Wayne K. Clatterbuck Associate Professor Forestry, Wildlife & Fisheries University of Tennessee
David S. Vandergriff, UT Extension Urban Horticulture & Forestry
Urban trees often need some degree of supplemental, physical support to reduce the risk of structural failure of the crown or root system. Cabling, bracing, guying and staking can provide a solution for these tree failures. Tree support systems help support the tree by limiting the movement of branches, leaders or the entire tree. This reduces the risk of injury to humans and damage to property by providing supplemental support for structurally weak areas of the tree.
Common Structural Deficiencies in Trees
The most common risk of tree breakage is the presence of one or more codominant stems. Codominant stems, or “v-crotches,” are structurally weak compared to a single stem. This is due to the lack of connective tissue anchoring a stem to the tree trunk and the presence of included bark between the stems. The greater the angle of the “v-crotch,” the greater the risk of structural failure. The best solution for problems associated with codominant stems is to buy and plant trees with a single leader. An alternative is to remove one of the codominant stems as early as possible in a tree’s life, allowing for the development of a single leader. Otherwise, bracing or cabling is required to strengthen the weak area of the tree.
Another condition that often results in structural problems is the presence of long, heavy or “overextended” limbs. These are limbs that are unusually long for the tree species or grow horizontally or downward, with most of the foliage concentrated toward the end of the branch. Breakage resulting from these conditions often occurs at the junction of the branch and stem. Alternately, the branch may crack due to the forces of tension and compression. These failures usually occur when the branch is under heavy loading such as wind, snow or ice. Installation of cables may be used to avoid making large pruning cuts. Early corrective pruning is the best course of action to prevent this condition.
A third structural problem is a weakly anchored tree. Poorly anchored trees are the result of transplanting a tree with a substandard root ball, a compromised root system with root damage or decay, or planting in shallow or compacted soils. Pruning, tree removal, installation of support devices such as cables, brace bolts and guys, or a combination of these techniques may be recommended.
Tree Support Devices
Brace rods are used when multiple leaders exist in the tree. These rods reduce the risk of the leaders spreading apart or moving sideways in relation to each other. Brace rods are also used to repair a crotch or branch that has split.
Brace rods are typically accompanied by at least one cable for additional support. Brace rods are installed as either a through rod (rod is bolted with a nut on other side of tree or branch) or dead-end (rod is threaded into the tree) configuration.
Cabling restricts the distance that a branch can move in relation to the rest of the tree. Cables are installed across a weak crotch to reduce the risk of a branch breaking. Cables are also installed on overextended branches to support the branch. More than one cable is often necessary in the installation and may be used in combination with brace rods.
Guying is where a cable is installed between the tree and an external anchor to provide supplemental support and to reduce tree movement. Trees with root problems may be guyed to keep them upright and protect potential targets if they fail. Established trees are guyed if they have had some degree of tipping from wind throw and require some support.
Tree staking is used to hold the tree upright and the root ball in place until the roots become established in the surrounding soil. Staking can also be used to straighten the trunk of a young tree or protect the lower trunk from injury. Generally, staking is discouraged because most trees with adequate root systems do not need to be staked at planting. Trees that are staked require constant monitoring and maintenance.
Trees can be staked aboveground or belowground. Aboveground staking should be installed as low in the tree as possible. The root ball can also be anchored belowground by driving metal or wooden stakes along the sides of the root ball about a foot deeper than the root ball. This eliminates the need for aboveground staking and maintenance.
Harris, R.W., J.R. Clark and N.P. Matheny. 2004. Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. Prentice Hall, Upper Saddle River, NJ. 578 p.
National Arborist Association, Inc. 2000. Support Systems (Cabling, Bracing and Guying Established Trees), ANSI
A300, Part 3. American National Standard for Tree Care Operations – Tree, Shrub, and other Woody Plant Maintenance – Standard Practices. Manchester, NH. 11 p.
Smiley, E.T. and S. Lilly. 2001. Tree Support Systems: Cabling, Bracing and Guying. Best Management Practices Series. International Society of Arboriculture, Champaign, IL. 30 p.
Watson, G.W. and E.B. Himelick. 1997. Principles and Practice of Planting Trees and Shrubs. International Society of Arboriculture, Savoy, IL. 199 p.