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Managing Risk in Urban Trees

fallen branches above playground

Trees provide a tremendous benefit to our homes and offices and should be checked annually or after severe storm events to minimize the risk of injury or property damage. It's important to learn how to recognize defects and damage in your trees in order to decide what steps are appropriate to take to manage the potential risks. When evaluating a tree a tree care professional will look to see if two criteria are met:

  1. A defect is likely to result in failure, meaning the tree could split, crack, fall, or break off, and
  2. the existence of a target that will be damaged, destroyed, or killed if the tree fails.

Safety should be the top priority when assessing trees for defects or performing any kind of tree work. If the work to be done is higher than you can reach from the ground, or the tree needs to be removed completely, we strongly recommend contacting a professional tree care company. If the tree and its branches are over or adjacent to power lines contact your local power company first. They may be responsible for the tree if it is within a right of way or easement. Special training is needed for trees near power lines and if the power company will not perform the work then you must find a professional tree care company. Visit our care and maintenance page for advice on finding an arborist or professional tree care company.

Evaluating the seriousness of defects and major damage is best left to a professional tree worker or certified arborist, but there are things you can look for to know when to contact a professional.

Assessing the Tree

First, look around the tree for potential targets - things that will be damaged or destroyed, or people or animals that may be injured or killed if the tree were to fail. If there is no target within striking distance, as when a tree is in a forest, then there is minimal risk. If there is a target present, then if possible remove the target in order to mitigate the situation. For instance, if a picnic table, patio furniture, or a trailer is located under a tree with significant decay move it to another location.

Next, take a look at the tree itself. Walk around it at a distance, and then again up close. Be on the look out for defects such as:

The more of these defects present, the more likely the tree might need professional evaluation of its risk level. For more information on each of these defects and what to look for, read on below about the types of defects in trees.

What Not to Do

  • Don't try to support a damaged tree with rope, cable, wire, bolts, or similar materials. The effort will probably NOT increase the safety of the tree. If cabling and bracing are necessary, they should be done only by a trained arborist.
  • Don't try to save a tree that was pushed over by a storm unless it was recently planted. The tree's roots will likely never develop well enough to adequately support the tree again.
  • Don't fertilize damaged trees. The use of nitrogen can make a stressed tree even more susceptible to insect pests and diseases and reduce the ability of the tree to deal with the damage that has occurred.

Types of Defects in Trees

Dead, Broken, or Loose Branches

If you see dead, broken, or loose branches in the tree, remove them as soon as possible if it is safe to do so. They could fall at any time and cause injury to people or damage to property, which means they pose a substantial liability to the land owner. If the branch is higher than you can reach from the ground, contact a professional tree care company to have the dead, broken, or loose branch removed. When removing the damaged or loose branch, make sure to follow best pruning practices.

bark missing from a tree

Missing Bark

Your tree might have missing bark or missing wood on the trunk or on branches. These defects can be caused by branch failures, wind, snow or other injuries and decay. Missing bark can allow decay into the tree and weaken it's structure - think of it like a cut in your skin, which can allow bacteria and viruses into your body. Trees can heal themselves, and some missing bark may not be a cause for concern. However if the tree is missing large amounts of bark or other signs of decay are present, then it may be time to contact a professional for evaluation.

Leaning Trunk or Raised Ground Around the Tree

If a tree is leaning, even if only slightly, and the soil around the base of the tree is somewhat raised on the far side of the lean, a root failure has most likely occurred. Urban environments often restrict structural root growth because of restrict growing space. Root systems don't have room to grow down and/or out due to underground utilities, soil compaction or structures, such as storm sewer pipes, subways, concrete barriers, etc. This situation leads to smaller anchoring systems trying to support large tree canopies. Particularly during storms, this unequal support system fails and the trees uproot themselves. Root failures in established trees are not reparable; the tree will not reestablish the structural anchoring necessary to be stable. The tree should be removed by a professional.

Presence of Cavities

cavaty in the side of a tree

Look for holes and cavities in the trunk and large branches of your tree. Cavities may be caused by broken branches, injury, or animal and insect activity. If the holes are visible or accessible from the ground, look for signs of moisture and decay inside them and notice how deep the cavity goes. Like wounds in your skin, cavities allow decay and disease into the tree which will weaken it over time. However trees can heal around injuries and remain stable for many years with cavities in them. There are two rules you can use to determine if the tree or branch is at high risk of failing due to a cavity:

  1. If it is a very large cavity or hole, meaning if more than 40% of the tree is missing, then the tree should be removed.
  2. If the wood around the outer circumference of the tree is less than 2 inches thick for each 6 inches of trunk diameter AND the opening in the tree (the cavity) is greater than 30% of the stem circumference then the tree or branch should be removed. For example, if a tree is 24 inches in diameter (2 feet) and has a cavity in it then the wood around the outside of the tree should be at least 8 inches thick (24/6 = 4, 4*2 = 8), if it less than that the tree is at greater risk of failure.

Cracks and Splits in the Bark or at Branch Connections

Trees with split trunks will likely fail completely in a later storm and should be removed. Vertical cracks can be caused by a variety of reasons, may be associated with decay and will eventually fail. Horizontal cracks indicate that the tree is already failing and should be removed as soon as possible.

Branches with strong attachments create a ridge of raised bark at the intersection of the branch and the trunk. If the ridge is absent and the bark grows into the branch attachment, then the upper side of the branch is unable to secure itself to the trunk. This creates a weak point where a branch can fail and fall during storms, wind, or other environmental stress. Epicormic branches (also called water sprouts or suckers) are formed as a response to bad pruning, topping, other injury or environmental stress. Epicormic branches always have weak attachments and grow very quickly.

a tree with a large crack in it

Another thing to look for is co-dominant stems - when a tree trunk splits into 2 or more stems of are approximately the same size and arise from the same point. Co-dominant stems and narrow "V" shaped attachments often are weakly attached, you may notice cracks or splits at these connections. These are the points at which failure will occur when the tree is under stress from high winds, snow or ice. The best time to correct a these weak attachments is while the tree is young.

Dead Wood

A dead top of a tree might indicated disease or insect damage and should be evaluated by a professional. Dead wood should be removed using best pruning practices. Dead trees should be removed if they were to pose a hazard by falling.

Presence of Fungus or Wood-Eating Insects

Decay causing fungi can attack a tree through a pruning cut, a broken branch, a lawn mower injury or other break in a tree's bark. Decay will cause the tree to rot, and rotted wood is structurally weak and prone to failure. The presence of mushrooms and other fungi growing on the tree's trunk or limbs is a sign of decay. Mushrooms growing at the base of a tree can be specifically be a sign of root or trunk decay and should be evaluated by an arborist or professional tree care worker.

Wood-eating insects, particularly carpenter ants, are also a sign of decay and poor health in a tree. Look for insect activity in and around injuries on the tree. High levels of activity may indicate a significant problem that should be evaluated by an arborist or professional tree care worker.

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