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Invasive knotweed identification and control
Information about the noxious weeds known as invasive knotweed (hybrid knotweed, itadori knotweed, giant knotweed, and Himalayan knotweed). Invasive knotweeds are also known by their Latin names, Fallopia x bohemica, Fallopia japonica, Fallopia sachalinensis, and Persicaria wallichii.
About this weed
Invasive knotweed is a selectively regulated Class B noxious weed. Because this weed is very widespread in King County, property owners are not required to control this species on their property, though it is encouraged, unless their property falls within the selected area.
Invasive knotweed is required for control in selected areas on the Green River and its tributaries and on the Cedar River and its tributaries, as described on the King County Weed List.
Invasive knotweeds are also on the Washington quarantine list, and it is illegal to buy, sell or offer them for sale in the state.
Invasive knotweeds are in the buckwheat family.
Knotweed varieties
In the Pacific Northwest, there are four similar species of invasive knotweed that can be difficult to tell apart. They share similar habitat, impacts and control methods. From most to least common:
Hybrid knotweed (Fallopia x bohemica)
The most common invasive knotweed in western Washington, also called Bohemian knotweed. Hybrid knotweed is a hybrid between giant and itadori (Japanese) knotweed and shares characters of both parent species.
Itadori knotweed (Fallopia japonica)
The most ornamental of the knotweeds. Shorter than hybrid or giant. It is also commonly called Japanese knotweed.
Giant knotweed (Fallopia sachalinensis)
The largest knotweed, up to 16 feet tall or more. Also known as elephant ear bamboo because of its very large leaves.
Himalayan knotweed (Persicaria wallichii)
The most distinctive of these species. It tends to be shorter, denser and more clump-forming and has pinker flowers and stems that are not hollow.
Why it’s a noxious weed
Knotweed creates monocultures (landscape filled with one species of plant) with the largest impact on waterways and wetlands. It displaces native vegetation, increases riverbank erosion, which reduces water quality. This lowers quality of habitat for fish and wildlife.
In a built setting knotweed also damages pavement, limits visibility on roads, obscures signage, etc.
Knotweed spreads rapidly. It can clone itself from very small root and stem fragments. Those fragments are spread by floods, animals, equipment,
etc. It is very difficult to control, particularly in sensitive areas.Plant description
Invasive knotweed begins growing in Mach and April. In a few months it will grow between 4 and 12 feet tall depending upon the species.
Plants are usually found growing as dense clumps. Canes are hollow (except for Himalayan knotweed), non-woody, and can be easily bent. The leaves of hybrid, itadori, and giant knotweed are spade to heart-shaped with smooth edges. Himalayan knotweed has a lanceolate (long and pointed) leaf with a pink midrib. Leaves alternate on stem. Leaf sizes vary among invasive knotweed species. Itadori knotweed leaves are 4-6 inches long while giant knotweed has leaves that can exceed 12 inches in length.
Flowers appear in late July. They are small, whiteish clusters which grow in the leaf joints. Knotweed mainly spreads vegetatively, with root and stem fragments being moved around by floods, erosion, animals, people, etc.
The knotweed plants die back at the end of the growing season, but their old reddish-brown canes often persist.
Be aware of look-alike plants
Knotweed is distinctive looking but can sometimes be mistaken for other plants. Red osier dogwood and young black cottonwood trees are sometimes mistaken for knotweed; however, they are both native and healthy for riparian (habitat along river and streams) ecosystems.
Dogwood and cottonwoods are both woody compared to knotweed which is herbaceous (no woody stem). Red dogwood leaves also grow in an opposite pattern, with each leaf being paired along the stem.
When in doubt, take photos and report them on iNaturalist.
What to do if you find it
Because invasive knotweeds are so widespread, property owners in King County are not required to control them. We currently map infestations of invasive knotweed on King County’s major river systems. We can provide advice on how to control invasive knotweed, but there is generally no legal requirement to do so. The King County Noxious Weed Control Board encourages property owners to remove invasive knotweed where possible, and to avoid introducing it to new landscapes.
Note: Public and private landowners are not generally required to control infestations of invasive knotweed that occur on their property in King County, Washington, except in selected areas on the Green River and its tributaries and on the Cedar River and its tributaries, as described on the King County weed list.
Control methods
Please visit our knotweed control page for videos on invasive knotweed control, including with and without chemicals.
Manual and mechanical control
Can work for small, isolated patches. The goal is to remove or deplete invasive knotweed’s energy stores in the rhizome (root system that spreads outwards and can grow new stems). If cutting, cut early and often. If digging, try to remove as much of the plant as possible. Repeat until you no longer see the knotweed return.
Cultural control
Tarping knotweed may work to suppress very small infestations of knotweed that are easy to access and monitor. Tarping needs to be loose and weighted down as opposed to tight and staked. When the knotweed begins to grow and push fabric upwards, stomp knotweed then make sure tarp is still adequately covering the infest. Repeat throughout growing season.
Knotweed escaping cultural tarp control. Photo by Peter Meininger, 2014 Saxifraga.nl
Biological control
Goats essentially cut/mow invasive knotweed by eating it. This works for suppression, but generally does not eliminate knotweed from a site.
Field trials are currently underway to determine the efficacy of biological control agents for invasive knotweed. Like goats, the goal of a biological control agent is not to eradicate a weed, but to suppress it.
Chemical control
For larger infestations of invasive knotweed, the most effective method is to apply herbicide. Apply herbicide in late July to early October. Knotweed can be treated by hollow stem injection with glyphosate and/or foliar applications with either imazapyr or glyphosate.
Note: Himalayan knotweed does not have a hollow stem, so hollow stem injection is not a suitable control method for that species of knotweed.
See the King County Noxious Weed Control Program Best Management Practices and PNW Pest Management Handbook for the most up to date and specific method for chemical control of invasive knotweed. To reduce costs and environmental impacts, please use an Integrated Pest Management approach to weed control. Always read the label before using herbicide and follow state and local regulations to determine active ingredients, correct locations for use, signal words, and personal protective equipment you should wear to protect yourself when applying herbicide.
Avoid spraying where there is a chance that herbicide will enter a waterway or wetland unless you are using a state-approved aquatic herbicide and have the required permits and licenses to do so. Use of pesticides in water is regulated in Washington state. See Washington Department of Ecology Aquatic Pesticide Permits for details.
For more information or a site-specific control recommendation in King County, contact the noxious weed program [LINK: noxious-weeds/infestation-form]. For information in other counties in Washington state, contact your local weed board or your local cooperative extension office.
Disposal instructions
Do not compost roots, root crowns, or underground stems. Seal in a bag and dispose in trash. Stems can be left on site to dry out and decompose if they are in a dry area. Do not let cut stems root in moist soil or get into waterways. Dried stems may be composted on site or put into yard waste.