Knotweed removal, Issaquah Creek

Keeping knotweed at bay along Issaquah Creek

Invasive weeds do not know where a public park ends and a private lot begins.

On both public and private lands, the banks of Issaquah Creek have been changing due to a highly invasive species known as knotweed (Polygonum sp). Knotweed spreads vegetatively from rhizomes, by seed disperal, and from plant fragments, rapidly distributing itself down rivers and growing aggressively once established. This causes native shrubs and trees to die and large monocultures of knotweed to form. These monocultures are not beneficial to the health of the stream because they degrade available habitat for salmon and other wildlife. Knotweed displaces native plants, which increases erosion, reduces shade and decreases large woody debris in the creek. Knotweed also clogs waterways and alters stream chemistry.  Reduced biodiversity of plants means fewer places for native land animals to live and less food for them to eat.

Due to the challenges that knotweed creates, the Mountains to Sound Greenway Trust is undertaking a large, multi-year project to restore Issaquah Creek by working to control knotweed on private properties. This project was started in 2007, when the Mountains to Sound Greenway Trust, King County, and the City of Issaquah, with assistance from the King Conservation District, launched an effort to control knotweed on the mainstem and tributaries of Issaquah Creek. Since 2007, the Greenway Trust and our partners, including the National Fish and Wildlife Foundation, have treated over 50 acres of knotweed and worked with 60 landowners.

Leah Hall, AmeriCorps volunteer for the GreenwayThis project is not solely interested in removing knotweed, though. The ultimate goal of the project is to reforest the banks of Issaquah Creek. This is particularly important as the hatchery dam on Issaquah Creek will be removed this summer, and many more salmon will be making their way upstream. We work with landowners, offering 18 different trees and shrubs at no cost to fit their creek-side needs. Greenway staff help landowners create a planting plan for their creek bank, and then one of our Washington Conservation Corps crews install the plants. This year alone, funded by a grant from the King County Flood Control District, we have worked with 9 private landowners to install 5,000 native trees and shrubs.

Lastly in this project, we are focused on educating the landowners with whom we work. We work to teach them about the problems that knotweed creates, the importance of growing native plants, and the value of maintaining healthy streams.

We are just wrapping up planting on Issaquah Creek for the year, and plan to work with more than 50 private landowners this summer to continue knotweed treatments. This will be followed by more plantings next winter. There is still a lot of work to be done, but we are making enormous strides in our restoration along Issaquah Creek.

The Greenway Trust, also with funding from the King Conservation District, and King County are working to restore the Raging River with a similar knotweed control project. On the Raging River, the Greenway Trust and our Washington Conservation Corps crews  have treated over 50 acres of knotweed and planted over 7,000 trees and shrubs on 10 private properties.

Please contact Jennifer McKeown or Leah Hall for more information.