Himalayan knotweed

(Persicaria wallichii)

Himalayan knotweed is known by many names, and is referred to in some sources as Polygonum polystachyum, Polygonum wallichii, Persicaria polystachya, Reynoutria polystachya or Aconogonum polystachyum. With its slender, elongated leaves, it bears greater similarity to Giant knotweed and Lesser knotweed, to which it is closely related, and is often mistaken for Lesser knotweed (and occasionally for Himalayan balsam). Native of the Himalayan region from Afghanistan to south-west China, it is one of the least common knotweeds in the UK, though is more prevalent in the southwest of England. It grows quickly to a height of up to 1.8m (6 feet). Stems are usually green (though leaf stems can contain the distinctive knotweed pink) and have the characteristic ‘zig zag’ from node to node. Stems are hairy, and a key identifier of the plant is the brown sheaths that persist at the bases of the leaf stalks. The dark green, alternate, leathery leaves are 10-20cm long (4-8 inches), tapered to a point. Short hairs can often be found on the veins, edges and undersides. Leaf shapes can differ within the species, with leaf bases varying from tapering to the leaf stem to developing a slight heart-shaped lip. White or pale pink flowers bloom from mid summer to late autumn and occur in loose, branched clusters around 20-35cm (8-14 inches) long. Flowers are hermaphrodite (ie contain both male and female parts). Himalayan knotweed is most commonly found in moist soils and poses a significant ecological threat to riparian areas where it can survive flooding and quickly colonise scoured shores and islands when the flood waters recede.

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