What is Japanese knotweed?

Japanese knotweed ( Fallopia japonica ) is a perennial herb from the buckwheat family, native to East Asia. It is known for its rapid and persistent growth, and is considered among the world’s worst invasive species. It can grow into a dense shrub reaching 3–4.5 meters (10–15 feet) in height. Its hollow, jointed stems look like bamboo, and its leaves, which can grow up to 15 cm (6 inches) in length, emerge red-purple, eventually turning green. In late summer, it sprouts clusters of small white or light green flowers. Japanese knotweed spreads easily and is notoriously difficult to eliminate once established. It is found across much of North America and Europe.

Where does Japanese knotweed originally come from?

Japanese knotweed ( Fallopia japonica ) is a perennial plant belonging to the buckwheat family (Polygonaceae) and is native to China, Korea, and Japan . It was brought to Europe and North America in the mid-18th and 1870s, respectively, for landscaping purposes. Appreciated for its rapid growth, it soon spread uncontrollably beyond gardens and parks. By 2019, it had spread to 42 U.S. states and 8 Canadian provinces and is considered invasive in much of Europe. The United Kingdom’s Environment Agency has identified Japanese knotweed as the country’s most invasive and destructive plant.

What does Japanese knotweed look like?

Japanese knotweed is a tall, dense shrub that can quickly reach 10 to 15 feet in height. Its hollow, jointed stems look like bamboo and turn orange as they mature. The plant's leaves are simple and ovate, growing up to 6 inches long, emerging red-purple and then fading to green. In late summer, it develops showy clusters of small white or light green flowers along the stems. The plant has small, winged fruits that produce tiny, triangular seeds that the wind and water easily disperse. Japanese knotweed dies back to its rhizomes in the fall.

Why is Japanese knotweed considered an invasive species?

Japanese knotweed is considered an invasive species because it rapidly colonizes disturbed soil, outcompetes native plants, and forms dense thickets that offer little to local wildlife. Introduced to Europe and North America in the 1800s, it was initially valued for its quick growth and use in landscaping. It tolerates drought, shade, high temperatures, and soil salinity, and it releases chemicals that hinder the survival of surrounding plants, giving it a competitive advantage. Its seeds are easily spread by wind and water, and it can regrow from small rhizome fragments, making it notoriously difficult to eliminate.

How does Japanese knotweed spread so quickly?

Japanese knotweed spreads rapidly due to several key traits. It thrives in disturbed soil, allowing easy seed germination and rhizome spread . The plant is also allelopathic , releasing chemicals that hinder the growth of surrounding plants, giving it a competitive advantage. Its rapid growth and easy asexual proliferation allow it to form vast, single-species stands, outcompeting native plants. Even a tiny rhizome fragment can lead to regrowth, making it difficult to eliminate.

What problems can Japanese knotweed cause for the environment and humans?

Japanese knotweed, native to East Asia, is an aggressive, invasive species that poses significant ecological and economic threats outside of its native range. It swiftly colonizes disturbed soil, such as roadsides and farmsteads, and its allelopathic properties allow it to release chemicals that suppress the growth of surrounding vegetation, leading to the formation of vast, single-species stands. Its rapid growth and resilience allow it to outcompete native plants, reduce biodiversity, and offer minimal support for local wildlife. The Environment Agency in the United Kingdom has recognized Japanese knotweed as the country’s most destructive plant because of its impact on ecosystem balance.

Japanese knotweed

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Japanese knotweed

plant

Also known as: Fallopia japonica

Written by

Anna DubeyAll

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Physical description

Japanese knotweed is a tall dense shrub that can rapidly grow to a height of 3–4.5 meters (10–15 feet). Arising in the spring from spreading underground rhizomes and a deep taproot, the hollow jointed stems are reminiscent of bamboo and assume an orange hue when mature. The simple ovate leaves are borne alternately along the stems and reach up to 15 cm (6 inches) in length. The leaves emerge red-purple in color and eventually fade to green. In late summer showy clusters of small white or light green flowers appear along the stems. The small winged fruits produce tiny triangular seeds that are readily dispersed by wind and water. The plant dies back to its rhizomes in the fall.

Japanese knotweed thrives in areas with disturbed soil, such as farmsteads, trails, and roadsides, where the conditions are prime for seed germination and for the spread of the plant’s rhizomes. The plant can tolerate drought, full sun, deep shade, high temperatures, and high soil salinity and is frequently found along streams and rivers. Japanese knotweed is allelopathic, meaning it releases chemicals into the soil that inhibit the survival of surrounding plants. Its rapid growth, easy sexual and asexual proliferation, and chemical edge are all traits that give the plant considerable competitive advantage among plants that did not evolve to withstand its encroachment—such as those in Europe and North America—and allow it to form vast monospecies stands.

History of invasion

Japanese knotweed was first taken to Europe from East Asia in the mid-18th century for use in landscaping. Gardeners appreciated the plant for its quick, reliable growth, and it gained popularity, though it soon spread beyond the boundaries of the gardens and parks where it had been planted. In the 1870s the plant was brought to North America as an ornamental and privacy hedge and to control erosion. Japanese knotweed quickly escaped cultivation in the eastern United States, and its spread spiraled out of control in both Europe and North America.

As of 2019 Japanese knotweed was present in 42 U.S. states and 8 Canadian provinces. The plant is also considered invasive in much of Europe, where it has entangled the Balkan Peninsula and Italy, Portugal, and Spain, among other countries. The United Kingdom’s Environment Agency has named Japanese knotweed the country’s most invasive and destructive plant. Despite determined efforts to eliminate it, Japanese knotweed has proven to be a persistent threat to ecosystem balance, successfully outcompeting many native plants and forming dense thickets that do little to support local wildlife.

Management

Japanese knotweed is notoriously difficult to eliminate. Foliar herbicides are typically not sufficient to kill its extensive root and rhizome network. Arduously digging out the rhizomes usually proves impractical and unrealistic, as the plant can regrow from even a miniscule fragment that remains in the soil. Such soil disturbance also churns the soil seed bank, promoting the germination of a new generation of Japanese knotweed seeds. Management thus often focuses on reducing the plants’ size in a slow and tenacious fight, rather than attempting to eliminate them quickly. Land managers chop off the stalks of established plants and may coat new sprouts in herbicides containing glyphosate or triclopyr. Cut stalks are then often painstakingly collected to limit new clones from sprouted stem fragments. Cutting the plants down at least three times a year prevents them from going to seed, and, more importantly, limits the amount of nutrition moving from the leaves to the persistent rhizomes. After several years, the rhizomes may become too depleted to regenerate in the spring, and the stand will eventually be brought under control.

Anna Dubey Melissa Petruzzello

Japanese knotweed

Physical description

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History of invasion

Management