Japanese Knotweed

Fallopia japonica, commonly known as Japanese knotweed, is a large, herbaceous perennial plant of the family Polygonaceae, native to Eastern Asia in Japan, China and Korea. In North America and Europe the species is very successful and has been classified as an invasive species in several countries.

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, cream or 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, Himalayan fleece vine, monkeyweed, monkey fungus, 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 Chinese medicine, it is known as Huzhang, which translates to “tiger stick”. There are also regional names, and it is sometimes confused with sorrel. In Japanese, the name is itadori.

It is listed by the World Conservation Union as one of the world’s worst invasive species.

The invasive root system and strong growth can damage concrete foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites. It can also reduce the capacity of channels in flood defences to carry water.

It is a frequent colonizer of temperate riparian ecosystems, roadsides and waste places. It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States. 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 −35 °C and can extend 7 metres horizontally and 3 metres deep, making removal by excavation extremely difficult.

The plant is also resilient to cutting, vigorously resprouting from the roots. 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 Haida Gwaii, 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.