Lightning and Trees

The lightning protection system for large trees may require as many as three to eight branch cables.

The lightning protection system for large trees may require as many as three to eight branch cables.
Bob Polomski, ©2022, Clemson Extension

A woven copper cable attached to a lightning rod at the top of this 5 ft. diameter bur oak channels the electricity from a lightning strike into the ground.

A woven copper cable attached to a lightning rod at the top of this 5 ft. diameter bur oak channels the electricity from a lightning strike into the ground.

Did you know that Lightning Safety Awareness Week was June 19 – 25, 2022?  When you hear statements from the National Weather Service like this one: “Continuous cloud to ground lightning is occurring with this storm. Move indoors immediately. Lightning is one of Nature’s leading killers. Remember, if you can hear thunder, you are close enough to be struck by lightning.” This means that you are at risk of being struck by lightning, and so are your trees.

Unfortunately, it is difficult to predict which trees will likely be struck by lightning.  A tree’s location and the surrounding environment may influence its susceptibility to lightning strikes.  For example, the tallest trees in a group, ones near lakes, ponds, streams, and solitary trees, seem to be more likely candidates for lightning strikes and would benefit from lightning strike protection. Studies have shown that ash, elm, oak, maple, poplar, pine, and spruce trees are stuck frequently, whereas beech, birch, holly, and horsechestnut are not struck as often. Also, some observers have also noted that trees with smooth bark seem to deflect lightning bolts better than rough-barked species.

With thunderstorm season upon us, it may be worthwhile to install lightning conductors in the tallest and most important trees in your landscape. Installing 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 tree’s highest point.  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 or outermost branches).  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 depending on the size of the tree and the complexity of its crown.

For more information, see HGIC 2359, Storm and Weather Damage.

Did You Know: When lightning strikes a tree, the charge follows the most conductive path between the top and the 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 off the tree.  Sometimes an entire trunk shatters, which can kill a tree outright, but even survivors are often severely disfigured, and the wounds become entry points for fungi and wood-boring beetles.  In some cases, the charge causes soil around the tree to erupt, killing the roots before dissipating in the soil.

The type of damage lightning does to a tree depends on a variety of factors. The anatomy and physiology of the tree seem to have a direct influence on lightning effects.  For example, lightning often follows the grain of the wood.  The vascular tissue of pine and apple is arranged in a spiral fashion, and on such trees, when lightning strikes, branches die in a spiral pattern up the tree.  Elm and oak, which have conducting tissue aligned vertically, may show branch damage on only one side of the tree.

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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