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How to Recognize Tree Hazard

A large bark inclusion and extensive internal decay. A qualified arborist could have detected this while the tree was standing.
Sounding is a very good way to quickly identify areas of concern within a tree.
Understanding how identify pathogenic wood decay fungi is critical to managing tree risk.
With enough time and load, bark inclusions will split apart.
Over-extended lateral limbs are a major structural concern for large older trees.
Over-extended lateral limbs need to be shortened to mitigate this type of failure.

(This article by Chuck Lippi was published in the Home & Garden Section of the Daytona News-Journal)

Trees add many benefits to our yards, neighborhoods and cities. Trees provide cooling shade, beauty, habitat for birds and animals and a softening visual break from the straight lines of urban buildings and roads. There are also psychological and social benefits of living in an area with trees. And many realtors understand a well-placed, healthy tree or a wooded lot can significantly increase property values.

But trees are living things that occasionally break or topple over especially during wind storms which are common during Florida’s summer months. When a tree or tree branch lands on people or property, that can mean trouble. Every tree has the potential to fail but only a few do each year. It is important for property owners to regularly check their trees for defects and problems. Even cities and counties are responsible for the safety of the many trees on public property. Sometimes the arborist can lessen the potential of the tree to fail. A property owner can check trees in the yard to determine if an arborist should be called.

If a tree has a history of dropping limbs, it should be examined closely. Another sign of trouble is a large dead or leafless branch. Often dead branches are symptoms of root problems or construction damage years earlier. Smaller dead branches, however, are common on a mature tree. The tree simply shuts down and sheds smaller unproductive branches that are shaded too much to produce sufficient carbohydrates for the tree. In both cases dead branches should be removed because they will eventually fall.

Narrow v-shaped crotches especially those located low on the trunk are prone to fail. The desirable and strongest shape for large shade trees is a single leader. When a tree has multiple leaders of the same size low on the trunk, some weight should be pruned off the leaders with reduction cuts (not topping cuts) to prevent the tree from splitting.

Cavities and decayed wood are other signs of potential problems. Most mature trees have some decay and cavities. The location and size of the decay determine if the tree is at risk. Some trees with rather large cavities are still structurally sound while other trees are not. Decay in a live oak, for example, is often less serious than the same size decay in a laurel oak or water oak. A consulting arborist with decay measuring equipment can best evaluate decay.

Although tree roots are below ground and invisible to most observers, there are indications of possible trouble. If you have ever noticed trees growing in a natural forest, all the trees have a widening at the base which we call the root flare. Some tree species have larger root flare than others but all trees have root flare. And the root flare should be visible. If not, fill dirt could have been placed around the tree during construction years before. The root flare is meant by nature to be above ground. When buried, the root flare can decay which will increase the possibility of tree just toppling over. That also brings up another root problem which is roots cut by construction for a house or sidewalk. Trees react very slowly to construction damage which may not show up for five or 10 years.

Excessive pruning or improper pruning can also make a tree more prone to failure rather than safer. Topping, the process of removing a large branch and leaving a stub, is an outdated pruning method that causes excessive and weakly attached sprouts to form just below the cut. Also a topping cut opens the tree up to decay. Proper cuts made back at a crotch or branch union allow the tree to form a protection layer that can slow or stop decay. Another widely used pruning technique that is bad is lion-tailing which is the removal of smaller interior and lower branches. A tuft of branches is left at the end of a larger branch much like a lion’s tail. Don’t allow an arborist to “clean out” those sprouts and smaller branches because these branches help make the branch thicker by depositing carbohydrates into the wood of the nearby larger branch. When a branch has been “cleaned out” or lion-tailed, then it will not thicken and grow stronger and most of the weight is out at the end making the branch more susceptible to breakage. Smaller branches should be evenly distributed throughout the tree canopy and not excessively “limbed up” or cleaned up.”

Also excessive crown thinning may also weaken a tree by removing too many leaves which produce carbohydrates that power the tree and by removing too much wood where the carbohydrates are stored. Usually you should not remove more than 10% to 20% of the canopy of a mature tree and not more than 30% of the canopy of a young tree.

There is some new research that indicates thinning a tree to reduce wind load during storms will actually increase the speed of the wind as it is funneled through smaller openings in the canopy much like making holes in a sail. The preliminary research indicates reducing the size of the canopy (sail) by reduction pruning (not topping) is the better way to reduce wind stress on trees.

Fungus growing at the base of a tree or on the trunk or branches is a sign that there is decay in the wood. Usually fungus at the base of a tree is in the form of brownish orange mushrooms or shelf-like fungal conks while fungus on the trunk or branches is in the form of conks, shelf-like spongy growths that measure from one to six inches wide. If you see fungus on your tree or right at the base, you should have your tree checked. External lichens and fungus growing on the bark, however, are common and not a problem for the tree.

Large trees with limited root space can be prone to tipping over as the tree grows larger. Roots anchor a tree sideways not deep. Long roots help brace a tree. When roots are not allowed to grow outward, the tree is in effect “pot bound” and can tip over.

Finally, a leaning tree, especially a tree that has recently started leaning and has soiled heaved up near one side of the trunk, should be checked. A sudden lean indicates pending failure. Other trees such as many live oaks, however, have grown up leaning and are strong and stable.

Given the natural variability of trees, an arborist may not be able to detect every possible way a tree or part of a tree may fail. Nor is there a way to predict when a tree will fail. Nor is there any complete guarantee that any procedures done to help the tree will prevent failure in the future.

Remedial steps an arborist can take to reduce a tree hazard

  • Move the target: Although you can’t move the house, sometimes a landscape feature, parking site, playground equipment or picnic table can be moved out of the danger zone.
  • Pruning: Remove the defective branches and reduce branches to reduce load leverage.
  • Cable and brace the tree: Provide physical support for weak branches.
  • Removal: Some hazardous trees are best removed