Japanese knotweed (Fallopia japonica or Polygonum cuspidatum), also known as fleece flower, Mexican bamboo and huzhang, is an invasive weed native to Asia. It was brought to both Europe and North America in the 1800s for use in landscaping and erosion control. It is an aggressive plant that crowds out native species and offers little benefit to wildlife. Gardeners, landowners and naturalists should learn to identify Japanese knotweed and eradicate it from their property whenever possible.
The leaves of Japanese knotweed are four to six inches long, arranged alternately on the stem, and are roughly oval or arrow shaped, with a flat edge near the stem and a distinct point at the tip. They are solid green in the summer and turn yellowish-brown in the fall. Japanese knotweed leaves often feel unusually thick and almost leathery.
Japanese knotweed stems are thick, but hollow like bamboo. The stems are rust colored and have many joints or nodes. They die back in the fall in colder climates, although the dead stems may remain standing all winter.
Flower and Fruit
Individual flowers of Japanese knotweed are small, but they occur in upright clusters up to six inches long. From a distance, the flower clusters appear frothy or foamy looking and may resembled white lilacs. The fruit of the Japanese knotweed plant is classified botanically as a nutlet. It is small, glossy dark brown or black, and three-sided, resembling a buckwheat kernel.
Japanese knotweed grows as a thick shrub or hedge and may reach heights up to 13 feet. It is an herbaceous perennial, which means that the above-ground parts die back after a hard freeze, but the roots remain alive underground, and the plant will put out fresh stems and leaves in the spring. Pulling out Japanese knotweed is not an effective method of eradication for large infestations, since roots are incredibly tenacious, and even the smallest broken piece can quickly regenerate into a dense thicket of knotweed.
Range and Habitat
Japanese knotweed is adaptable to a wide range of habitats, from full sun to full shade, in all types of soil, although it prefers riparian habitats near rivers or creeks. It is also a common weed of waste areas and brownfields. It is found throughout North America, although it is more heavily concentrated along the East Coast and in the Midwest. It has spread across the countryside of the British Isles and though much of mainland Europe, all the way from southern France to Norway. Japanese knotweed is also starting to invade Australia and New Zealand.