Himalayan Knotweed

Himalayan Knotweed (Persicaria wallichii) is commonly mistaken for Himalayan Balsam (Impatiens glandulifera). This is largely due to the similar colour flowers and long lance-like leaf shape. However, one main key difference is that leaf edge of the Himalayan Knotweed is not as serrated as Himalayan Balsam. Another contrast is its stems which are bamboo-like. Himalayan Knotweed has an elongated, tapering leaf. Whilst its stem is usually green, it can have interchanging red-green coloration. Brown sheaths can be seen at the very base of the leaf stem. The colouration of the loosely clustered flowers can vastly range from white to pink. It was originally introduced as an ornamental garden plant in Victorian times is still available commercially from some nurseries.

This invasive species spreads by rhizomes and vegetative propagation. It is well adapted to taking root in the post flood bare soil of river bank habitats and it can then survive future flooding. It is also increasingly found on roadsides and wastelands. It can grow up to a height of approximately 2 metre and form dense stands that outcompete native species. Its flowering takes place in late summer. Despite the flowers having both male and female parts they do not seem to produce fertile seed in this region. The species then dies back in the winter, resulting in brittle brown stems.

Horticultural control and management of the Himalayan Knotweed is crucial as it is an extremely persistent and rapid grower. This can dramatically reduce the quality of fish and wildlife habitat in riparian areas, as well as reducing availability of nutrients in the soil. It is typically spread by the use cutting machinery, passing vehicles, flowing water and the purposeful dumping of garden waste.