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Why do electric utilities prune trees?


Utility vegetation maintenance reduces electric safety risk to the public by:

  • Providing minimum separation between wires and vegetation to eliminate potential electrical shock
  • Reducing potential wildfire hazards from tree/wire conflicts and downed power lines caused by trees


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. Maintaining power to vital infrastructures and services such as hospitals, nursing homes, police and fire departments, airports, water and sewer pumping stations and traffic signals is paramount.

Utilities can better assure reliability of electric service by:

  • Having a preventative maintenance program that includes removing branches, vines and entire trees and brush from electrical equipment and energized conductors.
  • Removing vegetation that can potentially cause a safety hazard or an electrical outage when it has the potential to grow into the power lines or fall into them due to weather or poor tree health.
Does the utility follow any pruning guidelines?

An increasing number of utilities follow the American National Standards Institute (ANSI) A-300 Part 1: Tree, Shrub and Other Woody Plant Maintenance – Standard Practices, Pruning. These guidelines, endorsed by the International Society of Arboriculture (ISA), promote directional pruning methods which minimize pruning stress and focus on tree health while obtaining necessary clearance from power lines.

How often is utility tree pruning completed?

The time between maintenance activities varies from utility to utility and between different regions of the country. Sometimes the frequency is mandated by the state agency. The best interval lfor tree maintenance is based on:

  • Expected re-growth rates of tree species present
  • Amount of clearance that can realistically be obtained at the time of pruning
  • State or local regulations
  • Available program funding

Maintenance cycles are generally shorter where there is a longer growing season or a high percentage of fast-growing tree species. In areas with a short growing season or fewer fast-growing tree species, the cycle can be longer. Some utilities conduct ‘mid-cycle’ pruning and/or inspections to mitigate the fastest growing tree species midway through a cycle.

My trees are not even touching the wires, why do they have to prune them?

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 problems that can occur without warning even if the trees are not in contact with wires at this moment.

Pruning trees to direct growth away from wires before becoming problematic is ideal. It may take years before a tree presents a safety or reliability risk, but fast-growing trees near the power lines will need to be pruned or removed completely as they mature.

How much will be cut from my tree?

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
  • Power line voltage–higher voltages require greater clearance
What is a line clearance tree contractor?

As defined by Occupational Safety and Health Administration (OSHA), it is a company that is qualified to maintain trees near power lines. These companies employ qualified line clearance arborists who receive ongoing electrical safety training as well as provide equipment and tools that are maintained as non-conductive.

How are Utility Line Clearance Arborists trained?

Most tree care workers are not qualified to work near energized power lines. Qualified utility line clearance professionals meet OSHA qualifications and are the only ones who 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.

Note: 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.

Why won’t the utility put the lines underground?

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.

What is directional pruning?

Directional pruning removes branches growing toward the power lines while leaving those that are growing away. It is the most appropriate way trees for electric utility line clearance.

Trees that are pruned in accordance with ANSI A300 Standards will have branches pruned back to a lateral branch that is at least one third the diameter of the branch being removed. Additionally, cuts should be made close to the parent or main limb so as to not leave a stub or damage the main limb.

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 through-pruning). Trees growing alongside power lines may appear L shaped, or one side may be completely removed (side pruning). The tree may appear misshapen however; remember that this pruning is not performed for aesthetic purposes, but to safely clear the wires. In general, trees growing near the power lines will never have the potential to grow with a “natural” shape.

Is my tree a candidate for pruning or removal?

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 – high outage risk
  • Saplings (brush) with the potential to grow into or close to the lines
  • Large, previously topped trees under the lines
  • Hazardous trees (examples – leaning, in decline, severe dieback, cracked or split, hollow, etc)
How are trees pruned in a remote or wooded area?

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.


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