Japanese knotweed (Fallopia japonica, syn. Polygonum cuspidatum, Reynoutria japonica) is a large, herbaceous perennial plant, native to eastern Asia in Japan, China and Korea. In the U.S.A. and Europe the species is very successful and has been classified as invasive in several countries.
A member of the family Polygonaceae, Japanese knotweed has hollow stems with distinct raised nodes that give it the appearance of bamboo, though it is not closely related. While stems may reach a maximum height of 3–4 m each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down. The leaves are broad oval with a truncated base, 7–14 cm long and 5–12 cm broad, with an entire margin. The flowers are small, creamy white, produced in erect racemes 6–15 cm long in late summer and early autumn.
Closely related species include giant knotweed (Fallopia sachalinensis, syn. Polygonum sachalinense) and Russian vine (Fallopia baldschuanica, syn. Polygonum aubertii, Polygonum baldschuanicum).
Other English names for Japanese knotweed include fleeceflower, monkeyweed, Huzhang Hancock's curse, elephant ears, pea shooters, donkey rhubarb (although it is not a rhubarb), sally rhubarb, Japanese bamboo, American bamboo, and Mexican bamboo (though it is not a bamboo). In Japanese, the name is "itadori" (usually written in katakana). There are also regional names, and it is sometimes confused with sorrel.
Japanese Knotweed is a commercial source of resveratrol supplements. Huzhang root extract is a traditional Chinese medicinal treatment.
In the U.S.A. and Europe Japanese knotweed is widely considered an invasive species or weed. It is a frequent colonizer of temperate riparian ecosystems, roadsides and waste places. The success of the species has been partially attributed to its tolerance of a very wide range of soil types, pH and salinity. Its rhizomes can survive temperatures of and can extend horizontally and deep, making removal by excavation difficult. The most effective method of control is by herbicide application close to the flowering stage in late summer or autumn. In some cases it is possible to eradicate Japanese knotweed in one growing season using only herbicides. Trials in the Queen Charlotte Islands (Haida Gwaii) of British Columbia using sea water sprayed on the foliage have demonstrated promising results, which may prove to be a viable option for eradication where concerns over herbicide application are too great.
It can be found in 39 of the 50 United States (PUSDA) and in six provinces in Canada. The species is also common in Europe. In the U.K. it was made illegal to spread Japanese knotweed by the Wildlife and Countryside Act 1981, and it is listed by the World Conservation Union as one of the world's 100 worst invasive species. In the U.S.A. it is listed as an invasive weed in Ohio, Vermont, Virginia, New York, Alaska, Pennsylvania and Washington state.
The young stems are edible as a spring vegetable, with a flavor similar to mild rhubarb. In some locations, semi-cultivating Japanese knotweed for food has been used as a means of controlling knotweed populations that invade sensitive wetland areas and drive out the native vegetation.
Both Japanese knotweed and giant knotweed are important concentrated sources of resveratrol, replacing grape byproducts. Many large supplement sources of resveratrol now use Japanese knotweed and use its scientific name in the supplement labels. The plant is useful because of its year round growth and robustness in different climates.
Japanese knotweed is a concentrated source of emodin, used as a nutritional supplement to regulate bowel motility. The roots of Japanese knotweed are used in traditional Chinese and Japanese herbal medicines as a natural laxative. The active principle responsible for the laxative effect is emodin, present in its natural form as a complex of its analogs. Emodin has a mild laxative effect in doses of 20 to 50 mg per day.