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Japanese knotweed

Japanese knotweed

Reynoutria japonica

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How common: Common

Invasive alien species – remove it if you can

Japanese knotweed is a tall, fast-growing perennial that forms dense stands. It has hollow, bamboo-like stems and sprays of small whitish flowers late in the season. Native to East Asia, it is now widely seen along roadsides, riverbanks and waste ground, often spreading from old plantings. In many regions it is regarded as a harmful invasive species.

Did you know?

  • Its hollow, jointed stems look like bamboo.
  • In many places it spreads mainly from small pieces of its deep root system, not from seed.

About the species

Identification

Tall perennial forming dense patches, usually 1–2 m. Stems hollow, green to reddish, with knotty joints and few side branches near the top. Leaves 7–15 cm, broadly triangular to egg-shaped with a flat or blunt base and a long narrow tip; upper side green to yellow‑green, underside paler. Flowers in late summer–autumn: many small white to pale pinkish blossoms in airy clusters from the points where leaves join the stem. Seeds are small, three‑sided and dark brown but rarely ripen in cool climates. Larger relative giant knotweed has much bigger, heart‑based leaves; the hybrid is in between. Flowers in Finland in September–October.

Plant group

Herbaceous plants

Habitat

yards and roadsides, shore

Flowering

Sep–Oct

About the habitat

Thrives in sunny to half-shaded, disturbed places: riverbanks, roadsides, railways, dumps, embankments and abandoned gardens. Tolerates many soils, even heavy clay and poor ground. Mainly in southern Finland, scattered and local. Most records are in built‑up areas and settlements, often along roads and in new plantings; also as garden escapes.

Sources

Photo: W.carter, CC0 https://commons.wikimedia.org/wiki/File:Reynoutria japonica in Brastad 1.jpg

Text: Pinkka: A Pohjolan kasveja - Rabelera-Ruta, CC BY 4.0 https://laji.fi/taxon/MX.38240

Text: FinBIF species descriptions, CC BY 4.0 https://laji.fi/taxon/MX.38240

Text: Wikipedia (Finnish), CC BY-SA 4.0 https://fi.wikipedia.org/wiki/Japanintatar

See also