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BEST ADVICE FOR A HAZARD TREE IS TO REMOVE IT
Many people ask about how to repair or help trees that have been damaged by wind or hit by lightning. Others ask what to do with old trees that have cavities and decay in their trunk or branches but still seem to grow vigorously each year. Should something be done to cover the decay, fill the cavities, or "strengthen" the tree?
My response in most cases is to remove the tree. At first glance this advice may appear drastic but the truth is that a weakened tree is a hazard tree. The next stiff wind could topple the susceptible tree onto a house, garage, car, swing set or unsuspecting human.
Minnesota law defines a hazard tree as "a tree with a defect plus a target in the path, such as a sidewalk, a car, or a house."
Yes, I know, the tree in question may feel like part of the family. It provides shade for the picnic table in the back yard. Your kids used to climb it when they were younger. Perhaps you even had your wedding photos taken under that beloved tree.
In the wild, a weakened tree can go on standing and leafing out each spring for decades. Weakened large trees, however, don't belong in developed neighborhoods where houses are close together and where trees that fall can damage property or living things. And unfortunately, no amount of human intervention - with chains, cement, cables or foofoo dust - will make a hazard tree stronger.
Consider the issue of liability from a hazard tree. Based on past legal cases, if a neighbor's tree is unsound and threatens your property, the neighbor may be liable for any damage that occurs. The test is whether the tree owner knew or should have known that damage was likely. A tree owner is not expected to be a tree expert, but he or she is expected to recognize obvious symptoms of a problem, such as the unseasonable lack of leaves, a dead limb, visible decay, or a tree that leans dangerously to one side. If the potential for damage is foreseeable and if the tree owner fails to take corrective action, the courts will likely hold the owner legally responsible for damage caused to people or property.
What about trees that have been damaged but aren't a threat to people or property -- trees that don't have a target, in other words? Should large cavities in tree trunks be filled, for example?
No, according to Patrick Weicherding, University of Minnesota extension horticulturist. "A tree cavity is similar to a cavity in your tooth," he explains. "Without proper treatment, the situation will only get worse."
Weicherding says most tree cavities are the result of improper pruning, mechanical injury or storm damage. When bark injury has occurred, the exposed sapwood or heartwood is more susceptible to attack by fungi that initiate the decay process. Insects and animals such as raccoons, woodpeckers and squirrels that inhabit tree cavities utilize the tree wound as the front door to their new home. Carpenter ants excavate tunnels in the decayed portion of the tree. These tunnels allow water to accumulate, furthering the wood-rotting process.
In the past, Weicherding says, tree cavities were filled with cement, asphalt, masonry and even rocks and gravel. "Today we know that these materials are very abrasive. Natural tree movements, such as swaying and twisting, rub the inside surface of the tree cavity against the filling, further weakening the tree's defensive walls and allowing decay to expand. In addition, these materials do not allow the tree to bend so the tree becomes more susceptible to storm damage. Additionally, cavity-filling materials do not bond with wood and these gaps often trap water. This dark and moist environment allows decay fungi to proliferate."
Some "tree surgeons" advocate drilling holes into the bottom of tree cavities to facilitate water drainage. Weicherding says cuts, drilled holes or tubes only cause more damage to the protective walls, leading to further decay. "Biologically, there is no reason to drain water from the cavity. Draining a cavity allows fast-growing, oxygen-requiring fungi to invade. Water-saturated wood has little oxygen present and is inhabited by slow-growing organisms."
So how should trees with cavities be treated? Weicherding cites recent research that shows that it is better to leave the cavity open. "Remember, no type of drainage, sterilization, fill material, wound paint or scraping treatment stops decay," he says. "A healthy tree has the strength to compartmentalize and wall-off decay."
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