What's the best way to prune trees near power lines?
Never attempt to prune trees near electric transmission lines. Trees that grow near power lines pose a serious safety risk and pruning should only be done by qualified utility line clearance professionals.
Directional pruning, removing limbs growing into the power lines and encouraging growth away from the lines, is the recommended ANSI practice for utility line clearance. In the past utility companies topped, sheared or rounded over trees too big to be growing under the power lines. This creates a full looking crown that may be more visually appealing but is actually very damaging to the tree.
Indiscriminately topping or shearing the tree off below the power lines creates multiple wounds between growth points that the tree cannot seal off effectively. The wounded branches may either die out right or begin to decay, leading to the decline of the tree and possible failure (falling branches). Topping and shearing cuts also prompt the tree to send out epicormic growth (sucker branches) that grow quickly, straight up and directly back into the power lines as the tree tries to make up for the loss of crown area. This regrowth is weakly attached and much less stable than branches that develop from natural growth points.
Directional pruning reduces the total number of cuts (wounds) made to the tree. The branches are removed at a natural juncture that the tree can seal off with callus tissue. These cuts are far less likely to lead to decay. Because the cuts are made at a natural growth point, and less food producing surface area of the tree is removed, sprouting sucker growth is minimal. Directional pruning reduces the frequency of pruning by directing new growth away from the power lines.
The down side is the trees look unnatural. V-cuts and L-cuts around transmission and neutral lines are usually required to achieve necessary clearance on big trees. For those trees that have been repeatedly topped in the past there may be few, or no, sound lateral branches to prune back to and the pruning looks very severe. The City of La Grande partners with Oregon Trailr Electric Cooperative to remove the trees that are in the worst condition and replace them with smaller stature trees that won't grow into the power lines.