Why Cabling & Bracing?
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.
Bracing 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. The appropriate configuration is dependent on tree size, whether decay is present in the tree and the structural problem of the tree.
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 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. Cable systems include anchors, cables and the appropriate termination hardware for connection to the anchor. Cable anchors are installed in the tree at a point that is approximately 2/3 of the distance from the weak crotch to the ends of the branches. The exact location is determined by the location of lateral branches and the weak area of the tree being supported.
For this information and more detail, see: http://utextension.tennessee.edu/publications/Documents/SP659.pdf