Predicting which trees are likely to be struck by lightning is difficult. Studies have shown that ash, elm, oak, maple, poplar, pine, and spruce trees are struck frequently, whereas beech, birch, holly and horse chestnut are not struck as often. Your safest bet, however, is to protect the tallest and most important trees in your yard. When lightning strikes, the charge follows the most conductive path between the top of a tree and its roots. Typically this is the moist sapwood. Steam generated by the rapid heating of moisture in this wood results in bark or sapwood being blown oft the tree. Sometimes an entire trunk shatters. This can kill a tree outright, but even survivors are often severely disfigured, and the wounds become entry points for fungi and wood-boring beetles.
A lightning protection system does not prevent a tree from being struck by lightning; it simply conducts the electricity to the ground harmlessly. Such a system involves a lightning rod or air terminal installed above the highest point of the tree. From this solid copper or bronze rod, a woven copper cable runs the length of the trunk. Large trees may need as many as three to eight branch cables to protect major upper limbs. At the base of the tree, the copper cable is buried in a trench one to two feet deep that runs at least one and a half times the crown radius away from the trunk. (In other words, well beyond the drip line.) It is then attached to ground rods driven 10 feet deep into the earth.
Each tree, so equipped, provides a cone of protection in its vicinity. That is, lightning is unlikely to strike anything within a cone-shaped space that extends from the base of the trunk as far as the height of the lightning rod. Within this cone, lightning bolts will be drawn to the rod.
Designing and installing lightning protection in a tree is something that should be done by a professional arborist. The cost will vary from $800 to $2,000, depending on the size of the tree and the complexity of its crown.
This post is excerpted from the July/August 2000 issue of Horticulture.
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