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King County targets 3 new invasive plants


Three new species have made King County’s invasive weed “least wanted” list, officially known as the King County Weed List.


Three new species have made King County’s invasive weed “least wanted” list, officially known as the King County Weed List.

The list, adopted last month by the King County Noxious Weed Control Board, targets non-native plants that impact natural resources, agriculture, and human health in King County or in other counties in Washington.

The new additions join other ornamental escapees on the weed list that were introduced to this area for their beauty or practical uses, but that turned out to be so invasive that their harmful qualities overwhelmed their good points.  The County will be looking for and educating residents about all the plants on the weed list, including the three new species: Oriental Clematis, tree-of-heaven, and multiflora rose.

King County wants to help property owners find and control these new invaders as well as the rest of the county’s noxious weeds, including garlic mustard, giant hogweed and poison-hemlock, all of which can be found growing in the county this month.

The County’s Noxious Weed Program is offering a free class on invasive and noxious weeds in partnership with the City of Bellevue on April 5, from 6:30-8:30 p.m., at the Mercer Slough Environmental Education Center, and has three noxious weed control seminars scheduled in May for public agencies, contractors, and landscapers. Details on all the classes are available at

The County’s “least wanted” list includes plants such as garlic mustard, a Class A Noxious Weed, according to the Washington State Noxious Weed Board, and one of 52 state-listed noxious weeds that the King County Noxious Weed Program is working actively with landowners to control. Noxious weeds are non-native plants introduced to Washington that cause damage to the natural or economic resources of the state.

Large garlic mustard infestations were found in Coal Creek Natural Area in Bellevue and several infestations were found along the Cedar River and Soos Creek in 2010 and 2011. These discoveries generated concern with Noxious Weed Program staff because this particular weed had been primarily limited to just a few Seattle parks.

Garlic mustard is a fast-spreading biennial introduced to North America from Europe that moves quickly into forests, out-competing native understory species.  Perhaps even more insidious, garlic mustard has been shown to reduce beneficial forest soil fungi needed by native trees and shrubs, according to Steven Burke, the County’s Noxious Weed Program manager.

It is likely there are more garlic mustard sites scattered about the County, and Burke is asking for the public’s help in locating new sites. Garlic mustard photos and information are on the county noxious weed website,, where discovered weed sites can also be reported.

The King County Noxious Weed Program is part of a statewide effort to detect and respond to noxious weeds such as garlic mustard that harm natural and economic resources.  To help protect the state’s resources, the Washington State Noxious Weed Control Board adopts a statewide noxious weed list each year. Each County’s weed board then adopts its own noxious weed list that establishes which weeds require control by property owners and public agencies.

The King County Noxious Weed Control Board places a priority on preventing new infestations of the most serious noxious weeds, and encourages property owners to work together in stopping the spread of established noxious weeds.

Another high priority target for King County this year is the massive plant called giant hogweed, feared both for its invasiveness and the toxic juice that causes painful, watery blisters and burns on contact. Giant hogweed has leaves that are up to five feet wide and a central flowering stem that reaches 15 feet tall, topped by an impressive umbrella-shaped flower-head, stretching two feet across.

Since 1996, King County’s noxious weed hunters have found more than 1,800 sites with giant hogweed.

“By watching for new sites, returning to known sites every year, and helping landowners control it where needed, we have been able to keep giant hogweed from spreading and we are hopeful that it can be eradicated someday,” Burke said.

The hardest part of fighting uncommon noxious weeds like giant hogweed is finding them, and this is where the public’s assistance is most needed. Burke said park users, homeowners and neighbors report many new sites of giant hogweed and other noxious weeds each year, encouraged through workshops and outreach tables at community events.

The program seeks to teach homeowners how to recognize and control noxious weeds on their own property. Noxious weed information will be on display at several locations this spring and summer. The schedule is available at Community groups can schedule a presentation on noxious weeds or an information booth at their event by contacting education specialist Sasha Shaw, at 206-296-0290, or

Information on noxious weeds, including the King County weed list and the Washington noxious weed law can be found online at For more information on the King County Noxious Weed Board and Noxious Weed Program, call or email Burke at 206-296-0290 or

Related information

Noxious Weeds

King County Water and Land Resources