Utilities and their contractors probably prune more trees than any other group of professionals. The utility company typically writes, or hires a company to write, pruning specifications, and the contractor prunes trees according to these specifications. Electric utilities hire contractors to prune trees to ensure safe, reliable service to their customers, and to gain access to above ground utility structures. This practice is referred to as line clearance or utility pruning.
The most obvious utility pruning includes clearing both power lines and communication cables. Homeowners, horticulturists, arborists and others without electrical hazard awareness training must leave this kind of work to a line clearance tree trimmer or trainee. Call the utility company or a utility arborist to do this hazardous work. Never prune within 10 feet of a utility line or conductor unless you have the appropriate training.
The utility easement often includes only a portion of the tree crown, resulting in a one-sided crown after pruning. This complies with the industry pruning standards. When trees are planted directly under wires, trees are pruned in a V-shape.
Ice, snow and windstorms break trees and branches, causing them to fall into wires. The number one cause of outage in some regions is trees outside the utility easement falling across wires. Utility pruning within the easement focuses on removing branches that are dead, leaning, falling, cracked, diseased, poorly tapered, and those with inclusions and other poor attachments. Certain species are more prone to failure and could be treated differently. For more on these strategies, refer to chapters 12, 13 and 14 from “An Illustrated Guide to Pruning, Third Edition.”
The following discussion addresses pruning in and around urban and suburban areas of the U.S. These strategies may be impractical in rural areas.
Trees pruned with heading cuts generate sprouts that quickly grow back into the wires. However, heading may be the only option when there are not sufficient lateral branches, when the tree is directly under the line, or when the tree has been headed one or more times.
In the past, branches pruned with reduction and removal cuts can be encouraged to grow away from wires to a certain extent. This was called directional or lateral pruning. In addition to increasing service reliability, directional pruning could reduce pruning costs by increasing the interval between pruning efforts, in some cases. Fewer sprouts emerge from directionally pruned trees because this type of pruning conserves terminal buds on lateral branches (Goodfellow et al 1987). Pollarding and other forms of architectural pruning are high-maintenance options that require regular pruning and are rarely used.
Figure 1: If pruning trees next to wires, Option A is a poor choice, because sprouts from heading cuts quickly grow back into the wires and the headed branches can decay. For a lower-maintenance alternative, reduce the four branches indicated in red instead of heading them (bottom center). Sprout length should be less than from heading cuts. Some utility companies choose between Options B and C because they can provide clearance for a longer time than Option A. Option B is less damaging to the tree than the other options. Option C provides the best clearance, but this overpruning is unsightly and can result in sunscald, cracks, dead branches and decay in the trunk and roots. Utility wires are represented by three bullets (. . .).
Trees growing into wires from the side are pruned differently from those under wires. (See Figure 2.) Some utility companies prune as shown in Option A, while others prune less dramatically as in Option B. Only two pruning cuts were made in each example. The tree in Option B may need pruning again sooner than the tree in Option A, but Option B results in less debris removal, less trunk decay and better aesthetics. Many people would hardly notice that the tree was pruned.
Option A can cause more trunk decay because large branches or codominant stems were removed back to the trunk. Large branches and codominant stems lack a branch protection zone at their base.
Figure 2: When pruning next to wires, Option A provides more clearance from the utility wires, but option B looks better and is better for the tree. Unfortunately, there are few, if any, studies comparing costs for these two pruning doses. Trees pruned either way can be trained to one dominant leader, as shown, and directionally pruned away from wires.
Maintaining a tree as in Option B may require more frequent pruning (and thus will cost more). However, the long-term cost of Option A could surpass Option B’s costs if a severely decayed Option A tree fails and damages the line, or worse, injures someone. Since this is likely to result in litigation against the utility, customers could end up paying more for Option A. (This is largely hypothetical, as there are few published studies.).
In lieu of Option A in Figure 2, if all lower branches must be removed back to the trunk on the wire side of the tree, reduce the branches as in Option B and remove them over two or three pruning cycles, not all at once. If more than half of the foliage or current-year buds must be removed from a branch, or if sprouts will grow back into wires quickly, plan on removing the entire branch from the tree.
Although there are exceptions, some utility arborists try to make no more than three cuts to remove most of the foliage required to clear the line of branches. This is not always possible, but provides a guideline to work from.
Lateral branches on trees near wires are sometimes headed to provide clearance (Figure 1, option A). This usually does not conform to the ANSI A300 Part I Pruning standards or the “Best Management Practices” guidebook written to support this standard. Not only will sprouts quickly grow back into the wires, but they are also poorly attached to the tree because the headed branches often crack and begin to decay. A much better alternative is a combination of reducing some offending branches and removing others back to the trunk (Figure 1, Option B).
Utility companies sometimes choose to remove all overhanging branches from one side of the tree, depending on the importance of the line and the tree species and condition. Although this provides the most clearance, it raises aesthetic concerns and may be extremely damaging to the tree (Figure 1, Option C). Trees could have difficulty recovering gracefully from this type of pruning. Removing all branches from one side of the tree at one time is likely to initiate trunk decay and cracks, and may predispose the tree to sunscald, windthrow or trunk failure. Bark often dies on the side of the tree facing the wires and roots can decline.
Although trees pruned in this manner can remain standing for decades, some arborists would consider removing the trees if this type of pruning is required. As an alternative, reduce the offending branches (see Figure 2, Option B) and remove only one or two back to the trunk at the first pruning. Remove another one back to the trunk at the next pruning cycle and one more branch the following time. This will stress the tree less than removing them all at once. As good as this might be for the tree, it may not be practical in situations where a contractor is pruning hundreds or thousands of miles of rights-of-way.
It should be apparent that line clearance is a difficult task. All techniques have drawbacks, some worse than others. Moving the wires to a location away from the trees is a possible solution in some cases, but is prohibitively expensive. Removing large trees near wires and replacing them with small-maturing species is a good low- maintenance alternative, but it’s expensive as well. Communities, utilities and contractors will continue to evolve programs designed to provide reliable electricity and communication lines with minimum impact on trees.
Misunderstandings sometimes arise when property owners feel that their trees have been pruned inappropriately or too severely by the line clearance contractor. Citizens should contact the forester at the utility company, or the person in charge of line clearance pruning, to voice their concerns. Though sometimes mistakes are made, the contractor should be following contract specifications as directed by the utility. In the U.S., most large utilities include the ANSI A300 standard’s pruning (part one) clause in their specifications.
Removing incompatible trees and replanting with a small tree or large shrub is the best method of minimizing pruning requirements near wires. It’s the only sustainable option, for example, for palms growing within the influence of wires, because palm leaves grow back quickly when removed. Tree removal is rarely practical due to its high cost. Pruning is considered more economical than removal unless it costs as much or more to prune as to remove. In some instances, wires can be placed underground or moved away from the trees to allow trees to develop properly. However, high cost typically brings pause to associations and communities considering this option.
This article was excerpted from “An Illustrated Guide to Pruning, Third Edition,” with permission from the author. For additional literature cited, photography references, or supporting information, please source the original text.