Identification of Japanese Knotweed (Fallopia Japonica)

The following images and information will help you identify Japanese knotweed in and around your property. There are a number of similar knotweeds which are also becoming more prevalent in Ireland also. Broadly, the identification features on this page can be used to identify all. For specifics to tell the differences between knotweeds, follow the link here

Leaf and Flower

Leaves are roughly shield-shaped, quite variable in size and shape, usually 10-18 cm long and up to 13 cm wide. Leaves are arranged in a zig-zag growth pattern along the nodes of twigs, similar to other Fallopia species.

Distinctive features include

  • a flat base to leaves rather than a heart-shaped base (cf. Giant and Bohemian Knotweed),
  • short and pointed leaf tip rather than tapering (cf. Giant and Bohemian Knotweed),
  • Spring shoots have green/red/purple rolled leaves.

The off-white coloured flowers are small and clustered and hang from the joint of the stem and the leaf. Flowering occurs from August to October.

Distinctive features include

  • the female flower hang down in strings
  • the male flower are upward facing stands (only the female plant exists in Ireland)

Stems (Canes)

The stem structures are distinctive with a green hollow bamboo like appearance and dotted with dark blue / purple speckles during the growth season from approx March to October

During the winter die back the stems or canes the stems become woody and change to a orange/brown colour. At this time of year it may be difficult to identify Japanese knotweed stands as depending on how weathered the site is, the dead remains of the canes may no longer be standing, as in the picture to the right.

Distinctive features include

  • Green with blue / purple speckles in the growth season
  • Zig zag structure, with 'bamboo' rings on the stem nodes
  • Hollow
  • Brown and woody during winter die back - similar and often mistaken for the Dock plant
  • Wispy remains of flowers (see image right)

Roots (crowns and rhizomes)

Crowns refer to the large biomass (or knot of roots) located directly beneath a stand of knotweed. Depending on the maturity of the stand these can sometimes be quite large - even a number of feet across.

Rhizomes refer to the finger like roots that extend from the crowns, which the plant uses to sources nutrients and water. Stems will sprout from these to source light for energy, separate from the original stand to create a new stand. Thus it is often the case that the stands are interconnected with each other, creating a large underground system of roots. The rhizomes can extend laterally for 7 metres and vertically for 2 metres from the parent plant. Small fragments of rhizomes can resprout and the principal means of spread is through the deliberate or accidental movement of rhizome fragments or cut stems.

Distinctive features include

  • Rhizome has dark leathery surface that is smooth to the touch.
  • Rhizomes when broken reveal a bright two toned orange-coloured centre. Dock is more yellow and lacks the two-tone transition
  • When broken in half, they snap distinctively like a carrot. Dock doesn't break as easily and tends to tear rather than snap.
  • White fleshy tips on the rhizomes that get darker on older growth
  • Perform a scrape test on a pieces of root. The outer thin layer is easily removed when fresh. Inner flesh has pale strings running through darker orange flesh.