Invasive Plants in Northern Virginia: Japanese Knotweed

By Elaine Mills, Extension Master Gardener

Japanese knotweed.
Photo by K.L. Kyde, Maryland Department of Natural Resources

Fallopia japonica, syn. Polygonum cuspidatum, syn. Reynoutria japonica (Japanese knotweed) is a particularly aggressive buckwheat family member found throughout the Mid-Atlantic region. The plant was introduced to the United States from Japan in the early 1800s as an ornamental and erosion-control plant and has been considered as invasive, especially in riparian areas, since it escaped cultivation in the 1930s.

Japanese knotweed is a perennial herbaceous plant generally found in shady, moist to wet areas such as the banks of creeks, rivers, and ponds. Although it dies back each year, it emerges early in spring, growing over 3 inches a day to reach the shrub-like height of 15 feet in a single season. Sometimes called Japanese bamboo, it has hollow, jointed, reddish canes, 4- to 6-inch semi-triangular leaves, and sprays of greenish white flowers from August to September.

Knotweed spreads primarily through vegetative growth. Its large woody rhizomes grow 10 feet deep and can expand laterally up to 8 feet a year, quickly creating dense monoculture stands that block native plants’ access to light and leave underlying bare soil susceptible to erosion. In gardens, the plant has been known to grow through asphalt paving, patios, and concrete paths. It can also regenerate from stem or root fragments as small as a half inch that wash downstream from existing sites and start infestations at a new location. Some plants, including hybrids with Giant Knotweed, appear to produce viable seed that can be dispersed by water.

Japanese Knotweed can tolerate adverse conditions, such as full shade, high temperatures, high salinity, and drought. In addition, it has allelopathic properties, allowing it to inhibit the growth of potential plant competitors.

A combination of mechanical and chemical approaches is generally required to bring the plant under control. Biological control methods have been tested in Britain, but not yet in the United States. For details on control methods and proper disposal of plant parts see the Michigan Department of Natural Resources fact sheet Japanese knotweed.




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