Asiatic Knotweed (Reynoutria japonica) — identification, habitat & look-alikes

Asiatic knotweed, far better known in the US as Japanese knotweed, is an East Asian plant that has become one of the toughest invasive weeds in America, now reported in more than 40 states. Its hollow, jointed stems look like bamboo, and it spreads mostly from bits of underground stem.

Appearance

Appearance: A shrubby perennial, typically 4–8 ft (1.2–2.4 m) tall in the US, with hollow, reddish-brown stems that have raised joints like bamboo.
Leaves: Alternate, broadly oval leaves about 4–6 in (10–15 cm) long, with a flat, squared-off base and an abrupt tip.
Flowers: Small greenish-white to creamy-white flowers in branched sprays from the leaf bases. Male and female flowers grow on separate plants.
Flowering time: Late summer, roughly August to September in New York.
Fruit: Small, shiny brown, three-sided seeds about 1/8 in (3 mm) long.

Look-alikes

giant knotweedReynoutria sachalinensis

Giant knotweed is taller, with much bigger leaves that have a heart-shaped base and a paler underside; Japanese knotweed leaves have a flat, squared-off base.

Why they get mixed upBoth have bamboo-like stems and sprays of white flowers.

Bohemian knotweedReynoutria × bohemica

This hybrid falls between its two parents and is hard to tell from a photo; your county coordinator can help confirm it.

Why they get mixed upIt's a cross between Japanese and giant knotweed.

Where it lives

Typical habitats: stream and riverbanks
roadsides
gardens
waste places
Range: Native to Japan, China, and Korea. In the US it grows coast to coast in about 42 states, missing mainly from the dry Southwest, several Gulf Coast states, and the high Rockies.
Flowering: Flowers Aug–Sep
Flowers Aug–Sep

Growing it

Propagation: Spreads mainly by long, stout . Even small pieces of rhizome or stem can sprout, so it travels in moved soil, yard waste, and floodwater.

Name and classification

About the name: Often listed under older names Fallopia japonica or Polygonum cuspidatum.

Conservation

IUCN Red List: NENot EvaluatedNot yet assessed
Status: Washington lists it as a Class B noxious weed and bans its sale and transport.
Threats: Forms dense stands that crowd out native plants and can damage pavement and structures. Control typically takes several years, and any rhizome or stem fragment left behind can regrow.

Culture and stories

Culture and stories: Young shoots are eaten as a spring vegetable with a sour, rhubarb-like taste.
The rhizome is a common commercial source of resveratrol, an antioxidant sold as a supplement.