Knotweed Control

Since 2008, the Hood Canal Salmon Enhancement Group (HCSEG), in coordination with regional partners, has been working with local land owners 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?

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 smother native species and lower habitat biodiversity, water use and competition with native plants.  Knotweed spp. is known as an herbaceous perennial.  It tends to grow in wet areas particularly near streams and rivers but can also grow in dry areas.  Knotweed becomes easily established and spreads by seeds and vegetatively from rhizomes and roots. Plants die back in the end of the growing season but dead canes persist over the winter. It creates bank erosion problems and lowers the overall quality of riparian habitat for fish and wildlife.    If left untreated, it can overwhelm and eventually displace native vegetation.

What’s being done?

The goal of the Knotweed Control Project is to control knotweed using a combination of methods including prevention and chemical treatment which is applied over several years. The HCSEG has specially trained staff that are certified in knotweed removal.  Through education and landowner permission, we treat knotweed using an herbicide known as Glyphosate which is most effective on knotweed while least harmful in aquatic and terrestrial ecosystems.  All pesticide treatments follow recommended dosage guidelines.

Likewise, with landowner permission, this project also offers the opportunity to establish native plants along the knotweed controlled areas.

Since 2008, HCSEG has been working with over 80 landowners along the Union, Dewatto and Tahuya Rivers and has treated over 50 acres of knotweed.

To learn more about the HCSEG’s, Knotweed Control Project, please email Michelle Myers at myers@hcseg.org, and check out the Hood Canal Knotweed and Riparian Enhancement Project summary.

Additionally, here are links to four years of HCSEG’s Knotweed Control and Riparian Enhancement projects:
Knotweed Control & Riparian Enhancement – Years 1 – 3 and  Year 4.

Find out what’s being done at the regional level: Hood Canal Regional Knotweed Control Project.

Tips to Identify Knotweed

There are four types of knotweed; Giant, Japanese, Himalayan, and Bohemian and all are similar in appearance. It grows in large dense clusters and can reach 4-12 feet in height.  The canes/stems are hollow (similar to bamboo) and the flowers are small white showy plumes that appear in July.  Leaves are predominately heart-shaped and elongated, and can exceed 12 inches across, especially in giant knotweed.

For more information about Invasive Knotweeds, click here to download a handout from the WSU Kitsap County Extension.

Contact Information: 360-275-3575
Knotweed Project Manager – Mendy Harlow, mendy@pnwsalmoncenter.org
Knotweed Project Coordinator & Field Specialist – Michelle Myers, michelle@pnwsalmoncenter.org