Himalayan knotweed (Persicaria wallichii) is often mistaken for Himalayan balsam (Impatiens glandulifera) as it bears similar colour flowers and also has long lance-like leaves. However, the leaf edge is not as serrated as Himalayan balsam and its stems are bamboo-like, similar to other knotweeds. As its name suggests, Himalayan knotweed is native to the Himalayas, with a native range spanning from Afghanistan to South-Western China.
A comparison of Himalayan Balsam leaf (top) and Himalayan Knotweed leaf (bottom) and side by side are the leaves of Giant knotweed, Bohemian knotweed, Japanese knotweed, Himalayan knotweed
Himalayan knotweed has a long, tapering leaf. The stem is usually green, although can have alternating red-green coloration. Brown sheaths can be found at the base of the leaf stem. Flowers can range in colour from white to pink and are loosely clustered. It spreads vegetatively in the UK and Ireland from cuttings and fragments. Himalayan knotweed is not as common in Ireland as the other knotweed species, although it is considered to be in expansive mode at present.
Distribution of Himalayan knotweed in Ireland
Data from Biodiversity Maps held by the National Biodiversity Data Centre www.biodiversityireland.ie 2011