Japanese knotweed (Fallopia japonica) is a large invasive herbaceous perennial. Its stems are hollow and resemble bamboo, although it is not closely related. In a favorable location, the stems can reach about nine feet in height.
Japanese knotweed is native to Japan, China and Korea. In its native environment it is not considered invasive, since it is naturally controlled by insects and fungi.
Those controls do not naturally exist in Europe or North America, which has allowed Japanese knotweed to establish itself in dense stands with negative effects on native flora and fauna. It is widely found by roadsides and in other disturbed sites.
Japanese knotweed is on the Global Invasive Species Database of 100 of the world's worst alien invasive species, and is listed as an invasive or noxious weed in Alaska, New York, Ohio, Oregon, Pennsylvania, Vermont, Virginia and Washington state. It is illegal to plant or spread it in the United Kingdom.
Often classified as Polygonum cuspidatum, it was reclassified from the Polygonum genus to the Fallopia genus as Fallopia japonica. Its other names include Polygonum sieboldii, Reynoutria japonica, Fleeceflower, Himalayan fleece vine, monkeyweed, Huzhang, Hancock's curse, elephant ears, pea shooters, donkey rhubarb, sally rhubarb, Japanese bamboo, American bamboo, Mexican bamboo and (its name in Japan) itadori.
Introduction to the U.S.
Japanese knotweed was introduced into the U.K. in the mid-19th century as an ornamental plant. Its rapid growth made it useful for screening, and its dense stand of creamy-white flowers was considered attractive. It was brought from the U.K. to the U.S. in the late 1800's for use as a screening plant and for erosion control.
Japanese knotweed spreads partly by wind-borne seeds and mostly by underground rhizomes, which can reach 20 feet long. Non-chemical controls include digging out every fragment of the rhizomes, scrupulous removal of seedlings and frequent close mowing ("scalping") of the above-ground growth. It is resistant to many common herbicides, but repeated careful applications of glyphosate (sold as Roundup and other trade names) or triclopyr have been found effective, especially when combined with non-chemical controls.
Two biological controls are under consideration in the U.K. and the U.S. The psyllid Aphalara itadori is a small insect that feeds exclusively on the sap of Japanese knotweed. The leaf-spot fungus Mycosphaerella polygoni-cuspidati is also species-specific and devastating to Japanese knotweed.
Japanese knotweed is an important source of the nutritional supplement resveratrol, traditionally found only in grape skins and red wine. It also contains Vitamin A, Vitamin C, oxalic acid, potassium, phosphorus, zinc, manganese and rutin. In Japan it is used in traditional medicine as a laxative.