Controlling the spread of invasive plants such as knotweed is incredibly important to having a healthy river ecosystem. Invasive plants can out-complete native vegetation and are not beneficial to native wildlife.
The Upper Skagit Knotweed Program has been working since 2001 to control the spread of knotweed in the upper Skagit watershed. In 2018, the Skagit Fisheries Enhancement Group (SFEG) and partners in the CWMA surveyed knotweed along 83.6 miles of the Skagit River and major tributaries. SFEG worked with Americorps volunteers from the Washington Conservation Corps to treat knotweed along 52.3 miles of river shoreline along the Skagit and Sauk Rivers, plus various tributaries, traversing approximately 4,716 acres (7.36 mi²) within the 100 year floodplain.
Knotweed Facts
What is knotweed?
Knotweed is a class B weed on the state noxious weed list. There are three species of knotweed (Polygonum spp.) which have been found the upper Skagit River watershed since 2001 – Japanese, Giant, and Bohemian. Japanese, Giant, Bohemian, and Himalayan knotweed are perennial plants native to Asia, but planted in the Pacific Northwest as ornamentals. Common names include Mexican or Japanese bamboo, elephant ear, or fleece flower.
Scientific Names include: Polygonum cuspidatum, Polygonum sachalinense, Polygonum x bohemicum, and Polygonum polystachyum
Why is it a problem?
Knotweed is a particularly aggressive plant because of the ease with which it spreads and the massive root clusters it forms. In the Northwest, knotweed usually spreads when roots and stems are moved by waterways, by floods or in contaminated soil. Root and stem fragments as small as 1 inch can produce a new plant. It poses a significant threat to riparian areas, where it can survive severe floods and is able to rapidly colonize scoured shores and islands.
If invasive species such as knotweed are left untreated, healthy riparian areas are unable to develop and habitat functions will be lost. Knotweed and other non-native invasive plants are detrimental to freshwater ecosystems and natural riparian processes, which in turn negatively impacts fish and wildlife habitats and populations.
What does it look like?
Knotweed emerges in the spring, and reaches full height by summer. The knotweeds form dense stands of hollow stems that are green to red and resemble bamboo. The large leaf can either be egg- or heart-shaped, with a pointed tip. Minute, greenish-white flowers occur in branched sprays in summer. The plant is dormant in the winter, and the dead brown stems may remain standing.
Where does it grow?
Knotweed thrives in moist soil or river cobble, in full or partial sunlight. Most common in the flood zone along rivers and creeks, it also grows on roadsides, abandoned lots, yards, and other upland areas.
How does it spread?
Knotweed spreads by plant parts moving downstream during river flooding, high water events, or erosion. Treating knotweed in the upper watershed prevents infestation downstream and protects downstream areas from further infestation.
In the Pacific Northwest, knotweed usually spreads when roots are moved by floods or by people in either yard waste or in soil from construction sites. Even one patch can produce hundreds of new plants.
Questions? Please contact SFEG if you have any questions about knotweed. If you live in the Upper Skagit Watershed above Rockport, Skagit Fisheries Enhancement Group can treat knotweed on your property as part of the Upper Skagit Knotweed Program.
Contact Bengt Miller
(360) 770-0407
bmiller (at) skagitfisheries.org
CLICK HERE for the 2023 Knotweed Report!
Informational Resources
These internet sites provide information about knotweed and other invasive species.
– PNW Invasive Plant Council – Knotweed Information
– King County Knotweed Fact Sheet
– Washington Department of Agriculture Knotweed Information
– “Garden Wise” Brochure
– Noxious Weeds in Washington
– Stop the Spread of Knotweeds! Informational Brochure
About The Upper Skagit Basin Knotweed Program
In January 2010 SFEG formally entered into a partnership with The Nature Conservancy (TNC) to become the new leader of The Upper Skagit Knotweed Control Project. This innovative project is a strategic and thoughtful approach to controlling invasive knotweed in the Upper Skagit watershed that works in a top-down, watershed-scale approach.
The project focuses on surveying and treating knotweed found in the floodplain areas upstream of the town of Rockport along the mainstem rivers (Skagit, Sauk, Suiattle and Cascade) as well as their tributaries. The project is part of a long-term, basin-wide approach to knotweed control in the Upper Skagit watershed that has been advanced by the Skagit Cooperative Weed Management Area Working Group (Skagit CWMA) under TNC’s leadership since 2001.
This watershed-wide effort has been possible due to cooperation and partnership from over 15 organizations and approximately 100 private landowners. Since the project began in 2001, the effort has achieved approximately 50% control of knotweed in floodplain areas of the Upper Skagit watershed and the project is recognized as a model for invasive species work in Washington State.
The invasive, non-native knotweed species have become increasingly common in riparian corridors throughout North America. A recently published University of Washington study investigated the effects of knotweed invasion on the abundance and diversity of native plants as well as the quantity and nutrient quality of leaf-litter inputs in riparian forests in western Washington[1]. The study found that by displacing native species and reducing nutrient quality of litter inputs, knotweed invasion has the potential to cause long-term changes in the structure and functioning of riparian forests and adjacent aquatic habitats.
The floodplains in the Upper Skagit watershed (including the Sauk, Suiattle and Cascade Rivers) are a high priority area for protection and restoration actions in order to recover critical habitat for threatened Chinook salmon in Puget Sound. There is a coordinated effort to protect floodplain habitat in the Upper Skagit watershed through the members of the Skagit Watershed Council.
As part of its role in protecting floodplain properties, TNC completed a Site Conservation Plan in 2001 that identified the primary threats to biodiversity in the Upper Skagit ecosystem. Invasive species were identified as a threat, and in particular, three species of knotweed were singled out above other invasive species as posing a significant threat to the Upper Skagit watershed floodplain system.
SFEG is committed to continuing the work with local partners and landowners, to control knotweed through survey and treatment in a prioritized manner. Additionally, SFEG is hopeful that through outreach we may be able to use knotweed control as a stepping stone to assist landowners with other habitat restoration projects. We are very excited to lead this program and look forward to the challenge of expanding our salmon habitat improvement efforts in the Upper Skagit watershed.
SFEG does utilize herbicide to control invasive weeds on project sites where that is the only effective means of control. For more information please read the SFEG WEED CONTROL POLICY
