Himalayan knotweed

Persicaria wallichii

Overview

Photo credit:©GBNNSS; RPS group PLC
Persicaria wallichii
Habitat:
  • Himalayan knotweed is a terrestrial plant found across many habitats, including disturbed areas, roadsides, forests, and grasslands.
  • It may also be found in some freshwater habitats, like riparian zones and irrigation channels.

Description:
  • Himalayan knotweed is a shrubby perennial plant, measuring 40-120cm in height, and rarely up to 180cm.
  • Its reddish-brown stem is erect and branching. The stem can range from smooth to densely haired.
  • The lanceolate to elliptic-lanceolate leaves are smooth to densely hairy on top, and smooth to merely densely hairy or covered with brown woolly soft hairs below.
  • The flower spike is usually richly branched, wide and measuring up to 11cm long and up to 5.5cm wide. The flowers are usually creamy white to pinkish. The flower lobes range between 3-5, with 5 lobes being more common. The flowers are slightly fragrant, and have styles of different lengths.
  • Download N.I.E.A. ID guide


Origin and Distribution:
  • Himalayan knotweed is native to China, Bhutan, Afghanistan, Nepal, Pakistan and Myanmar.
  • It has been introduced to Austria, Belgium, Czechia, Denmark, France, Germany, Ireland, Italy, Norway, the United Kingdom (Great Britain and Northern Ireland), Canada, the United Kingdom and New Zealand.

Impacts:
  • Himalayan knotweed is highly invasive as it spreads vegetatively, grows quickly, shading out native species, and is a habitat generalist.
  • It can cause declines in native plants, including rare or localised species, due to their quick growth and shading out other plants.
  • Like other knotweed species, it is difficult and expensive to control.

How did it get here?:
  • Himalayan knotweed was brought to Great Britain in the early 20th century as an ornamental plant, and has since spread from containment.
  • It could further be spread by contaminated soils.

Is it found in Ireland or Northern Ireland?
  • Himalayan knotweed is currently in both Ireland and Northern Ireland.

You can help by reporting any sightings:
Prevent Spread/Arrival
  • Follow Check Clean Dry protocols to ensure that this plant isn't accidently introduced or spread further.
  • Report all sightings.

Current legislative position 
  • This species is on Schedule 9 of the Wildlife (Northern Ireland) Order 1985. It is against this order to plant or allow to grow in the wild any plant (or hybrid of any plant) listed in Schedule 9.

Species Related Files:

Invasive Species Ireland

Invasive Species Ireland