As the winter storm season is fast approaching, the question is often asked: Just how safe are the trees in our landscapes?
Is there a chance they could lose major branches or even blow over? It’s not unusual to find an assortment of large trees in residential landscapes throughout our coastal region. Many of these trees are native to the area and undoubtedly not much thought was given to their eventual size when they were planted. As a result, towering seventy-five foot plus Douglas firs, Western red cedars and gigantic big-leaf maples along with other species often dominate the garden landscape.
Homeowners are often amazed when trees that appear to be healthy, with plenty of foliage, suffer major damage during a windstorm. More often than not, when a large limb or branch comes crashing down, inside portions of that limb or trunk are usually revealed to be rotten. The internal wood may be soft, dark in color, or completely hollowed out. These symptoms are all caused by wood rot fungus diseases that more than likely infected the tree in previous years through wounds to the trunk.
The puzzling part of this is that many trees have no visible symptoms of internal decay problems. Once wind damage has occurred however, the damage becomes very visible. Keep in mind the vascular system of trees is just under the bark, so trees infected with wood rot, can appear fine because water and nutrient exchange continues between the leaves and the roots. However, as wood rot fungi invade the heartwood or center wood of the trunk, trees become structurally weakened. Depending on the tree species and the fungus disease, some trees can live for years with no visible symptoms of internal decay. Once a tree is infected with rot, it is almost impossible to control it and usually the best advice is to remove the tree.
An on-going inspection of trees in your landscape will help to prevent potential storm-damage problems. Decaying trees can be prone to failure, but the presence of decay, by itself, does not indicate that the tree is hazardous. Advanced decay (wood that is soft, punky, or crumbly) or a cavity where the wood is missing can create a serious hazard. Often, trees infected with wood rot will produce fungus structures commonly referred to as “conks”. These are highly visible and often increase in size as the decay progresses. Other symptoms of wood rot include missing bark and discolored areas on a trunk or branch.
A tree usually decays from the inside out, eventually forming a cavity, but sound wood is also added to the outside of the tree as it grows. Trees with sound outer wood shells may be relatively safe, but this depends upon the ratio of sound to decayed wood, and other defects that might be present. Evaluating the safety of a decaying tree is usually best left to trained arborists.
Some trees are prone to failure due to weak branch unions. These are places where branches are not strongly attached to the tree. A weak union occurs when two or more similarly sized, usually upright branches grow so closely together that bark grows between the branches, inside the union. This ingrown bark does not have the structural strength of wood and the union is much weaker than one that does not have included bark. The included bark may also act as a wedge and force the branch union to split apart.
Trees with a tendency to form upright branches such as maple often produce weak branch unions. When the angle between the two branches is less than 30 degrees, a weak union will almost always develop. In such a case, it is best to prune out the smaller branch while still small. Ideally the crotch angle will be between 45 and 60 degrees. Weak branch unions also form after a tree or branch is topped where the main stem or a large branch is cut at a right angle to the direction of growth leaving a large stub. The stub inevitably decays, providing very poor support for new branches that usually develop along the cut branch.
Now is an excellent time to inspect trees in your landscape, especially those that lose their leaves in the fall. They should also routinely be inspected after severe storms. Examine all parts of the tree, including the roots, trunk flare, main stem, branches and branch unions. Be sure to examine all sides of the tree. Use a pair of binoculars to see branches high off the ground.
Trees in poor health may have many dead twigs, dead branches or small, off-color leaves. Trees in good condition will have full crowns, vigorous branches and healthy, full-sized leaves; however, green foliage in the crown does not ensure that a tree is safe. Tree trunks and branches can be quite defective and still support a lush green crown. Evaluating and treating trees as a potential risk of damage should they lose branches or fall over in a windstorm is complicated, requiring professional expertise. Never hesitate if you think a tree might be hazardous to have it evaluated by a professional. Consult your phone book under “Arborists” or “Tree Service.” The Pacific Northwest Chapter of the International Society of Arboriculture (pnwisa.org) has a listing of professional arborists in our area.
This column is printed as a service of the Washington State University Extension Office at Elma. Donald D. Tapio is the regional gardening and farming specialist for Grays Harbor and Pacific counties. He can be reached at (360) 482-2934 or firstname.lastname@example.org.