Plant 292

Reynoutria japonica Houtt. (Polygonaceae)

Japanese knotweed

Reynoutria japonica (synonym: Fallopia japonica) is a large, herbaceous perennial, with separate male and female individuals, which is native to Japan, China and Korea. It is in the same family as rhubarb (Rheum) and the docks and sorrels (Rumex). In Britain, Reynoutria japonica is most familiar as a pernicious weed.

In 1850 a single living specimen of Reynoutria japonica was collected from Japan by the German botanist and physician Philipp Franz von Siebold. It was transported to the Royal Botanic Gardens at Kew for display. Subsequently, genetic clones of this female specimen have spread throughout the UK, initially intentionally by people for use as an ornamental, then later accidentally through disturbance and transport of infected soil and the spreading habit of its underground stems (rhizomes). The fact that all specimens in the UK are genetically identical has probably limited its spread since it cannot reproduce sexually or disperse via seed. However, new plants can be produced from a very small quantity of rhizome, meaning it will spread rapidly.

The plant can be easily identified by its unusual spade-shaped leaves and the ochrea, a sheath of tissue around each leaf node; a trait shared by many in its family. The small, white flowers are produced in erect inflorescences in summer and early autumn. Young leaves are a deep red and the stem is also speckled with red spots.

In its native Japan, the plant has long been eaten as a spring vegetable foraged from the wild. It is said to have a very sour, rhubarb-like taste. Known as Itadori meaning 'tiger stick'. the plant has also been used in traditional medicines as a painkiller, although no active chemicals have been found in the plant to support this claim.

In its native habitat, the species is predated upon by a variety of invertebrates and diseases; one species of psyllid has evolved to feed specifically on knotweed (Aphalara itadori). The presence of these predators helps control knotweed's spread. The other plant species with which Reynoutria japonica shares its habitat also provide competition, preventing it from taking over as it does elsewhere. In introduced environments these pressures are less intense allowing it to become one of the world's most invasive species. Due to its extreme invasiveness and resistance to removal, houses where Reynoutria japonica is found growing become much more difficult to sell as removal of the species is a long and expensive process.

Further reading

Beerling DJ et al. 1994. Fallopia japonica (Houtt.) Ronse Decraene. Journal of Ecology 82: 959-979.

Forman J and Kesseli RV 2003. Sexual reproduction in the invasive species Fallopia japonica (Polygonaceae). American Journal of Botany 90: 586-592.

Hollingsworth ML and Bailey JP 2000. Evidence for massive clonal growth in the invasive weed Fallopia japonica (Japanese Knotweed). Botanical Journal of the Linnean Society 133: 463-472.

Robert Taite