September is the Time to Control Japanese Knotweed
Japanese knotweed (Reynoutria japonica) is one of the most damaging weeds in the world, and has been designated as one of the 100 worst global invaders. This characterization give you a pretty good idea of just how unwelcome this invasive weed can be when it invades a community.
Japanese knotweed is a fast-growing and hardy plant. Native to Japan, China, and the Korean peninsula, it was introduced to the United States in the 1800s as an ornamental plant. But don’t let the heart-shaped leaves, bamboo-like stems, and small white flowers fool you! Japanese knotweed is an aggressive invader that is transforming the landscape. Large infestations can be found across Oregon, including every major drainage in Clackamas County.
Also known as “donkey rhubarb” and “fleece-flower”, Japanese knotweed is an Oregon Class B noxious weed. This means it is a weed of economic importance that is regionally abundant but may have limited distribution in some Oregon counties. It also means that the propagation, transport, and sale of this plant is prohibited by law.
Japanese knotweed is an herbaceous perennial. It is most frequently found along streams and rivers, where root fragments, seeds, or stems are washed downstream by the current, creating new infestations. It is also found near quarries and constructions sites, where it spreads with contaminated soils and equipment. Yards and gardens are also common invasion sites, where unsuspecting homeowners planted knotweed, not realizing its potential for damage and spread. From these introductions, it continues to spread along roadsides, forest edges, and vacant lots.
This fast-growing invasive weed can have annual growth of up to 10 feet. Stems are green to reddish-brown and hollow, segmented similar to bamboo, but are much weaker. It has alternate, leathery, thick, triangular-shaped leaves. Flowers are greenish-white to cream in large plume-like clusters at the ends of the stems. They bloom from late July to October.
Why Should I Care About Japanese Knotweed?
Japanese knotweed spreads quickly along stream banks and in wetlands where it forms tall, dense stands that compete for light and resources. This harms our native plants which provide valuable food and habitat for pollinators and wildlife.
Knotweed also competes with important native streamside vegetation. Unlike our native riparian plants, knotweed dies back each winter leaving streamside soils exposed. This leads to increased erosion of the streambank, which in turn damages fish habitat and the nutrient cycles within streams. Salmon are particularly impacted by the damage done by Japanese knotweed.
Knotweed grows not only from seed but also from root and stem fragments. Even the smallest fragment of a stem or root from Japanese knotweed can yield an entirely new colony downstream. To make matters worse, Japanese knotweed will grow even more quickly and densely in response to cutting, so many well-meaning landowners have unknowingly increased its spread in their attempts to control it.
Additionally, Japanese knotweed can decrease property values by damaging asphalt, concrete, and building foundations from the rapid spread of its roots and rhizomes. A long-term investment is needed to be successful in the management of these plants.
How Can I Manage Japanese Knotweed?
Getting rid of Japanese knotweed is not easy and requires a persistent effort to be successful.
Manual and mechanical approaches for controlling knotweed use a combination of approaches including cutting, covering, and digging. These approaches focus on starving the knotweed of sunlight and removing as much material as possible. These approaches are very labor-intensive and require regular and repeated effort for success. Due to the massive time and effort involved with these practices, they are only recommended for very small infestations.
When cutting or digging out Japanese knotweed, plants should be bagged up and disposed of as trash. They can also be piled up and dried before burning, but a barrier should be placed between plants and the ground to prevent re-rooting. Pulled or dug knotweed should not be composted as this can spread it to new areas.
For larger knotweed patches, herbicides are one of the few effective tools. Even with herbicides, careful timing is important to ensure success. September is a perfect time to control Japanese knotweed. Herbicide applications are best when carried out just as the knotweed leaves are starting to turn from green to yellow. During this time, the knotweed plants are pulling energy back down into their root system. This makes it more susceptible to the effects of herbicides. A carefully timed herbicide application can provide superior control to any other control method. A landowner guide to controlling knotweed is available to learn more about these strategies.
Successful control of this invasive weed will take more than one year. While there are mechanical or manual control options for small patches, large sites will almost certainly require integrating herbicide use into your control strategy.
Report Japanese Knotweed!
Have you noticed invasive Japanese knotweed in your area? If so, please report your sightings to the Oregon Invasive Species hotline or contact us directly. Your help in identifying and reporting locations of Japanese knotweed in our community will help to stop the next invasion before it starts
For more information about knotweed check on these resources:
Biology and Management of Knotweeds in Oregon
Knotweed Best Management Practices: Columbia Gorge CWMA
Plants Profile for Polygonum cuspidatum (Japanese knotweed)
Knotweed profile – State of Oregon
Knotweed– Washington State Noxious Weed Control Board
Invasive Species Compendium Datasheet- Fallopia japonica (Japanese knotweed)
King County Noxious Weeds: Japanese knotweed Best Management Practices
Weed Research & Information Center: Japanese knotweed Weed Report
Controlling Knotweed in the Pacific Northwest
Knotweed Gallery
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- Young knotweed sprout
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- Large infestation of knotweed along a stream
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- A small knotweed seedlong
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- A large infestation of knotweed
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- A large knotweed plant in fall with changing leaves
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- A resprouting knotweed plant growing in river cobble
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- A flowering knotweed
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- knotweed leaves
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- knotweed flowers
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- A large knotweed plant
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- knotweed stems with seeds in winter
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- knotweed leaves
