knotweed control

cooperative weed management area

Since 2008, the Hood Canal Salmon Enhancement Group (HCSEG), in coordination with regional partners, has been working with local landowners to conduct surveys and treatment within the Hood Canal watershed for the noxious weed known as knotweed (Polygonum spp.).   In Washington State, “noxious weed” is a legally defined term.  The Washington State Noxious Weed Control Board determines which plants are placed on the Washington State Noxious Weed List (WAC 16-750).  These plants are non-native, aggressive and invasive, but have the potential to be eradicated or adequately controlled within the state.

What is Knotweed and why is it a problem?

There are four species of invasive knotweed in the Hood Canal including Bohemian (Polygonum x bohemicum), Japanese (P. cuspidatum), giant (P. sachalinense) and Himalayan (Persicaria wallichii). All four are collectively referred to as “knotweed”. Knotweed is an extremely aggressive, non-native plant that was imported from Asia as a garden ornamental.

Salmon ecologists are concerned about knotweed because of its capability to out-compete native plant communities and replace them with a monoculture of one species, reducing functional and species diversity. Knotweed is an herbaceous perennial that dies back with the first hard frost leaving dead canes and often bare soil devoid of native plants. Knotweed creates bank erosion problems and interferes with salmon habitat forming processes in riparian ecosystems by outcompeting native tree seedlings important for large woody debris recruitment. The rhizomatous roots can spread 23 feet from the parent plant and penetrate more than 7 feet into the soil, however the large root mass provides poor erosion control (King County BMP, 2015). On top of this, flood waters pick up, transport and deposit stem and root fragments downstream spreading them to new areas where they will invade and displace native plant communities. In addition to reproducing by seed, knotweed can regenerate from both stem and root fragments, studies have shown that fragments as small as 0.7 grams or 0.5 inches can form new plant colonies (Urgenson, 2006), (King County BMP, 2015). Although salmon physically inhabitant the aquatic zone, their habitat encompasses the whole riparian ecosystem. If left untreated knotweeds will overwhelm and displace native vegetation foundational to the functioning of riparian systems, which the survival of salmon depends on.

Tips to identify knotweed:

Giant, Japanese, Bohemian, and Himalayan knotweed are similar in appearance. Giant, Japanese, and their hybrid Bohemian all have thick green to reddish hollow stems that are jointed and swollen at the nodes, similar to bamboo (King County BMP, 2015). Large bright green leaves ranging from egg to heart-shaped with a pointed tip. Leaves are alternate with smooth edges. Himalayan is more divergent in appearance with solid stems, and elongated triangle shaped leaves. Knotweeds sprout in April, and grow 4-12 feet tall by July.  Spikes of white flowers appear from late July and begin to form seeds by mid-August (King County BMP, 2015).

Urgenson, L. S. (2006). The Ecological Consequences of Knotweed Invasion into Riparian Forests. College of Forest Resources, University of Washington, 1–65. Retrieved from http://depts.washington.edu/pnwcesu/reports/J9W88030027-Urgenson-MSThesis.pdf

Invasive Knotweeds. (2015). King County Noxious Weed Control Program: Best Management Practices, 1–15. Retrieved from https://your.kingcounty.gov/dnrp/library/water-and-land/weeds/BMPs/Knotweed-Control.pdf

Contact Information: 360-275-3575
Knotweed Project Coordinator & Field Specialist – Alex Papiez, alex@pnwsalmoncenter.org

What is being done?

The goal of the Hood Canal Summer Chum Riparian Enhancement and Knotweed Control Project is to control knotweed using a combination of methods including prevention, chemical treatment, and enhancement of native plant communities through streamside plantings. HCSEG annually surveys and treats over 45 miles of rivers and streams; and plants 10,000 native plants each year with an emphasis on conifers. HCSEG has specially trained staff that are certified in knotweed removal. Through education and landowner permission, we treat knotweed using aquatically approved herbicides including Polaris, active ingredient imazapyr, and AquaNeat, active ingredient glyphosate. Both herbicides are approved by the Washington State Department of Agriculture (WSDA) and the Department of Ecology (DOE). HCSEG and the Washington Conservation Corps (WCC) apply Polaris via foliar application using backpack sprayers to precisely target knotweed. Field crews also use AquaNeat to directly inject the stems of knotweed to avoid damage to adjacent native plant communities or ornamental trees and shrubs. HCSEG receives a National Pollutant Discharge Elimination System (NPDES) permit each year from WSDA and DOE which allows us to apply near streams with the most effective herbicides for knotweed that are the least harmful in aquatic and terrestrial ecosystems. All pesticide treatments follow recommended dosage guidelines. Without these specialty herbicides and permit, it is illegal to apply herbicides near or in streams/waterbodies.

Likewise, with landowner permission, this project also offers the opportunity to establish native plants along knotweed controlled areas. Streamside landowners are encouraged to contact our Stewardship Coordinator- Alex Papiez for free native plant installation.

Since 2008, HCSEG has been working with over 297 landowners along the Union, Dewatto, Tahuya, Big Anderson, Big Beef, Big and Little Quilcene, and Dosewallips Rivers.

To learn more about the HCSEG’s Knotweed Control Project please email Alex Papiez at alex@pnwsalmoncenter.org.

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