• Trees with cavities = trees with disease. Because the defect is on the inside, it is harder to see and judge the danger from the outside.
• A reasonable policy can't be based on emotion. Extremes like "It's a beautiful tree, leave it alone!" or "It's a rotten tree, cut it down!" usually just cause more trouble.
• The happy medium is a policy that has two goals: 1) to protect trees from unnecessary removal, and 2) to protect people and property from unnecessary harm.

• Does the tree have a likely target?

-- Even a "bad" tree is not a hazard tree if has no significant target to hit.

-- Remove a tree with a cavity when it has a stationary target like a house.

• What kind of tree is it?

-- Different species have different mechanical strengths and disease strategies.

-- Be more cautious with species of high hazard potential such as silver maple, cottonwood, willow, basswood, boxelder, black locust, and tree-of-heaven.

• What is the tree's condition?

-- Look for good overall balance, a full and normal crown, and at least 4" annual growth.

-- A declining or leaning hollow tree is much riskier than a vigorous and upright one.

• How extensive is the rot?

-- Traditional methods include "sounding" (hitting the trunk) and probing a hole. Both are good techniques for finding the largest cavity, and to get a rough idea of its size.

-- A more scientific way is to drill with a 3/16" long-shafted bit a few places around the trunk at the most likely point of failure. Measure how far up the bit you are when resistance drops, the shavings become discolored, or nothing more comes out.

-- Research shows that a cavity is unlikely to make a tree fail if there is 1" (2" is better in high hazard species) or more of sound wood per 6" of diameter -- i.e., average sound wood thickness ¸ trunk diameter = 0.15 (high hazard, 0.30) or more.

How can I save a tree with internal rot?

• You can't. You can try to slow its decline, but you can't stop the disease.
• The best--and cheapest--intervention is usually to help the tree do its own work of containing the disease. Supply 1) weekly water during dry spells, 2) 2-6" of organic mulch, and 3) a low-nitrogen slow-release fertilizer in early fall or spring.

Nelda P. Matheny and James R. Clark, 1994. A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas, 2nd ed., Savoy, IL: ISA. Pp. 19-33. Or call us (716-461-1000).