Invasive Species Ireland > Species Accounts > Established > Terrestrial > Himalayan knotweed

Himalayan knotweed

Koenigia polystachya

Overview

Photo credit: ©GBNNSS; RPS group PLC
Koenigia polystachya
Widely Spread Species:
  • Under Article 19 of Invasive Alien Species Regulation (1143/2014) Himalayan knotweed has been identified as a Widely Spread Species in Northern Ireland and as such, management measures will be put in place to minimise its impacts.
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 and their 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 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 Northern Ireland?
You can help by reporting any sightings: @ the Centre for Environmental Data & Recording (CEDaR) - Or via the iRecord App.
Prevent Spread/Arrival
  • Follow Check Clean Dry protocols to ensure that this plant isn't accidentally introduced or spread further
  • Import only clean soil from known sources
  • Ensure all vehicles and equipment are cleaned to avoid cross contamination
  • Be aware of the threat of colonisation from upstream areas washing Himalayan knotweed material downstream
  • Promote native species and biodiversity - use alternative, native plants
  • Know what you are buying/growing and source native Irish seed and plants
  • Do not swap plants and cuttings
  • Never collect plants from the wild
  • Safe disposal of plant material and growing media
  • Report all sightings
Current legislative position (Listed on 02 August 2022)
  • This species must not intentionally be brought into the Union; kept; bred; transported to, from or within the United Kingdom, unless for the transportation to facilities in the context of eradication; placed on the market; used or exchanged; permitted to reproduce, grown or cultivated; or released into the environment.
For further queries, you can contact the Invasive Non Native Species (INNS) Team in the Northern Ireland Environment Agency on 028 9056 9558 or Email: invasivespecies@daera-ni.gov.uk

Species Related Files:

Download FileNIEA-ID-Guide-Persicaria-wallichii-Himalayan-Knotweed-V3-Nov24
Download FileRA_Koenigia_polystachya_Himalayan_knotweed
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invasivespecies@daera-ni.gov.uk
Invasive Species Northern Ireland

Invasive Species Northern Ireland