Since You Asked: Trees near power lines need their growth stunted

I live outside of Rogue River and arrived home to see two Pacific Power notices hanging on my fence. The yellow one said they were going to trim my trees. The white one said they were going to treat my trees with "tree growth regulators."

They say they will be performing this service at no charge. But what is this stuff? And will it harm my trees?

— Cindy-Lou S., Rogue River

We're assuming your trees must be near overhead power lines to be on the trimming block, Cindy-Lou. In any case, we at Since You Asked called Pacific Power and asked Tree Maintenance Spray Foreman Jon Hjort about your concerns.

Tree growth regulators (TGR) have been around for awhile and are not harmful to the trees, according to Hjort. In fact, he said, they can be beneficial, particularly for nursery stock or fast-growing trees located near or under power lines.

Paclobutrazol is the active ingredient in this TGR. A hormone inhibitor, paclobutrazol "tricks the tree into growing slower," Hjort said.

"Instead of letting the tree send its energy into creating new growth at the top of the tree, it concentrates the tree's growth into the root mass," he said. "We've been doing it for many, many years now."

The TGR can be applied to the soil around the base of the tree, or it can be injected into the tree itself. The application is generally done once every four years or so, he said.

The net result is the tree will be slightly smaller, with slightly darker in color leaves and deeper roots. For the Pacific Power tree crews, it means less trimming work, he said.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by e-mail to We're sorry, but the volume of questions received prevents us from answering all of them.

Share This Story