E-Flora BC: Electronic Atlas of the Flora of British Columbia

Reynoutria japonica (Houtt.) Ronse Decr. var. japonica
Japanese knotweed
Polygonaceae (Buckwheat family)

Introduction to Vascular Plants

© Bryan Kelly-McArthur  Email the photographer   (Photo ID #74902)

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Distribution of Reynoutria japonica var. japonica
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Japanese knotweed is an introduced and invasive species in British Columbia that is native to eastern Asia (Japan, China and Korea). It was imported as an ornamental plant and is sometimes used as a fence. It is a herbaceous perennial that thrives in disturbed sites, spreads rapidly, and forms thickets that are difficult to eradicate. New plants can grow from small pieces of root.

Japanaes knotweed is similar in appearance to Bohemian knotweed (Fallopia x bohemica) and giant knotweed (Fallopia sachalinensis) (though usually smaller) and hybridizes with giant knotweed.

Japanese knotweed is considered one of the top ten invasive plant species in British Columbia and is targeted for eradication. "The invasive root system and strong growth can damage foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites." (Wikipedia 2012).

Species Information

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Perennial herb from a rhizome; stems erect, numerous, branched, 1-3 m tall, reddish-brown.
Basal leaves lacking; stem leaves alternate, 10-15 cm long, the blades egg-shaped to more or less round, the bases rounded or squared off, the tips abruptly pointed, long stalked; stipules short, tubular, entire, usually deciduous.
Inflorescence of numerous flowers in many panicles; perianths whitish or greenish-white, about 2 mm long at anthesis, 5-lobed nearly to the base.
Achenes, 3-angled, black, smooth and shiny, 2.5-3 mm long.
Easily confused with P. sachalinense.

Source: The Illustrated Flora of British Columbia

Habitat / Range

Moist to mesic ditches and disturbed sites in the lowland zone; frequent in the lower Fraser Valley, rare in the Queen Charlotte Islands, Powell River and SC BC; introduced from Asia.

Source: The Illustrated Flora of British Columbia

Synonyms and Alternate Names

Fallopia japonica

Taxonomic and Nomenclatural Links

Additional Photo Sources

General References