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Giant knotweed is an Asian perennial, one of the largest knotweeds, that grows 7–13 ft (2–4 m) tall with leaves often a foot (30 cm) long. Brought to Europe and North America as a garden plant, it has escaped and spread along streams and roadsides, where its dense patches crowd out native plants and are very hard to remove.

Appearance

Growth form
A herbaceous perennial with hollow, jointed, bamboo-like stems that are swollen at the nodes. It dies back in winter and regrows from thick underground stems ().
Leaves
Huge, alternate, heart-shaped leaves, about 6–15 in (15–40 cm) long and 4–11 in (10–28 cm) wide, with a wavy edge.
Flowers
Small greenish-white flowers in loose clusters from the leaf joints on the upper stems, in late summer to early fall.
Fruit
Small, three-sided, winged fruits.

Look-alikes

Japanese knotweedReynoutria japonica

Giant knotweed is taller with much larger leaves that have a heart-shaped base. Japanese knotweed leaves are smaller with a nearly straight base.

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

Bohemian knotweedReynoutria × bohemica

This hybrid of giant and Japanese knotweed is common and can be hard to tell apart in photos. Leaf size and leaf-base shape are the clues to check.

Why they get mixed upIt has giant knotweed as one parent.

Where it lives

Typical habitats
  • Stream banks
  • Roadsides
  • Railroad edges
  • Neglected gardens
Range
Native to northern Japan, Sakhalin, the southern Kuril Islands, and Korea's Ulleungdo island. Introduced in Europe and North America, including scattered places in the United States.
Preferred conditions
Favors moist ground: stream banks, damp waste places, roadsides, and railroad rights-of-way.

Growing it

Propagation
Spreads mainly by rhizomes. Even small pieces of stem or root can grow into a new patch.

Name and classification

About the name
Also known as Sakhalin knotweed. Older names include Polygonum sachalinense and Fallopia sachalinensis.

Conservation

IUCN Red List
NENot EvaluatedNot yet assessed
Status
Class B in Washington (listed 1999) and on the state's quarantine list, so buying, selling, or moving the plant there is prohibited.
Threats
Forms dense patches that crowd out native plants, and established stands are very hard to remove. Control usually takes several years.

Culture and stories

Culture and stories
  • The young shoots are edible. In western Europe it was widely tried as a livestock forage crop after a drought around 1893, but it proved less useful than hoped and was mostly given up.

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Giant Knotweed (Reynoutria sachalinensis) — identification, habitat & look-alikes | Moeyo