Lightning
by Dr. Robert Black, Consumer Horticultural Specialist

This is the time of year when a number of trees are lost due to lightning.

Lightning injuries to trees are extremely variable and appear to be governed by the voltage of the charge, the moisture content of the part struck and the species of tree involved.

The woody parts of the tree may be completely shattered, then may burn. A thin strip of bark parallel to the wood fibers down the entire length of the trunk may be burned or stripped off, the internal tissue may be severely burned without external evidence, or part or all of the roots may be killed. The upper trunk and branches of evergreens may be killed outright, while the lower portions remain unaffected. In crowded groves, trees close to the one directly hit may also die. In many cases, grass and other vegetation growing near the stricken trunk will be killed.

So-called "hot bolts" with temperatures over 25,000 degrees Fahrenheit will make an entire tree burst into flames, while "cold" lightning can make it literally explode as it strikes at 20,000 miles per second. On occasion, both types fail to cause apparent damage, but months later the tree dies from burned roots and internal tissue damage.

Tall trees, those growing alone in open areas, trees with roots in moist soils or those growing along bodies of water are most likely to be struck.

Though no species of tree is totally immune, some are definitely more resistant to lightning bolts than others. Birch, for example, is rarely struck, whereas elm, maple, oak and pine are commonly hit. The reason for the wide variation in susceptibility is not clear.

Some authorities attribute the variation to the composition of the trees. Trees high in oils (birch and beech) are poor conductors of electricity, whereas trees high in starch content (ash, maple, and oak) are good conductors. In addition, deep-rooted or decaying trees appear to be more subject to attack than are shallow-rooted or healthy trees.

It is commonly believed that lightning never strikes twice in the same place. This is not true for some trees have been struck by lightning as many as seven times, judging from the scars on their trunks.

Inspect an injured tree carefully before any attempt is made to fix the damage. Many trees are severely injured internally or below ground, despite the absence of external symptoms and will soon die regardless of treatment. Consequently, expensive treatments should not be undertaken until the tree appears to be making a good recovery.

Where external damage is not great or where the tree is particularly valuable, several immediate measures are justifiable. Some benefit is derived by tacking on and covering with burlap any long thin pieces of bark that have been split or lifted from the sapwood. Shattered limbs and torn bark should be removed carefully. In addition, the tree should be fertilized to stimulate vigorous growth.