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Seattle City Light Using Directional Pruning to Keep Power Lines Clear

The spring surge in plant growth means work for Seattle City Light. The utility uses directional pruning to encourage trees to grow away from power lines and discourage re-growth into power lines.


It happens every Spring.  You can count on it. 

The temperature slowly starts rising. The sap starts flowing through the trees again. Soon buds are forming and bees start warming up for their pollen-packing dance. Then you wake up and POW – there are flowers and green leaves EVERYWHERE.  The cherry and plum trees start first, along with the Camellias and Rhododendrons; the Magnolias not far behind.  Then it’s our native Pacific Dogwood (Cornus nuttallii) followed by their slightly smaller cousin the Flowering Dogwood (Cornus florida), and before you know it everything that looked dead a month ago is either green, flowereing, or both.

Right about the same time, Seattleites start emerging from their winter’s hibernation too. They stand in the planting strip, or backyard and for the first time in almost six months … they look up.  They see the trees in their neighborhood, reaching over their house or lining the road.  They notice the branches weaving through the thick black telecommunication lines like some kind of linear crows-nest gone horribly wrong. They note the majestic Douglas Firs (Pseudotsuga menziesii) looming over it all.  All seems well as the white and orange bucket-trucks go rolling past.  That is, until they hear the chainsaws.

They cock their heads and squint their eyes to better see where it’s coming from. When they do,  the horror sets in – “Those chainsaw-wielding mercenaries are cutting MY TREES!!!!!” (Actual quote from an unenthusiastic customer.)  

Where once stood “natural” shaped trees among the power lines, there now stand conifers the shape of capitol Ls and Ts. Deciduous trees cut into Vs and Js curve and bend away from the power lines like limbo dancers.  Why do we do this?

City Light’s tree trimming program manages vegetation on more than 1,700 overhead miles of lines throughout our service area.  We are on a four-year trim cycle. That means that during a four-year period, City Light will provide appropriate vegetation clearances for all the power lines in our distribution system.  When you see our contractors trimming the trees they must be sure to remove enough material to keep the trees safely away from the lines for the next four years of growth. 

Every cut we make on a tree creates a wound that threatens the health of the tree to a greater or lesser extent.  The tree responds in two ways: by putting up walls on the inside and adding “callous wood” on the outside.  This is the tree’s way of minimizing the spread of decay.  The tree also will send out “water-sprouts” or “suckers” to try to compensate for the loss of foliage (leaves – which feed the tree).  With our primary goal of ensuring safe and reliable power to our customers, we need to make sure we’re not creating future tree hazards. This is where directional pruning comes in.

Directional pruning is a concept that was born out of the research of Dr. Alex Shigo, former Chief Scientist for the USDA Forest Service, who is widely regarded as the father of modern arboriculture (tree care).  He identified how trees go about putting up walls inside the trunk to prevent the spread of decay and pioneered the idea of “natural target pruning,” which focuses on making pruning cuts at the branch/trunk union where the tree is best able to close over the wound.

The idea is pretty simple: make the fewest cuts possible to encourage the tree to grow around the power lines and to discourage re-growth into the lines. 

The key to making this work lies not in what you cut, but what you leave behind. When removing a branch that is threatening our lines, our contractors are trained to cut back to a limb that is vigorous enough both to survive and to suppress competing water-sprout growth.  When done properly, this encourages the tree to continue to put growth on the remaining limb (which is already growing away from our lines) while discouraging it from trying to establish new growth that will cause trouble down the road. This is a win-win for the trees and the utility because:

a)     Less re-growth into our lines means fewer cuts needed in the future;

b)     Fewer cuts on the trees means less cost for the utility to pass on to customers;

c)      Cuts made back to viable, vigorous limbs are more likely to thrive;

d)     This method avoids having to “top” the trees – a cruel, slow death-sentence for trees.

Years ago, before the tree care industry and/or the utility line clearance industry (or anyone else for that matter) understood the ways trees’ internal systems work, line clearance workers regularly “rounded over” and “topped” trees to try to make them look more symmetrical and pleasing to the eye.  We now understand that these practices do far more harm than good to the trees, encouraging the development of water-sprouts and interrupting the trees’ ability to slow decay. In short order the trees return to a wild, unkempt look due to the proliferation of water-sprouts and/or begin to decline from the heavy-handed cutting they received.  The worst part is, once these cuts are made there’s no going back – trees that were mishandled years ago will continue to be an issue for the rest of their lives.

Directional pruning on the other hand is recognized as a best management practice for utility line-clearance pruning and is promoted by the International Society of Arboriculture, the Tree Care Industry Association and the Utility Arborist’s Associations. There’s a significant body of peer-reviewed research that supports the principles behind and the practice of directional pruning for utility line clearance – just contact City Light’s Arboriculturist if you would like more information, or got to our website at

Finally, we want to work with our customers to promote a thriving canopy throughout our service territory.  The best way we can do this is to ensure that the right tree is planted in the right place to begin with, so our line clearance crews never have to touch them! You can download “The Right Tree”, a publication available on our Urban Tree Replacement Program’s website  This publication can help you decide which trees are compatible with our power infrastructure. 

By working together, we can promote a healthy urban forest AND safe, reliable power delivery.