Himalayan knotweed

Meet the plant

Himalayan knotweed (Persicaria wallichii) is rhizomatous perennial plant, growing up to 2m tall with small pinkish or white flowers. The leaves are slender, lanceolate shaped and up to 20cm long, with sheaths surrounding the stem at the base of their stalks. It grows in damp grasslands and streamsides.

Native to the Himalayas, it was first introduced as an ornamental garden plant and was in cultivation in Great Britain by 1900. The first record of Himalayan knotweed in the wild was in 1917 in North Devon. The plant is still available from some suppliers.

Impacts

Himalayan knotweed forms dense stands, with individual stems up to 2m tall, which exclude all other vegetation and native flowers. It is capable of colonising roadsides and riverbanks to the detriment of native flora.

In Great Britain, Himalayan knotweed grows in rough grassland, roadside verges and along riverbanks. It rarely sets seed and spreads predominately by vegetative means – by fragments of rhizome or pieces of stem containing nodes. Plant fragments are often moved by machinery or flowing water – or deliberately dumped in garden waste.

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Himalayan knotweed management

We’ve found Himalayan knotweed scattered across our project area, with the majority of sites in the Tay and Spey catchments. It is less prevalent than our other target species but highly invasive – we aim to get on top of the problem before it can get any worse!

Usually we find this plant on the mainstem of rivers, or key tributaries, where we believe it has spread due to previous unsuitable control efforts. It can only be treated by herbicide application – cutting, mowing or strimming will spread the plant as cut stems and fragments can regrow and establish in new sites. Control by these methods is ineffective in any case as the plant simply regrows from the undamaged and robust underground rhizome left behind.

Himalayan knotweed control

Pesticide application – spraying

Himalayan knotweed can only be treated by herbicide application, using a backpack sprayer to apply foliar spray (Glyphosate). Optimal treatment time is August to September, when the plant begins to flower.

The plant seems to respond very well to treatment and we often see infested sites nearing eradication after 2-3 years of treatment. Monitoring is then required for several more years to check for regrowth.
None!

Unlike Japanese knotweed, the stems of Himalayan knotweed are not hollow so stem injection is not possible.

*Himalayan knotweed can regenerate from plant fragments – so DO NOT CUT! Cutting, strimming or mowing will contribute to it’s spread and is ineffective as the plant simply regrows from the robust underground rhizome.*