Japanese knotweed

Japanese knotweed, (Fallopia japonica), herbaceous perennial plant of the buckwheat family (Polygonaceae) native to China, Korea, and Japan. Persistent and aggressive, Japanese knotweed is a noxious weed in many areas outside its native range and ranks among the world’s worst invasive species.

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.