Pacific Northwest Weed Management Handbook

cover photo
Horseweed, or marestail, (Conyza canadensis) in an eastern Oregon hybrid poplar plantation. Horseweed is prevalent and sometimes difficult to control in many perennial cropping systems, especially when few or no preemergence herbicides are used.
Photo by Rick Boydston, USDA, used with permission.

This handbook is designed as a quick and ready reference for weed control practices and herbicides used in various cropping systems or sites in Idaho, Oregon, and Washington.

This handbook will be useful to Extension agents, company field representatives, commercial spray applicators and consultants, herbicide dealers, teachers, and producers.

Recommendations are based on research results from the Agricultural Experiment Stations and Extension Services of Oregon, Idaho, and Washington. A few suggestions are included from research conducted in other states, and from U.S. Department of Agriculture research centers. In all cases, authors make every effort to list only registered herbicides, and to ensure that the information conforms to product labels and company recommendations.

Revision and Availability This handbook is updated quarterly. Individual sections are revised once each year; revision dates are listed at the start of each section. Most sections are also available as PDF documents on the weed handbook website:

Some sections may include additional online content, such as photos and links to related websites, publications, and other resources.

Important Points to Remember

  • This handbook is not intended as a complete guide to weed control or herbicide use.
  • Before using any herbicide, read the label on the container. Before a herbicide can be recommended for a specific use, it must be thoroughly tested. The recommendation on the manufacturer’s label, when followed, can prevent many problems arising from the improper use of a chemical.
  • Information is supplied here with the understanding that no discrimination is intended and no endorsement by the Cooperative Extension Service is implied. Trade names (brand names) of some commercial pesticides are used in portions of this handbook to help identify the common name used by the Weed Science Society of America (WSSA). Authors have assembled the most reliable information available to them at the time of publication. Due to constantly changing laws and regulations, authors can assume no liability for the recommendations. Any use of a pesticide contrary to instructions on the printed label is illegal and is not recommended.


Ed Peachey, Oregon State University

Associate Editors

Andy Hulting, Oregon State University
Tim Miller and Drew Lyon, Washington State University
Don Morishita and Pam Hutchinson, University of Idaho