Cooperative Extension Service 
Institute of Food and Agricultural Sciences
Hendry County Extension P. O. Box 68 LaBelle, Florida 33975-0068 Phone (863) 674-4092

Hendry County Horticulture News

Lightning Strikes - Florida Leads the Nation

Here in south Florida, we are living between the Temperate Zone to our north and the Tropics to the south.  This fact greatly influences our local weather patterns.  The tropical influence is apparent in our rainfall pattern which is divided into five or six months of wet weather and six or seven months of dry and our long sultry summers which seems to start in April and endure until late October.

The influence of these two climactic zones sometimes results in violent weather conditions which can have dramatic effects on gardening and outdoor activities.  Key West is only 50 miles north of the Tropic of Cancer, the northern edge of the tropics and we are greatly influenced by the torrid zone.  Typical of our tropical rainy season are frequent brief down-pours, which can deliver several inches of rain in a few hours and account for most of our average annual accumulation.  Equally characteristic are the sometimes violent thunderstorms which frequently occur.  Florida has the highest incidence of lightning strikes in the continental United States and shares the dubious distinction of being one of the most lightning prone spots on earth.

Thunderstorms happen when warm moist air rises from the earth surface to some 30,000 feet, where the surrounding air is cold enough to cause water vapor to condense into ice crystals and rain drops.  Rising and falling ice particles and water droplets collide producing a negative electric charge in the cloud.  When a lightning strike occurs, negative charges jump to the positively charged ground creating a charged path.  A powerful spark then leaps along this path from the ground to the cloud.  This is the lightning bolt.  Lightning can also jump from cloud to cloud or within a cloud.

Summer 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 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.

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.

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.

While there is little that can be done to protect plantings from the random effects of lightning, an understanding of this spectacular natural phenomena may help us be able to better evaluate stricken trees and assess their chances for survival.  Good luck and good gardening.

Gene McAvoy is the horticulture agent with the Hendry County Extension Service.  Direct your horticulture questions to PO Box 68, LaBelle, FL 33975,  e-mail - or phone 863-674-4092 or  863-983-1598.  You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle.   Office hours are from 8:00 - 5:00.

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