A tall, slender chestnut oak tree leans precariously close to the back of a house in a wooded Northern Virginia neighborhood.
If the oak and its neighbors had been pruned when young, it would have grown straight and strong. But the tree is decades old, 70 feet tall, and its oblique angle and a large, old wound at the base suggest it is structurally unsound.
Many old shade trees have cavities from past injury or disease; nature allows them to compartmentalize the damage. But if a tree has lost a third or more of its living tissue within its trunk and is likely to do damage if it falls, tree surgeons consider it a hazard and recommend its removal.
But how can you tell?
Enter arborists Craig Herwig and Jeff Aten, and a piece of new equipment called a Resistograph. Resembling a cordless drill attached to a typewriter roller, the device's drill bit can extend about 18 inches into a trunk. While the drill does its work, a needle with ink maps the bit's path. High resistance means solid wood; low resistance reveals a void. The probe's progress is recorded on graph paper.
On the oak, drilling at the base reveals a cavity. Two more holes higher up show a column of dead wood inside the trunk; Aten and Herwig estimate 20 percent structural loss. Factor in the angle at which the tree leans, creating a potential weak spot in the trunk, plus an existing crack, and the tree's future becomes clouded.
In this instance, new technology helped the diagnosis. Still, the best way to protect and maintain old, valuable shade trees is through well-established practices. It all boils down to pruning but the right pruning at the right time.
"No care is better than bad care," said J. Peter Deahl, a third arborist with the company Care of Trees. "A lot of topping still goes on," he said, referring to the practice of arbitrarily removing large limbs. The tactic reduces the mass of the tree but creates ugly regrowth and promotes pests and diseases. It's better to be conservative and remove only dead wood, seriously conflicting branches and watersprouts fleshy stems erupting from old branches.
Other tips: It is far easier to shape a tree when it is young, say, a year or two after planting, than after the tree is near maturity. Also, to keep an ornamental tree, say, at 12 feet high, prune it as it approaches that size, not at 20 feet.
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