Invasive Knotweed – October 2016 Weed of the Month

When people ask for the worst noxious weed in King County, invasive knotweed is on the top of the list. It has so many of the qualities that make invasive plants troublesome. It grows and spreads rapidly, out-competes most other plants, is very difficult to eradicate, harms both natural ecosystems and built landscapes and structures, increases erosion on rivers, and offers few benefits to the ecosystems it invades compared to the native plants it replaces. What’s not to dislike about this plant?

Knotweed island in the South Fork Snoqualmie River. Photo courtesy King County.

Knotweed island in the South Fork Snoqualmie River. Photo courtesy King County.

You may notice knotweed along roadsides, rivers, trails and perhaps even your own backyard. In October, the leaves start to turn bright yellow as the plant goes dormant for the winter, but that may not happen until after a hard frost. Knotweed can keep on growing well into October and later if the weather is mild.

Bohemian knotweed in the fall. Photo courtesy of King County.

Bohemian knotweed in the fall. Photo courtesy of King County.

In the fall, knotweed builds up its massive root storage for next year’s growth and moves all its sugars and nutrients out of its leaves and stems and into its underground reserves. Knotweed is so efficient at moving nutrients out of its leaves, that it gives very little back as leaf litter to the streams and forests it populates, especially compared to red alder and the other native trees and shrubs it replaces.

Knotweed stems. Photo by Erin Haley.

Knotweed stems. Photo by Erin Haley.

Taxonomically, knotweed is somewhat complex. The plant commonly called Japanese knotweed could be any one of three different species: two parent species (Japanese and giant) and their hybrid (Bohemian). In British Columbia and the UK, you are most likely to see true Japanese knotweed, but in mainland Europe and most of the United States, the most abundant invasive knotweed species is the hybrid, Bohemian knotweed. You will also see small amounts of the third species giant knotweed most of the places where the others occur.

Bohemian knotweed flower stems in the fall after petals dropped off. Photo courtesy of King County.

Bohemian knotweed flower stems in the fall after petals dropped off. Photo courtesy of King County.

To make things even more complicated, there is disagreement about the Latin nomenclature for this group of plants. In the United States, most people (including the Washington State Noxious Weed Board) put these species in the genus Polygonum (P. cuspidatum, P. bohemicum and P. sachalinense), but in Europe and Canada, they are most often put in the genus Fallopia (F. japonica, F. bohemica, and F. sachalinensis). When searching online, it works best to use both names to get all the latest information.

Japanese knotweed in Japan. Photo courtesy of Steve Burke.

Japanese knotweed in Japan. Photo courtesy of Steve Burke.

Japanese and giant knotweed are native to Japan and other parts Asia but were introduced as ornamentals many decades ago to both Europe and North America, and have since spread and become serious pest plants most places they occur. The hybrid Bohemian knotweed appears to have originated in Europe where the parent plants co-occurred, and then was sold overseas to North America as an ornamental species.

There is more information on the differences between the species and how to identify them on our website and on the website of the Washington State Noxious Weed Control Board.

Knotweed in a commercial lot in Seattle. Photo by Sasha Shaw.

Knotweed in a commercial lot in Seattle. Photo by Sasha Shaw.

If you have knotweed growing on your property or next door, you may want to do some research about how to get rid of it effectively. It’s really best to get started as soon as possible, before it gets even worse. An alarming story from England describes the nightmare of having knotweed as a neighbor. The homeowner in the story hasn’t been able to sell her property because of the damage caused by her neighbor’s Japanese knotweed.

Giant knotweed growing through asphalt. Photo courtesy of King County.

Giant knotweed growing through asphalt. Photo courtesy of King County.

In Washington, knotweed is classified as a Class B Noxious Weed due to its negative impacts and how difficult it is to control. However, because it is very widespread, most counties west of the Cascades don’t require landowners to control knotweed, except in a few limited areas.

Old knotweed root crown found on upper Green River growing a new shoot. Photo by Sasha Shaw.

Old knotweed root crown found on upper Green River growing a new shoot. Photo by Sasha Shaw.

In King County, control is required only along the Green River and the Cedar River, and tributaries, where our program and partners are actively controlling knotweed as part of large-scale, cooperative, externally funded projects. Even for these rivers, the regulated area is only on the riverbanks where knotweed has the most direct impacts on water quality and salmon habitat.

King County weed specialist surveying for knotweed on the Duwamish River. Photo by Erin Haley.

King County weed specialist surveying for knotweed on the Duwamish River. Photo by Erin Haley.

However, just because knotweed control isn’t required, that doesn’t mean that there isn’t a lot of knotweed control going on in our area. Many cities and county agencies are starting to chip away at the massive infestations of knotweed on public lands and more and more private landowners and conservation groups are controlling knotweed as well.

Roadside knotweed dying after treatment. Photo by Monica Walker.

Roadside knotweed dying after treatment. Photo by Monica Walker.

In addition to the Cedar River and Green River, our program has been tackling knotweed on the upper Snoqualmie River and its tributaries, the upper Skykomish River, the Duwamish River, Miller-Walker Creeks and other high priority areas. Organizations such as Forterra, Mountains to Sound Greenway Trust, Sound Salmon Solutions and others have also been actively targeting knotweed along rivers and creeks in King County.

Knotweed growing on a sidewalk in a north Seattle neighborhood. Photo by Sasha Shaw.

Knotweed growing on a sidewalk in a north Seattle neighborhood. Photo by Sasha Shaw.

If there is knotweed in your neighborhood, consider forming a cooperative group of your own to start tackling it. We can walk you through the process, but the most important thing is to get all the property owners and public agencies working together.

Knotweed doesn’t respect property lines and has to be tackled strategically if you really want to get rid of it. You can think of it as an old-fashioned barn-raising and work together to get the job done! Contact our knotweed team for more information and advice.

Mapping knotweed on a river in King County. Photo by Frances Lucero.

Mapping knotweed on a river in King County. Photo by Frances Lucero.

To get started learning about knotweed and how to control it, you can watch our knotweed videos or read our other online resources on knotweed. Also, all of our program personnel are very knowledgeable about knotweed, so feel free to contact our program with any knotweed questions.

Staying dry under knotweed on a rainy day on the Skykomish River. Photo by Erin Haley.

Staying dry under knotweed on a rainy day on the Skykomish River. Photo by Erin Haley.



Categories: Noxious Weed of the Month, Program News, Tips, Weed Identification

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