In 2015, California EPMT discovered Japanese knotweed along Lagunitas Creek. 2 years later, treatment cut knotweed by 93% on National Park Service land, a major gain now

A plant does not care where one piece of land ends and another begins. That is the problem facing conservation crews across Washington’s Olympic Peninsula. Japanese knotweed has spread along riverbanks in and around Olympic National Park, crowding out native vegetation. Now several groups are working together to keep the invasive plant from damaging important salmon habitat.

The effort brings the National Park Service, the Quileute Tribe and Clallam County together. Their partnership has continued for more than a decade across boundaries that mean little to a spreading plant. The goal is straightforward, though the work is not. They want to remove knotweed while protecting waterways used by coho salmon.

Why is Japanese knotweed so difficult to control?

Japanese knotweed is not simply another unwanted plant along a trail or riverbank. It can grow aggressively and form thick stands that leave little room for native vegetation. Its underground rhizomes also make established populations difficult to eliminate completely. Plant fragments can spread through waterways and help create new infestations farther downstream.


That becomes especially troublesome in riparian areas, where land meets rivers and streams. Native plants help stabilize riverbanks and provide shade over the water. They also contribute organic material and nutrients to freshwater ecosystems. When knotweed replaces that vegetation, the changes can affect the conditions salmon depend upon.

At the Quillayute River, the infestation had already altered parts of the river corridor. Knotweed covered the ground with large green leaves and began affecting the stream environment. According to the National Park Service, the plant can reduce nutrients entering streams and slow water movement. Those changes can contribute to warmer water, creating problems for salmon eggs and young fish.

Why does salmon habitat make this a bigger story?

Coho salmon spend different stages of their lives in both freshwater and the Pacific Ocean. Adults return to freshwater streams to spawn after spending time at sea. Their young then depend on suitable freshwater conditions before eventually moving toward the ocean. That makes the health of rivers and their surrounding vegetation an important part of their life cycle.

Coho salmon have also declined in abundance in many areas over recent decades. Their status is therefore a concern for conservation agencies and communities connected to these rivers. For the Quileute Tribe, salmon carry an importance that extends beyond wildlife management. Protecting the waterways also means protecting a resource with deep cultural significance.

That connection gives the knotweed project a human dimension that can be easy to miss. Crews are not simply removing an aggressive plant from public land. They are trying to protect a living river system that supports fish, wildlife and people. The work also shows why environmental problems often cross political and property boundaries.

Why can’t one agency handle the infestation?

The Olympic Peninsula contains a patchwork of federal, tribal, state, county and private lands. Rivers can pass through several of those jurisdictions within a relatively short distance. An invasive plant can move between them without stopping. Treating only one section therefore leaves open opportunities for the infestation to return.

That reality helped drive the creation of the Olympic Invasives Working Group. The first twice-yearly gatherings began in 2002 and brought together people managing invasive plants across the peninsula. Federal and state agencies participated alongside tribal representatives, county weed coordinators, private landowners and businesses.

Those meetings created relationships that eventually supported specific projects. The knotweed effort became one example of what happens when land managers treat an ecosystem as connected. The approach also means people can share knowledge, labor and information instead of tackling the same threat separately.

Cheryl Decker, who worked as a liaison with the National Park Service’s North Coast/Cascades Network Exotic Plant Management Team, saw that value firsthand. She described the partnership between the National Park Service, Clallam County and the Quileute Tribe as crucial because they were confronting infestations in neighboring riparian areas and estuaries.

What was the fieldwork actually like?

The work has not been particularly comfortable. When treatment began along the Quillayute River, the infestation was extensive enough to require more than a month of work each year. Crews sometimes included more than eight people searching and treating affected areas. Thick Pacific Northwest vegetation made some locations especially difficult to reach.

Then there were the wasps.

One particularly challenging site had what the National Park Service team described as a disproportionate number of wasps. During Decker’s first week, the vegetation was so dense that visibility became another problem. The crews wore orange clothing so they could keep track of one another while working through the vegetation.

Those details may sound small compared with the ecological problem. They show what invasive species control actually looks like on the ground. It is slow, physical work that depends on people returning to the same places repeatedly. Success rarely arrives after one treatment.

Has the knotweed problem improved?

There are encouraging signs along the Quillayute River. The area was last treated in 2017, and later searches found substantially less knotweed, according to Garrett Rasmussen, Timber Fish and Wildlife program manager with the Quileute Tribe. But fewer plants does not mean the infestation can be forgotten.

Rasmussen said the area still needs to be searched every year. The partnership recently spent eight days working on the project, with much of that time devoted to searching the large area. That distinction matters because controlling knotweed requires finding surviving plants before they have another chance to spread.

The same principle has worked elsewhere. In California, the National Park Service’s Exotic Plant Management Team helped address Japanese knotweed along Lagunitas Creek in Marin County. The effort eventually helped create the Marin Knotweed Action Team, bringing multiple land managers together. Treatments on National Park Service land reduced knotweed numbers by 93% in 2017.

The Olympic Peninsula partnership has already expanded its work beyond the original location. Last year, the groups moved to another riparian area along the Bogachiel River. That river also crosses boundaries, creating many of the same challenges seen during the earlier project.

The next target could involve another invasive species rather than knotweed. The larger partnership is designed to remain useful regardless of which plant becomes the next threat. That may be its most important contribution to conservation on the peninsula.

Invasive species management rarely produces a dramatic finish. A successful year can simply mean fewer plants and another round of monitoring. For the crews working beside these rivers, that quiet progress matters because healthy habitat is built through repeated effort.

The knotweed will keep testing the boundaries. The people fighting it have learned not to do the same. By working across jurisdictions, they have a better chance of protecting the rivers that coho salmon need. In this case, the strongest defense against an invasive plant may be cooperation that spreads farther than the plant itself.