September Invasive Weed of the Month: Japanese Knotweed

Japanese knotweed plants turn yellow in September. Time to act!

Japanese knotweed plants turn yellow in September. Time to act! (Photo: S. Leininger)

Japanese knotweed (Fallopia japonica) has been called a “nightmare” and a “botanical bully,” and is one of the 100 worst global invaders. These characterizations give you a pretty good idea of just how unwelcome this invasive weed is in our community.

A fast growing and hardy plant, knotweed is native to Japan, China, and the Korean peninsula, and was introduced to the United States in the 1800’s as an attractive ornamental. Like many invasive weeds, it readily spread into natural areas and has become a menace worldwide, including many infestations in Oregon, including much of Clackamas County.

Also known as Mexican bamboo, elephant ear bamboo, and fleeceflower, Japanese knotweed is an Oregon Class B noxious weed. This means it is a weed of economic importance which is regionally abundant, but may have limited distribution in some Oregon counties. It also means that the propagation, transport, and sale of this plant are prohibited by law.

How Can I Identify Japanese Knotweed?

Stems and leaves emerge in the spring.

Stems and leaves emerge in the spring.

These long-lived perennial plants are called knotweeds because their rhizomes or roots appear “knotty.” The plants have woody, hollow, bamboo-like stems that are pale green with purple speckles. The stems are much weaker than those of bamboo. Young shoots may look similar to asparagus.

During the late summer and early fall, the plants are recognized by their towering stems, arching branches, large triangular-shaped leaves, and clusters of small, white flowers. The flowers bloom from late July through October and grow in dense, plume-like clusters at the end of the stems.

After hard frosts, the plant dies back to the ground, but bare, reddish stems might still be visible.

Why Should I Care About Japanese Knotweed?

WeedWise staff Jeff Lesh with flowering knotweed.

WeedWise staff Jeff Lesh with flowering knotweed.

Japanese knotweed spreads quickly along streambanks 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 this degraded habitat.

This invasive plant grows not only from seed, but also by root and stem fragments. Even the smallest fragment of a stem or root from Japansese 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, this noxious weed contributes to decreased property values from the potential of asphalt, concrete, or foundation damage from the rhizome, as well as the long term investment in management of the plants.

How Can I Control Japanese Knotweed?

Knotweed emerging in the spring

Knotweed emerging in the spring

Getting rid of Japanese knotweed is not easy and requires a persistent effort to be successful.

Manual and mechanical approaches for control combine cutting, covering, and digging. These approaches focus on starving the plant of sunlight and removing as much material as possible. This can be very labor intensive and requires 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 patches, herbicides are one of the few tools that can be effective. Even with herbicides, careful timing is important to ensure success. September is a perfect time to control Japanese knotweed. Herbicide applications should be timed just as its leaves are starting to turn from green to yellow. During this time, the plants are reallocating their above ground growth back down into its roots system. This reallocation makes it more susceptible to the effects of herbicides. A carefully timed herbicide application can provide superior control to any other control methods. 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. For more information on how to control Japanese knotweed, please check out our knotweed brochure or contact our WeedWise team at 503-210-6000.

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 the District’s WeedWise program by giving them at call at 503-210-6000. Your help in identifying and reporting locations of Japanese knotweed in our community will help to stop the next invasion before it starts!

Invasive Knotweed Brochure
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