Storm Damage to Landscape Trees: Prediction, Prevention, Treatment
Copyright © 2012 Regents of the University of Minnesota. All rights reserved.
In the upper Midwest, wind and ice storms are common, and often cause tremendous damage to individual trees and urban forests. These storms can cause weakly attached branches or leaders to separate and rip trees apart.
Trees with stem girdling roots or inadequate root systems may blow over or break off at the ground line.
Whatever the case, these effects can cause both personal and property damage.
Predicting tree failure
Figure 6. This branch bark ridge indicates strong branch attachment. Photo by Gary R. Johnson
The key to preventing damage is to predict circumstances that could result in tree damage and take action to correct potential problems before storms strike. Look at the form of the tree, any decay that may be present (and the extent of the decay, if present), maintenance practices involving the tree, the presence of stem girdling roots, and existing site problems. There may be other predictable situations present, but these factors are the most common.
Figure 7. Codominant leader with included bark = poor form and potential hazard.
Photo by Gary R. Johnson
Form: Trees may suffer from several natural form imperfections that lead to damage under storm conditions. Inclusion of bark at branch unions is a common weak point in trees. Branch unions should have a rough, protruding branch bark ridge where the branches meet. Without this proper union, branches or leaders have a tendency to separate during storm situations. Trees may also have codominant leaders. This occurs when a tree has two or more branches or stems that are trying to become the centerpoint of the tree. Codominant leaders typically exhibit inclusion at their unions. Some species that are notorious for having included branch unions and codominant leaders are:
- European mountain ash
- Green ash
- Red maple
- Amur cherry
- Silver maple
- Littleaf linden
Decay is the natural degradation of tree stem, branch, and root tissue. Degraded tissue has very little strength and is the most common contributor to tree failure (based on University of Minnesota Storm Damage Research). Location of decay within a tree is critical in assessing the tree's potential for failure. Decay located within a single branch is not always a critical situation since weak branches can easily be removed.
Figure 8. Decay and codominant leaders led to failure. Photo by Gary R. Johnson
But decay located within the main part of the trunk can be seriously dangerous. Determining the extent of decay within the tree determines the tree's decay and potential to fail. There is a test for determining the strength loss within a branch or stem: for every 3 inches of branch or stem diameter, solid wood should comprise at least 1 to 1.5 inches. Anything less than that often indicates a branch or stem that is more likely to fail during a storm. Species that commonly have decay problems are:
Maintenance: Poor maintenance practices may encourage decay anywhere in the tree. Mechanical damage, including wounds from lawnmowers/grass trimmers, and poor pruning practices, often leads to decay. Piling mulch several inches against the stem may lead to stem girdling roots and other dysfunctional root systems. Incorrect irrigation can oversaturate a soil and can encourage shallow root systems, stem girdling roots, and unstable trees. Improper use of staking equipment (wires around stems) often girdle and weaken stems.
Poor pruning practices that encourage decay or the formation of weak branches include: flush cutting (pruning too close to the lead branch or trunk), leaving long stubs, "topping trees," or stripping bark when pruning. Trees should have between 4'-6' of coarse mulch, none of which is resting against the trunk of the tree. Staking equipment that is not properly installed or is left attached to the tree too long will damage stem tissue and encourage decay. Never attach staking wires or ropes to tree stems. Use wide, padding materials around the tree stem and connect these materials to the tree wires or ropes. There are several types of padding available at many garden centers or hardware stores, but strips of carpeting, old belts, or inner tubes also work well. No staking or guying materials should be left on a tree for more than 1 year without inspection and adjustment for increased stem growth.
Figure 10. Stem broke at the point of compression from stem girdling roots. Photo by Gary R. Johnson
Stem Girdling Roots: Stem girdling roots are roots at or below the soil surface that partially or completely encircle the trunk of the tree. Over time, they begin to stress the health of the tree, including the root system. The girdling roots eventually cause compression of the lower trunk, creating a weak point that is often the point of failure in high wind storms. Many stem girdling root problems can be prevented by root pruning pot bound trees before planting and planting all trees at the correct depth–the first branch roots just below the soil surface. Some trees that seem to have a tendency to form stem girdling roots are:
- Green ash
- Norway maple
- Silver maple
- Littleleaf linden
- Red maple
Figure 11. Blow over as a result of shallow root system. Photo by Gary R. Johnson
Trees that are native to floodplain areas (elms, maples, etc.) have the tendency to form "stem" roots where they are frequently buried by floodplain material. These trees commonly form encircling roots when planted in the urban environment, particularly when they are planted too deep. The species listed above are those that most often develop stem girdling roots in urban situations.
Common site problems that may contribute to tree failure are: shallow soils, compacted clay soils, saturated soils, confined rooting areas, and inappropriate species for the location (such as large trees in small boulevard strips). Most of the problems are directly or indirectly related to lack of oxygen reaching the roots of the trees. When oxygen is lacking, root systems decline and tree stability declines. It is important to know your site situation, including soil types and rooting volume, before trees are selected and planted. Don't plant large trees that require well-drained soil on these sites. Use smaller trees and/or trees native to wet sites such as black alder and larches, or correct the soil problem. Plant trees that mature to less than 40' in height if boulevards are less than 10' wide.
Figure 12. Flush cutting, i.e., cutting too far into living tissue, can provide sites of decay. Photo by Gary R. Johnson
Inventory: By keeping track of trees on your property and their condition, predicting storm failure is much easier. Create a list of "key trees and key problems." Key trees would be those that are most important to the property. Key problems would be those that are most likely to damage or weaken those key trees.
Monitor: Check key trees regularly. When minor damage occurs, correction (such as pruning or wound "painting" on oaks during oak wilt season) may prevent it from causing extensive damage throughout the tree. If extensive damage has occurred, immediate corrective action should be applied to prevent further damage.
Figure 14. Planting too deep hid a girdling root that caused stem compression. The stem failed several inches below ground. Photo by Diana Bolander
Proper Pruning: Pruning either corrects problems or creates them. If pruning is done improperly, it can create places for decay to enter and the wound will only increase in extent. Done correctly, pruning wounds should close over naturally, keeping decay from starting and expanding in the wound area. A general rule for pruning wounds: the smaller, the better.
Protection From Mechanical Wounding: Mulching, planting trees in landscaped beds, and even staking can give trees the necessary protection from mechanical injury. Wounds caused from lawnmowers and grass trimmers can promote areas of decay in the tree. Cars, snowplows, staples, and any stacked materials that wound stems and branches can cause long-term damage in a short time.
Figure 15. Proper way to plant a tree. Illustration provided by the National Arbor Day Foundation, Tree City USA Bulletin No. 19.
Figure 16. Tree failure can have severe consequences--many of which can be prevented. Photo by Gary R. Johnson
Best Planting Practices:
Planting too deep may be the most common planting mistake that leads to tree failure. Literature is available on proper planting techniques. Most importantly, do not plant the tree too deep. The first set of roots should be just below the soil surface. Figure 15.
Maintain Health: By watering properly and frequently, and by fertilizing when nutrients are deficient, tree health can be maintained at a high level. When trees are stressed, they become more susceptible to problems ranging from aesthetic (leaf scorch), to decay, to severe, uncorrectable damage from diseases or insects.
Assistance: City foresters, county Extension offices, and tree care professionals are available to answer questions about tree care problems.
There are several treatments a homeowner may use to correct the minor damages that storms inflict. Before any of these techniques are attempted, consider hiring an arborist. Except for the cases of pruning small branches, and straightening slightly leaning, small trees, consult an arborist. The following are the most common types of corrective treatments:
Corrective Pruning: Small branches that have been damaged extensively should be removed to the next branch, but never cut off the branch collar. Use the proper pruning techniques to safely remove broken branches. If a branch is too heavy to support with one hand, a three-cut method should be used.
Figure 17. Proper three cut method for pruning limbs. © USDA-Forest Service.
Straightening, Staking and Guying: For minor uprooting of smaller trees(<25-ft.), straightening and/or guying is an option if correction takes place immediately after damage has occurred. When staking an uprooted tree, be sure that the roots remain covered and moist. Stakes should be placed evenly around the tree and attached securely without pulling on the tree. Thin rope or wire should not be used against the trunk of the tree.
Figure 18. Proper staking procedures and methods. Note the slack in the forms of attachment. © Florida Cooperative Extension Service, Circular # 858.
Wound Repair: Torn bark may be removed to reduce entry sites for diseases and insects or for aesthetic purposes. Split, cracked, torn branches should be removed to points of no damage. Bark should not be removed from areas greater than the damage already present. When pruning branches or repairing wounds, it is usually unnecessary to paint the wounds. The exception is during oak wilt season (April, May, June). During this period, wounds made on oaks should be painted immediately with a latex paint or shellac to deter insects carrying the oak wilt disease fungus.
Cabling and Bracing: Cabling and bracing are frequently applied treatments following storm damage, but only trained professionals should perform these installations. Most tree care companies will provide this service. Cabling and bracing are most effective as preventative measures, and provide extra support for weakly attached branches or stems.
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Knowing When to Hire an Arborist: Possibly the most important question to address when evaluating storm damage is whether to hire an arborist to do the work or to attempt it yourself. If you need a chainsaw or ladder to do the pruning, if there are any downed and potentially energized lines in the area of the tree, or if you are wondering if the tree is worth saving, you need an arborist. In any situation where there is the potential for personal or property damage (broken limbs hanging high in the tree or unsupported branches hanging over sidewalks), it is very important to immediately call your city forestry department or a reputable tree care company to remove the potential danger.
The directories Tree Care Companies Providing Emergency Storm Service in Minnesota, and Companies with Certified Arborists in Minnesota are available through the Minnesota Society of Arboriculture.
For a list of University of Minnesota Extension Service publications on tree selection and care, contact the Extension Office, Forest Resources Department, 116 Green Hall, 1530 Cleveland Ave. N., St. Paul, MN 55108, 612-624-3020.
Funding for Storm Damage to Landscape Trees was provided by:
University of Minnesota Extension Service [the Renewable Resources Extension (RREA) program of the University of Minnesota Extension Service and the U.S. Department of Agriculture--Cooperative States Research, Education and Extension Service (CSREES)].
College of Natural Resources, University of Minnesota.
The authors thank the following individuals for their technical assistance and review of the publication:
Katie Himanga, consulting urban forester, Heartwood Forestry.
Patrick Weicherding, extension horticulturist, University of Minnesota Extension Service, Anoka County.