by Mike Kuhns, Extension Forestry Specialist

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One of the main reasons to prune trees is to remove branches and twigs that are becoming obstructions or nuisances. Examples of this are low branches that head out over a driveway or sidewalk and branches that are rubbing on the side of a house or the roof. The most common reason for this type of pruning, however, is to clear powerlines and other overhead utility lines.

Trees and power lines are both important assets for our communities. Trees provide beauty, shade, wildlife habitat, and wind protection, while power lines bring us electricity for lights, heat, and appliances.

Unfortunately, trees and power lines often end up in conflict. Many power outages are caused by trees that have grown into the lines. Trees growing into power lines cause increased line clearing costs. The pruning done during line clearing may also decrease tree health and cause trees to become dangerous. Fortunately, modern power line pruning techniques have been developed that result in healthier, safer trees and reduced line clearing costs. However, they also make a tree look much different than it would if traditional techniques were used.

Topping

Utility lines traditionally have been kept clear of tree branches by topping, also called hedging or shearing. In this method, the entire crown of the tree is cut back to a certain distance below the wires, much like the shearing of a hedge. Little time is spent making proper pruning cuts so this method can be fairly fast. Unfortunately, many stubs are left on the tree because most of the cuts are made between the points where branches meet. Sprouts usually grow out quickly from the stub and the old branch becomes rotten below the stub cut. So in one or two years, instead of clear lines and a healthy tree, we end up with sprouts back in the lines and an unhealthy tree that is fast on its way to becoming a hazard. Line clearing costs rise and the trees need to be topped heavier the next time to remove the rot. This vicious cycle is repeated until the tree is dead or gone.

Directional Pruning

Directional pruning is an alternative to topping that leaves trees healthier and ultimately reduces line clearing costs. In directional pruning, also called natural pruning or the "Shigo method" (after a well-known tree researcher), only branches that head toward the utility lines (or other obstructions) are pruned. Those that are growing down or out away from the wires are left alone to continue their growth. Also, no stubs are left since branches are always pruned at the point where they reach another branch or at a "crotch". Such pruning cuts heal well and minimize sprouting and decay when properly made.

Proper cuts are made by removing all of the branch without leaving a stub, but at the same time not disturbing the branch collar and the branch bark ridge (see diagram). Flush cuts are no longer recommended since they make larger wounds that lead to dieback and decay. Wound dressings should not be used since they have been found to increase decay. The branch that is left after cutting must be healthy and vigorous and at least 1/3 the diameter of the branch that is removed. Branches that are too small or growing poorly will be not be able to suppress sprouting.

Trees that are directionally pruned will not have the familiar hedged look of a topped tree. Instead, they will have a V-shape if lines are directly overhead or an L-shape if lines are to one side. Though this may appear to create an unbalanced or weak crown, the tree will grow to correct any lack of balance and will end up healthier than if it had been topped.

Directional pruning has many advantages over topping. First, it leaves a tree that is nearly as healthy as before it was pruned. Trees do not need to be pruned as often since there are less sprouts and the branches that remain are growing away from the wires. This saves money and reduces utility bills. Fewer wounds are created on the tree and wounds that are made heal much better. Directional pruning can also be started when a tree is young. As a tree's crown grows, branches that are pointed away from the lines can be kept while those pointed toward the lines can be removed.

Directional pruning works best if started when a tree is young and may not work on trees that have been topped many times. Such trees often are too rotten and poorly formed to benefit. Trees with crowns that spread are the most easily pruned. Conifers and other trees with strong central leaders or trunks are more difficult to train with this method.

Tree Removal

Directional pruning will not solve all line clearance problems. It doesn't work well on older trees that have been topped many times. Trees with narrow, upright crowns like Lombardy poplars and some conifers are also difficult to treat this way. In these cases tree removal and replacement is preferable to topping. Replacement trees should be small and selected to fit under the lines when mature or should be planted to one side. Some utility companies have even instituted tree replacement programs to encourage the planting of more appropriate trees near utility lines.

Planting Precautions

The best way to prevent conflicts between trees and power lines is to plant trees in the correct location. Though some large trees grow under power lines naturally, many are planted there because people do not realize how large they will get. Remember that a four foot tall, two foot wide tree might end up being 60 feet tall and 30 feet across. Learn the mature size and crown characteristics of any tree you buy and plant accordingly.

Avoid planting trees of any size directly under main power supply lines since utility crews may need access to these areas. Small trees (up to 20 feet tall) can be planted adjacent to power lines but off to one side. Medium or large trees should be spaced 20 to 30 feet horizontally from power lines. The following table shows the spacing from power lines for some trees commonly planted in Utah.

Tree Size and Power Lines

Small Trees: Can be planted adjacent to power lines
Tree Type		Mature Height
Dwarf Fruit Trees		20 to 25 feet
Flowering Crabapple		10 to 25 feet
Redbud		15 to 20 feet
Purpleleaf Plum		15 to 20 feet
Hawthorns		15 to 20 feet
Ornamental Junipers		10 to 20 feet
Medium-Small Trees: Plant at least 15 feet horizontally from power lines
Tree Type		Mature Height
Callery (Bradford) Pears		20 to 35 feet
Mountain-Ash		20 to 30 feet
White Spruce		30 to 45 feet
Medium-Large Trees: Plant at least 20 feet horizontally from power lines
Tree Type		Mature Height
Green Ash		40 to 60 feet
Linden or Basswood		40 to 60 feet
Austrian, Scotch, Ponderosa Pines		45 to 65 feet
Norway Maple		40 to 50 feet
Honeylocust		40 to 50 feet
Buckeye and Horsechestnut		30 to 45 feet
Blue and Norway Spruces		50 to 70 feet
Large Trees: Plant at least 30 feet horizontally from power lines
Tree Type		Mature Height
Silver Maple		50 to 70 feet
Oaks		50 to 70 feet
Hackberry		50 to 70 feet
Sycamore and London Planetree		70 to 80 feet
Cottonwoods, Poplars		70 to 90 feet

For Assistance or additional information on tree selection, planting, and care contact your County Extension office, District Forester, or local nursery. For more information about trees and power lines contact Utah Power or your local power provider.