is not extremely important, it does provide some structural stability for the plant. When missing, the tree can be weakened and is more prone to failing, or falling over, in adverse weather conditions.

Heartwood is lost because of various factors. The most common being from wood decaying organisms such as fungi and bacteria, followed by insects then animals. Usually, one leads to the other. Unfortunately, little can be, or should be, done to stop the loss of heartwood. The tree has a natural defense system that tries to wall off the infected area to prevent further decay. Any effort to clean the infection may result in a disruption of this barrier formation. So it is best to leave the tree alone and do nothing to the wound.

To identify a hollow tree, visually inspect it for openings. Not only on the main trunk but on branches higher up. Decay can start high in the tree where a branch broke off or where two branches come together called a crotch. Likewise, look for loose bark, cracks, or other imperfections on the trunk that may be hiding a problem within. Finally, use a hard object to lightly tap on the trunk. There will be an audible difference between a tree that is solid and one that is hollow. If you find growths such as mushrooms, conks, or cankers on the tree, there is a good chance that a wood decaying organism is hard at work.

There are two options when it comes to dealing with a hallow tree — cut it down or leave it alone. But remember, any tree has the potential of falling at any time. With that said, research has proven that trees with at least three or more inches of good wood all the way around are still somewhat structurally sound. Less than that, and maybe the tree should be removed. Likewise, a tree is less stable if there is a hole that allows you to see into the hollow cavity. If the gap is more than one-third the diameter of the circumference of the tree, consider having the tree removed.

As a final thought, consult with a certified tree arborist if you have any concerns about a hazardous tree in your landscape. With the summer drought and adverse growing conditions, already weakened trees have been made even more dangerous.