Protecting Your Trees From Lightning
Every year when thunderstorms roar through Virginia, long- cherished trees are destroyed by lightning strikes. People and homes below the trees are also put in jeopardy. As an arborist, my job is both to protect trees, whenever possible, and to reduce the damaging consequences to trees that have suffered a strike.
Lightning strikes trees because they reach into the sky, providing a pathway for positive charges from the ground to lead toward the negative ions contained in thunderclouds. Although wood is a poor conductor, tree sap conducts much better than air, completing the circuit from ground to sky. Along the path of the strike, sap boils and the gas in the wood expands, often with explosive consequences. If only one side of the tree shows the evidence, a stripe spiraling from top to bottom, the chances of the tree surviving and eventually closing the wound are good. Trees are usually killed when the strike passes completely through the trunk, leaving a path of splintered bark on each side.
Species of tree, height, geographic location, and proximity to structures are all factors affecting the likelihood of a tree being struck. Whether a tree succumbs after suffering a strike depends on its species, health, and age. Keeping trees well mulched, watered, fertilized, and trimmed increases their ability to compartmentalize wounds and survive the vascular system damage caused by lightning.
Trees can be protected from lightning by installing a copper cable system that extends from air terminals near the top of every major trunk down to 10-foot copper ground rods driven beyond the tree's drip line. Special fasteners hold the cable away from the tree to protect the trunk. These systems, properly installed, can be expensive, costing up to $1,000 for a large tree. They are, however, relatively inconspicuous, non-injurious, permanent, and very reliable. Installing a makeshift system of aluminum material or cable fastened directly into a tree with fencing nails can be worse than nothing, attracting strikes directly down the side of the tree.
If a tree is struck, there is little that can be done immediately to improve the chances of survival. If in leaf, trees that are going to die quickly usually wilt within a few days. If this does not occur, I recommend watering and organically deep-root fertilizing the tree. If it survives long enough to leaf out the following spring, the prognosis for recovery is good. The loose bark and splintered wood should be cleaned from the path of the strike to remove insect and fungus habitat and to help the tree compartmentalize and surround (with new growth) the wound. Once cleaned, the stripe in the bark may be painted with shellac. Asphalt tree paint should never be used for it retards wound closure.
Tall poplars and oaks on hilltops close to houses are good candidates for being struck by lightning. Whether they are worth the expense of protecting depends on the situation. For all that trees give us-shade, wildlife habitat, beauty, oxygen, and just plain joie de vivre, lightning protection for our older and taller arboreal friends just makes sense. It is often too late once the fire-from-the-sky strikes.
Article contributed by Benjamin White, a certified arborist and licensed tree expert, owns Growing Earth Tree Care in Fairfax, VA.
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