When I inspect large trees, the homeowners usually ask if their trees are safe. As an arborist involved with trees for more than 38 years, I realized not to use the word “safe” when evaluating a tree’s structure.
Arborists assess risk looking for signs or indications the tree could shed a limb or fall over. Arborists understand trees are subject to environmental stresses beyond our control.
We cannot predict with absolute certainty a tree’s structural integrity. A healthy, structurally sound tree can blow over by the uplift of the surrounding root system during a winter storm. This type of failure is called wind throw and occurs when the soil is saturated and doesn’t have sufficient strength to keep the root system firmly anchored in the ground.
The International Society of Arboriculture (ISA) has developed a Tree Risk Assessment Qualification. The arborist attends a three-day course focusing on tree risk assessment, mitigating risk and using the two-page ISA Basic Tree Risk Assessment Form. This comprehensive form considers many factors to establish an overall tree risk rating of low, moderate, high or extreme.
Last summer I took the course and passed the exam to become a qualified tree risk assessor.
I recently inspected a deodar cedar for a Curtis Park resident who was concerned about the tree toppling over because of a trunk lean. This particular cedar has a phototropic lean, which is a natural growth response to the surrounding environment. In this particular case, the tree was growing toward the sun away from an adjacent cedar.
My inspection determined that this cedar was not loose in the ground. The overall risk assessment of this tree is moderate.
Why wasn’t it rated high due to the size of the tree and the existing lean? Because this tree is growing between two other deodar cedars of similar size. The adjacent cedars buffet the leaning one from the prevailing winds.
Furthermore, the root systems of all three trees are grafted to each other, which provides better anchoring spread over a much larger area than if it were a single tree.
This homeowner also had the tree pruned in the past, which reduced the wind sail.
But no matter what risk rating a tree receives, I recommend that homeowners should inspect their trees after the wind exceeds 20 mph. Look for soil mounding, broken or hanging branches, and cracks in the wood.
Addressing these problems before the next wind could prevent damage to your car or home and, in certain circumstances, save the tree.