How to Install a Direct Cabling System in Ornamental Trees
Direct cabling, also
called cable bracing, is oftentimes done on valuable ornamental trees that have
a sharp V-shaped or split crotch. When properly installed, a direct
cabling system can add years to the life of a valuable specimen tree, like the
one shown in the accompanying photo. Direct cabling is the most basic of
all installations. More complete installations are essentially multiples
of the single unit.
The main purpose of direct cabling is to functionally tie together the crown of the tree as a cohesive unit to withstand the stresses and strains of wind, ice, and the weight of limbs and foliage. In direct cabling, two limbs at least 6 in diameter or more are connected to each other by means of a copper-covered steel cable to provide equivalent support to both branches.
In simple crotch cabling, as a general rule, cabling takes place approximately 2/3 the height of the tree from the crotch to the ends of the limb. If the cable is installed too high, a ballooning effect may occur below the area of support. On the other hand, if the cable is installed too low, no or very little support will be given to the upper branches.
This job is not for amateurs but should be skillfully done by a qualified professional arborist. As most tree-related accidents take place 10 to 15 feet from the ground, every effort should be made to properly install the cable system without compromising safety.
Depending on the size of the lag bolt that is being used, a 1/16 undersized hole should be bored in the center of each branch that is directly in line with the cable that is to be installed. If the cable is not directly in line with the lag bolt, the lag may break, the wood may split/weaken the branch, and the lag may work itself out of the limb. The lag bolt should be installed in the branch where they will not completely penetrate the limb. One should use the largest lag possible at the point of attachment without going through the limb. Lags should be screwed in up to a point where another partial turn will bring the point against the bark.
The following table shows the sizes of bits, lags, and cables that are commonly available:
BITS LAGS CABLES
9/16 5/8 3/8
From a practical standpoint, a 1/4" cable plus an accompanying 1/2" lag hook can be used for limbs up to 6 in diameter. For limbs 6 to 10 in diameter, a 5/16 cable along with a 5/8 lag hook is used.
*Thimbles vary as to size of cables.
After the lags have been inserted, the limbs should be drawn together by means of a running bowline, tackle block, or come along. Devices for pulling branches together should be placed approximately 3 feet above the point where the tree lags are inserted. Judgment must be carefully taken to avoid immediate or later slack caused by the size of limbs, foliage load, distance to the crotch, etc. Once again, this is not a job for amateurs!
The distance between the lags is carefully measured with a tape and the appropriate sized cable is prepared. Bend the end of the cable into a loop to complement the shape of the thimble. Leave 10-12 of end to do the splicing at the end of the cable. Splicing is an article in itself.
The thimble on one end is placed over the lag bolt and a 1/4 or 1/2 turn is made on the lag to lock it into the bark. The other end of the cable is installed and secured in the same manner. The rope sling or other tightening device is released and the splices are now painted with a metal preservative.
Periodic inspections are necessary to take the slack out of sagging cables. While there are no hard and fast rules available on the average life of a cable, the US National Park Service indicates that cables should be checked every time work is performed on the tree.
Thompson, Robert A. Tree Bracing- Tree Preservation Bulletin No.3. Washington, D.C. 1963.
Cavity Treatment Cabling and Bracing. Instruction Book Number 8 With Examination Questions. The Davey Institute of Tree Service, Kent, Ohio.
John A. Morley