Did you know that King County has an entire program team dedicated to invasive knotweed? In fact, nine staff members spend their summers treating 120 river miles of knotweed along the Cedar, Snoqualmie, Skykomish and Green-Duwamish rivers. In addition, the program lends out injector kits to King County residents.
This group is referred to as the Riparian Team within King County’s Noxious Weed Control Program. They focus on “invasive knotweeds,” targeting four species: giant knotweed (Fallopia sachalinensis), Itatori knotweed (Fallopia japonica), hybrid knotweed (Fallopia x bohemicum), and Himalayan knotweed (Persicaria wallichii).
Twenty years in, the Riparian Team is a well-established and impactful program, but back in 2004 awareness of knotweed was minimal, funding for control work didn’t exist, and methods to remove knotweed were just being developed. So how did we get here? How could one plant be so important? Read along to find out.
Knotweed Biology 101
In 2004, before we had a dedicated riparian team, knotweed was designated as “control recommended” by the weed board because it was a landscape scale issue; full regulation would require massive resources and there were no effective control methods known.
Part of the problem is how effectively knotweed spreads. Here in the Puget Sound region, knotweed is not spreading by seed. There are the rare plants here and there that have viable seeds, but the vast majority of plants spread vegetatively. Knotweed most commonly spreads through its root system. It reproduces clonally through the fragmentation of stems and rhizomes (see definitions below). Just one piece of rhizome the size of your pinky nail can form a whole new plant. The longer the knotweed is growing, the larger the rhizome mass becomes, and the harder it is to remove.
Rhizome: horizontal underground plant stem that is capable of producing the shoot and root systems of a new plant
Monoculture: growth consisting of a single plant
Cloning, also known as vegetative reproduction: any form of asexual reproduction occurring in plants in which a new plants grows from a fragment or cutting of the parent plant or specialized reproductive structures
During the growing season, knotweed forms a dense monoculture. But in the winter, the above ground growth dies back, and the plant goes dormant, often leaving nothing but dry hollow stems and bare earth in its place. On rivers, this leaves bare banks that are highly prone to erosion during winter floods. Even though there is a ton of rhizome in that soil, it’s very brittle. Flood events easily break up the knotweed and move clone material downstream and out into the floodplain.
In knotweed’s native range it grows in the lava fields alongside active volcanoes in Japan. In this region several controls keep it in check. Knotweed plants in this native range are usually smaller due to poor soils and growth is limited due to repeated coverings of volcanic ash and landslides. Once you know a young knotweed plant can punch up through layers of volcanic strata, it is easier to understand how it is able to grow through a concrete driveway.
Program History and Impacts
The team’s focus on knotweed started as a response to community reactions in 2004. Likely because of the widespread flooding in ‘95-’96, knotweed suddenly appeared to be everywhere.
Knotweed was a known problem but hadn’t become an issue for most people until it started to crowd out their riverbanks. Once established on the river, knotweed outcompetes native vegetation, lowers habitat quality for fish and other wildlife, increases bank erosion, and reduces water quality.
But in 2004, not much was known regarding effective knotweed management. The landowners were not interested in excuses, it was a clear problem, and it was our job to solve it regardless of if we knew how. So, we listened and began to build trust with landowners, developed methods that worked, and searched for funding that could sustain the efforts needed.
In partnership with Washington State University (WSU), we started small in Soos Creek trialing various methods to determine what would control knotweed effectively. A huge part of the early work was building trust. Many landowners were not thrilled with the idea of the county visiting their property for decades to come. It took a lot of community ambassadors a lot of work to start those relationships and a key component of the program is continuing to foster those relationships.
Through trial and error, the riparian team (alongside partners like WSU) determined foliar spray met our needs to be efficient, effective, and to use minimal herbicide. Our motto for safe and successful treatment is to use “the right product at the right rate at the right time.” The project started upstream, working our way down to minimize work being undone by fragments floating downstream and starting new infestations.
Herbicide use is the least harmful method due to knotweeds extensive root system that can grow 25+ feet out and 7 feet down. Effective manual removal would involve excavating the entire area which destroys the living soil, animal habitat, and river structures.
As we built the program and learned what the best funding sources and partners are we expanded along the Green River, then the Snoqualmie, the Cedar, and the South Fork Skykomish River.
Present Day
Now we survey and/or treat over 120 river miles a year. Knotweed is legally required to be controlled in unincorporated king county along the Green and Cedar River, though our project area works well beyond those parameters. Our team is continually adapting to meet the new challenges and changes in management these rivers require. Because knotweed has been so well controlled in some areas, when returning for routine maintenance our riparian team is able to control other weeds that plague river sides as well, such as garlic mustard and spotted knapweed.
In the late summer and early fall the members of King County’s Noxious Weed Control Program focus primarily on knotweed control and management.
Lending Program
King County’s Noxious Weed Control Program was created by state law and is funded to support, and control regulated noxious weeds. Because knotweed is so widespread, it is only regulated along very specific river areas. So, while we do not have the jurisdiction or resources to help most landowners struggling with knotweed, we do have a program in place designed to help.
Knotweed stem injection is one of the most effective control methods for knotweed control. Rather than spraying a low concentration herbicide on the leaves, it involves injecting each stem with a more concentrated herbicide solution. This targeted approach is ideal for those with small to medium patches on their property.
Learn more about how knotweed injectors work in the video below!
That said, knotweed injectors are costly and less readily available than spraying equipment, that is why we offer the knotweed injector lending program. This program is free and available to all residents of King County from July through October and allows residents to borrow an injector for 10-day periods.
Thanks to our borrowers approximately 7500 sq ft of knotweed was controlled in 2023 alone that would not have been otherwise. Please reach out and we can discuss if injectors are the right method for you.
How to Borrow a Knotweed Stem Injector (kingcounty.gov)
If you’re on the fence, here’s some comments from our feedback form:
“This was so helpful, the training videos were clear and the injector was easy to use. I’m excited to get the knotweed out of our yard’s mini-ecosystem!”
“This program is so vital and necessary, and to be made available to home and property owners to control the spread of knotweed is a simply brilliant use of time and resources.”
“It was actually kinda fun to do!”
Knotweed Control Restores Ecosystems
The program’s goal is not to fully remove knotweed from the ecosystem (unrealistic due to its spread), but to eliminate its impacts on a watershed scale. When monocultures are removed and are replaced with diverse native plants, the banks are not washed out every year. This results in varying river structures which are critical for a thriving food web of the salmon, birds, plants, and everything else they are connected to through the ecosystem. A restored riparian buffer with a healthy root system improves water quality, minimizes erosion hazards, provides habitat, and improves flood control. Stream health in King County is improving and our knotweed projects are one piece in maintaining a healthy watershed.
Rivers are infrastructure that need to be tended. This requires maintenance, we can’t walk away, or the problems of knotweed will return. Vegetation management is a key component of that care. As we expand our work, we can complement other watershed scale restoration projects. Part of that work is prioritizing and using site specific strategies. We are constantly adapting and happy to exchange information with others looking to do a long-term project like this one.
Resources
King County Best Management Practices – Invasive Knotweeds
Invasive Knotweeds – King County Noxious Weed Factsheet
Invasive knotweed control videos – King County, Washington
Story of Knotweed | Salish Magazine
Reach out if you have any questions about knotweed control on your property at noxious.weeds@kingcounty.gov or 206-477-WEED(9333) and happy weeding!
As told by Sayward Glise, Noxious Weed Control Specialist
Thanks! This is an excellent, educational article with the history and control of knotweed.