What began in 2013 with a few dead trees has become a major die-off along the highway that winds through the Cascade Range, connecting the verdant floor of the Willamette Valley to the high desert of central Oregon.

Over 1,400 ponderosa pines, some centuries old, have been killed, and more are expected to die. State and federal investigators blame their deaths on roadside spraying of the weed-killer aminocyclopyrachlor, or ACP, by Oregon’s Department of Transportation.

Toxic levels of ACP can still be found in area trees six years after the nearby roadside was last sprayed. The chemical is a known tree-killer whose label warns against letting it come into contact with the roots of desirable trees. But weed control specialists question whether other factors, like road de-icer and drought, might be involved.

The grove of dying trees has thrust a chemical known for its destruction on Midwestern golf courses and backyards nearly a decade ago into the spotlight in Oregon — exposing divides between state and federal pesticide regulation, and between environmental challenges on either side of the Cascades. Now, Oregon regulators are clamping down on a chemical that federal regulators banned under one name, but continued to license under another. [Read more…]