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Noxious Weed Control Board

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Cupola House
380 Jefferson St
Port Townsend, WA  98368

(360)379-5610 Ext. 205

9:00am - 4:00pm



Knotweed in Jefferson County

Most Jefferson County waterways are infested with invasive knotweeds.  The Big Quilcene River is particularly bad and Snow Creek and the Dosewallips River both have some large stands.  An infestation on the Hoh River has been considerably reduced over the past eight years.

Knotweed on the Big Quilcene River—photo credit to Luke Cherney of the Hood Canal Coordinating Council

Knotweeds are originally from Asia and were introduced as ornamentals to Europe and then to the US in the 1800’s.  They thrive in disturbed areas—in its native land Japanese knotweed is a primary colonizer of volcanic slopes.  Our county’s rivers provide the perfect ‘disturbance’ zone for knotweeds to thrive and migrate in.

Three species of invasive knotweeds occur in Jefferson County—Japanese knotweed (Polygonum cuspidatum), giant knotweed (P. sachalinense) and Bohemian knotweed (P. bohemicum), which is a hybrid between Japanese and giant knotweeds. 

The leaves of Japanese knotweed are usually less than 7 inches in length, pointed at the tip and flattened at the base.  Giant knotweed leaves are large—12 to 20 inches long, less pointed at the tip and deeply indented at the base.  Leaf size and shape in Bohemian knotweed are intermediate between the two parent species.  Most of the invasive knotweed plants in Jefferson County are Bohemian knotweed, although large infestations of giant knotweed occur on the Big Quilcene River and some has been found on the Hoh.

Giant Knotweed on the Hoh River
—photo credit to Jill Silver of the 10,000 Years Institute

All three are extremely invasive, particularly in streamside (riparian) areas where they can completely crowd out native plants, creating significant impacts to the riparian and stream ecosystems.  Knotweed dies down completely in the winter, leaving almost bare ground which offers no food or shelter for insects, amphibians, birds or mammals.  Bare ground can be easily eroded, and sediment which is eroded into streams can smother salmon eggs and insects in the gravel, and cause damage to the gills of fish.

Knotweeds do not out-compete established trees, but dense knotweed inhibits growth of tree seedlings.  Over time this leads to fewer trees in infested riparian areas, less shade and therefore higher water temperatures which impact salmon health.  Lack of trees will also eventually mean less large woody debris, which is an important component of healthy salmon habitat, creating pools and cover from predators.

Knotweed on Gravel Bar—photo credit to Luke Cherney

Knotweed is a large plant (Japanese can grow up to 10 feet tall, giant up to 15 feet) with an extensive root system.  Dense infestations clog small streams and wetlands that provide critical rearing habitat for young salmon, and many other species.  When streambanks are lined with native trees and shrubs, bud scales and small insects drop into the water and are important food sources for juvenile salmon.

Once established, knotweed is extremely difficult to get rid of.  Although some knotweed plants produce viable seed, reproduction is almost always by means of plant fragments.  Fragments of root or stem as small as 1 inch can generate a new plant and fragments are constantly being moved downstream, starting new infestations.  The fragmenting root systems spread many feet down and out from each plant (15’ x 30’), such that hand pulling or digging is rarely successful and herbicide application is almost always necessary. Knotweed growing on terrestrial sites does not spread as prolifically as plants growing in or near water, and the impacts are less severe.  For these reasons, control efforts have been focused on knotweed in riparian areas.

The Washington State Department of Agriculture has made grant funding available through the Knotweed Program
Knotweed/Knotweed.aspx  Since 2005, Jefferson and Clallam Counties have jointly received state funding and control work has been done on the Big and Little Quilcene, Dosewallips and Duckabush Rivers and Snow and Salmon Creeks, as well as on rivers and streams in Clallam County.  Much work remains to be done in all of these watersheds and Chimacum Creek has not yet been surveyed.  Grant funding is awarded annually and we are hopeful, but not certain, of receiving funding for 2010 to 2011.

Injecting knotweed stems with herbicide is an effective control strategy, but is time-consuming.  After one year of treatment stems are usually too small to inject and have to be sprayed.  Our crews always use herbicides which are specially formulated to be safe in water and crew members are trained and licensed in the use of aquatic herbicides.

Crew Member Injecting Giant Knotweed on the Big Quilcene River—photo credit to Ron Wong of the Quilcene National Fish Hatchery

Knotweed After One Year of Treatment—photo credit to Eve Dixon, Jefferson County Noxious Weed Control Program

The 10,000 Years Institute, a local non-profit group, has received separate WSDA and foundation funding and has done a great deal of control work on the Hoh River since 2002 (
10k_asp/knotweed.asp ).  All that remains there now are small, isolated plants.  Continued monitoring and controlling of these small plants is essential, or many years of hard work will be undone.

Spraying Knotweed on the Hoh River—photo credit to Jill Silver

Clallam and Jefferson Counties and 10,000 Years Institute are all members of the Olympic Knotweed Working Group, a loose consortium of governments, tribes and non-profits that pools information and resources and shares the common goal of eliminating knotweed from the Peninsula.  Jefferson County’s involvement with this group has been very productive.

For more information on knotweed identification and control, see the Written Findings of the Washington State Noxious Weed Control Board,



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