Reynoutria x sachalinensis Reynoutria sachalinensis Reynoutria sachalinensis Reynoutria sachalinensis

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Japanese Knotweed

Reynoutria japonica

  • Latin synonym: Fallopia japonica, Polygonum cuspidatum
  • Name also: Asian knotweed, Crimson Beauty, Donkey Rhubarb, German Sausage, Huzhang, Itadori, Japanese Bamboo, Japanese Fleece Flower, American Bamboo, Kontiki Bamboo, Mexican Bamboo
  • Family: Dock Family – Polygonaceae
  • Growing form: Perennial, dioecious bushy herb. Forms dense stands.
  • Height: 1–2.5 m (40–100 in.). Stem green–reddish, hollow, branching from upper part, curved from joint.
  • Flower: Regular (actinomorphic), approx. 5 mm (0.2 in.) wide. White–pink tepals 5. Stamens 8. Pistil of 3 fused carpels. Dioecious (male and female flowers on separate plants). Several whorled flowers forming raceme. Inflorescence an axillary, approx. 10 cm (4 in.) high cyme comprised of several racemes.
  • Leaves: Alternate. Leaf blade short-stalked, widely ovate–triangular, shortly and sharply tapered, with entire margins, glabrous, 5–15 cm (2–6 in.) long.
  • Fruit: Achene 3–4 mm (0.12–0.16 in.), brownish. Rarely forming.
  • Habitat: Yards, gardens. Ornamental and escape.
  • Flowering time: September–October.
  • Harmfulness: Harmful invasive species.

Japanese knotweed belongs to genus Rynoutria (earlier Fallopia) and the Dock family. Its scientific family name Polygonaceae comes from the Greek words poly (‘many’) and goni (‘generation’ or also ‘joint’), so ‘many-jointed’, and Japanese knotweed is a good example of this prominent family trait. An even better example of this is common knotgrass (Polygonum aviculare), which has a very jointed growing form.

Fast-growing Japanese knotweed is native to the Far East and is cultivated in Finland as an ornamental, but it can survive and even extend its habitat if it is left untended and it can escape to nearby places. In most part of Europe and in several states of the USA, Japanese knotweed is considered an invasive species. Japanese knotweed grows in rubbish dumps and other places where land from building sites has been dumped.

Giant Knotweed & Bohemian Knotweed

Reynoutria sachalinensis (Fallopia sachalinensis) & Reynoutria x bohemica (Fallopia japonica x sachalinensis, F. x bohemica)

Another quite common ornamental knotweed that is grown in Finland is giant knotweed (also called Sakhalin knotweed), which grows taller than Japanese knotweed. Its leaves are bigger, more cordate and often red-veined. Japanese knotweed and giant knotweed can cross-breed, and the hybrid they produce is called Bohemian knotweed. Like Japanese knotweed, also giant knotweed and bohemian knotweed are in Finland classified as harmful invasive species.

invasive alien species

Also elsewhere

According to the IUCN (International Union for Conservation of Nature) Japanese knotweed is listed as one of the world’s 100 worst invasive species.

“Reproduction occurs both vegetatively (rhizomes) and seeds, making this plant extremely hard to eradicate. The dense patches shade and displace other plant life and reduce wildlife habitat.”

Invasive Plant Atlas of the United States

Compare leafy spurge, purple loosestrife and broad-leaved pepperweed.

Other species from the same family

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