This summer, the City of Olympia will spray an herbicide to control knotweed, a state-listed invasive species.  This weed is aggressively taking over native habitat, yards, and other open areas. Once it gets established, it spreads quickly and vigorously.  The State has mandated that knotweed be controlled on both public and private land.  

The City of Olympia has received notice from the Thurston County Weed Board that we must begin a program to eliminate this weed on all property within the City’s limits.  City staff have identified stands of knotweed on City property that will be sprayed by a licensed pesticide applicator within the next two months.  Our spray program will begin after July 21, 2007, and continue through September, 2007.  Each stand will receive one spray treatment.

The City of Olympia uses an Integrated Pest Management (IPM) approach to managing undesirable species on City property.  An IPM approach employs chemical herbicides only as a last resort.  Extensive research has been conducted on effective methods of control for this species.  Due to its large root system, knotweed is a persistent plant.  Cutting does not effectively eliminate it as it can re-grow from one-inch long stem fragments.  Digging the root ball is also not effective as it is extremely difficult to capture the entire root system.  Herbicide application is the most effective control method.

The City will spot-treat knotweed stands with three different products. All three products effectively control knotweed with the least impact to human and environmental health.  All herbicide applications will be made by a licensed pesticide applicator.  The majority of stands will be sprayed with the herbicide imazapyr.  This herbicide is formulated into two different products; Arsenal and Habitat.  Arsenal will be used on dry sites and Habitat will be used near water. We will apply a glyphosate product called Aquamaster on stands within our Drinking Water Protection Areas. 

Only products containing glyphosate as the active ingredient should be used for the control of knotweed.  Glyphosate is recommended because it is the most effective product available with out a pesticide applicator license.  Since the City has licensed applicators on staff, we are able to use the more effective herbicide imazapyr.
 
The City’s goal is to protect the health of people and the environment while eradicating this noxious weed.  More information is available online at the Thurston County website. 

If you have any questions or concerns, please contact Victoria DeCillo, Senior Program Specialist, at (360) 753-8463, (360) 753-8270 (TTY), or by email.