The following FAQs were developed by the Utility Arborist Association to provide an overview of how an electric utility uses tree maintenance techniques to achieve their goal to provide safe, reliable service.
It’s a matter of safety and providing reliable power for the members we serve.
SAFETY—Utility vegetation maintenance reduces electric hazard risk to the public by:
- providing separation between wires and vegetation to eliminate potential electrical shock
- reducing potential wildfire hazards from tree/wire conflicts
RELIABILITY—Trees are among the most common causes of utility service interruptions. Trees that are too close to power lines can interfere with electric service; especially when weather brings lightning, wind, ice, or wet snow.
Typically, a qualified utility forester or vegetation manager prescribes the amount and type of pruning necessary based on:
- tree growth rate and structure
- wind direction
- tree species: strong or weak wooded
- tree health or vigor
- environmental factors
- water sources
- proximity of tree to wires and line configuration— higher voltage lines require greater clearance
Utility companies are proactive and try to prune trees BEFORE they pose a risk to the power lines. Because trees are dynamic, factors such as swaying in the wind, sagging with ice/snow weight, and uprooting in storms are examples of how problems can develop without warning even if the trees are not in contact with wires at this moment.
Directional pruning removes branches growing toward the power lines while leaving those that are growing away. It is the most appropriate pruning method for utility line clearance.
How will a tree look after it is directionally pruned?
Trees growing directly under power lines may appear U or V-shaped (crown reduction or throughpruning). Trees growing alongside power lines may appear L- shaped, or one side may be completely removed (side pruning). The tree may often appear misshapen but this pruning is being performed to provide for safety and service reliability, not for aesthetic purposes. In general, trees growing near the power lines will never have the potential to grow with a “natural” shape.
DO NOT TOP TREES! Also called ‘roundingover,’ this is not directional pruning and is not an acceptable pruning practice. It involves cutting branches to stubs or lateral points that are not large enough to grow successfully. It can severely weaken the tree and even kill some species.
Undergrounding of lines is very expensive and results in more difficult (and longer) repairs in the event of a power failure. Also, converting an overhead system to underground typically causes substantial damage to existing trees’ root systems.
Situations where tree removal may be preferable to line clearance pruning include:
- Tall or fast-growing species growing directly under the power lines that require frequent pruning and will never have a natural form
- Saplings (brush) with the potential to grow into or close to the lines
- Large, previously topped trees under the lines
- Trees with a high risk of failure (examples – leaning, in decline, severe dieback, cracked, split, hollow, etc)
The ANSI A-300 Part 1: Tree, Shrub and Other Woody Plant Maintenance—Standard Practices, Pruning are the accepted guidelines and are endorsed by the International Society of Arboriculture (ISA). They promote directional pruning methods which minimize pruning stress and focus on tree health while obtaining necessary clearance from power lines.
In remote/rural locations, utilities often utilize mechanical equipment to increase efficiency and worker safety. Large saws mounted on high-reaching booms can be used to prune the sides of right-of-way corridors. In some cases, saws are suspended from helicopters. When using this equipment it is understood that the quality of the cuts can be less than those made by hand. Nevertheless, efforts are made to avoid unnecessary damage to the tree.
Chemical application is another method of side pruning where herbicides are applied to the foliage of selected branches growing into the right-of-way corridor. The treated branches eventually die and are shed by the tree.
Only qualified utility line clearance professional arborists who meet OSHA qualifications are legally permitted to work within 10 feet of power lines or work on a tree that has branches within 10 feet of power lines. Line clearance arborists are trained to prune trees according to American National Standards Institute (ANSI) A-300 pruning standards and follow industry best practices, which helps preserve the health of trees.
DANGER: Homeowners should never hire a private tree contractor to work within 10 feet of power lines or attempt to do the work themselves. The utility should always be contacted for information first.