To Prune or to Cable?
To properly cable trees, arborists should attach hardware in the upper one-third of the canopy. These two limbs are attached with 3/16-inch-diameter high-strength cable.
Urban foresters are never quite sure what they'll find when they visit a property. While there is no shortage of insect and disease problems that can plague home landscapes, just as often we encounter cultural or structural problems. Not only are these problems more varied in their cause, but the solutions are equally unique. That said, when it comes to the structural integrity of trees, there are two common courses of action: subordination pruning and cabling.
Many clients want to know how they can reduce the chance of tree damage due to storms and high winds. Maple, ash, linden and oak trees are particularly concerning because of their propensity to develop codominant leads.
Codominant leads occur when two vertical leads begin to grow at the same point of attachment. When the leads begin to grow into each other, they create a weak point in the structure of the tree due to included bark, pressure on the attachment point, and the weight load of the limbs.
As trees with codominant leads mature, the chance for splitting due to storm damage increases dramatically. Heavy snow loads, wind and ice are major contributing factors for storm damage on trees in the urban environment. Proper pruning techniques can repair minor storm damage to limbs. Unfortunately, in a majority of cases, trees are damaged beyond repair. Once this happens, the only thing to do is remove the tree and grind the stump.
As urban foresters, we should be proactive about pruning trees when they are young to help them develop proper structure. Dr. Edward Gilman, professor at the University of Florida, has done extensive research into the reasons for subordination pruning, and his tutorials can serve as a guide for arborists.
Structural pruning, also called subordination pruning, is the reduction in height of aggressive, codominant leads to a sustainable, outward-growing lateral. Shortening all large lower branches when the tree is young will force more growth into the trunk, thus strengthening the tree trunk.
Removing rubbing and crossing limbs and raising low limbs can also improve the structure of your clients' trees and limit the chance of storm damage. Trunk wounds from pruning young trees will be small and will result in little decay when these shortened low branches are removed later.
A 1/4-inch forged eyebolt attaches to a 5-inch-diameter limb to prevent storm damage in a tree with codominant leads.
Another advantage of this type of pruning is improvement of the aesthetic beauty of the tree.
For smaller, immature trees this type of pruning should be done every three to five years, depending on the growth habit and size of the tree. Urban foresters can, and should, help decrease the development of codominant leads to reduce tree mortality during storms.
Over the years, we have seen great results from structural pruning, and the benefits of this type of pruning should not be ignored by city foresters responsible for the care of trees in the public domain.
Most communities want to do the bare minimum when it comes to pruning their trees. This may include raising low limbs and removing deadwood, but otherwise leaving trees to Mother Nature. However, the benefits of subordination pruning in communities have been proven.
A local HOA was willing to take the extra time to have the 350 maples planted in common areas examined to ensure they developed properly. In addition to raising low limbs and thinning these trees, structural pruning was done to all of the maples as needed.
The trees ranged in size from 3 to 10 inches in diameter and from 10 to 30 feet in height and were pruned every two years. Over the last five seasons, through snow, straight-line winds, hail and everything in between, the trees have sustained little damage. In fact, the results were unusual for such a large stand of maples - after two rounds of structural pruning, only one maple suffered broken limbs after a storm.
Subordination pruning can garner great results when performed on young trees. However, when it comes to large, mature trees, it is not always the best approach. In these cases, cabling is the best course of action.
Cabling and bracing
Over the past 10 years, board-certified master arborist Matt Evans has developed the cabling program for the urban forestry department at Ryan Lawn & Tree, a lawn and tree care company based in Overland Park, Kan., with four branches across Kansas, Missouri and Oklahoma.
Following ANSI and ISA standards, company arborists install cables in trees with codominant leads to add supplemental support. This allows the cabled leads to move as one unit during winds instead of oscillating. Additionally, extra support is provided in the upper part of the canopy, allowing it to successfully shoulder additional weight loads from snow and ice.
Cabling requires installing specifically sized forged eyebolts and cable in the upper one-third of the canopy to attach two or more codominant leads. The company uses 1/4 to 5/8-inch diameter forged eyebolts and common-grade (galvanized) or extra-high-strength cables 1/8 to 1/4 inch in diameter, depending on the diameter of the limbs that need to be cabled. For example, a 5-inch-diameter limb requires a 1/4-inch-diameter forged eyebolt with a 1/4-inch-diameter common-grade cable.
Attaching one lead to another is called a direct cabling system. A tree with three leads may require a triangular system, where all three leads are supported by three cables attaching each lead together. It is important to attach the cables in the upper third of the canopy and to install the cable at the same angle as the eyebolt for optimal support.
This style of cabling is interesting for several reasons. First, by installing a forged eyebolt directly through the tree with a long drill bit, little damage is done to the vascular tissue of the lead being cabled. Secondly, as the laterals grow in diameter, they eventually grow around the eyebolt and part of the cable, which, over time, will strengthen the tree.
Matt Evans began to cable trees because he wanted to offer his customers the best service possible. "I found it to be important to have this service available to customers that had the need for cabling," Evans said.
It is important to remember that as arborists we deal with organic material, and nothing is ever guaranteed. When we offer cabling on a tree, we are recognizing that the tree has a structural defect. We never promise that this will fix the problem; we are simply offering to provide the tree additional or supplemental support.
Pruning trees proactively when they are young is always the best practice for aesthetics and storm prevention, but if your customer has a mature tree, cabling is your best option to help reduce the chance of storm damage.
Arborists pruned aggressive codominant leads back to sustainable laterals on two sycamore trees. This will increase the structural integrity of the tree and help prevent damage from high winds.
Urban foresters cannot be shortsighted in their role as stewards of proper tree care. Fixing disease and insect problems is important, but many cultural problems must be addressed. We must educate our customers about the proper growth habit of trees, and teach them that pruning trees when they are first planted will help ensure that they grow tall and strong.
Learning best practices for subordination pruning and cabling will help us better serve our customers, and help solidify arboriculture as an important and professional career choice for people who love to work outdoors.
ISA-certified arborist Tom DePaepe is a consulting arborist with Ryan Lawn & Tree.