Fallopia japonica | Japanese Knotweed



Conservation status

Not Assessed

Legal status

Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted). Vector materials for this species (soil or spoil taken from infested sites) are also covered under Regulations 49 & 50. Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

First reported in the wild



Invasive species - risk of High Impact

Irish status


Introduction pathways - 1

Escape from Confinement

Introduction pathways subclass - 1

Ornamental purpose

Invasive score


NAPRA Ireland risk assessed


Species Biology


Tall, vigorous herbaceous perennial, growing in dense patches up to 3m tall (shorter than F. x bohemica & F. sachalinensis), with distinctive red bamboo like stems (Booy et al., 2015). Leaves 10-15cm long, shield shaped with a flattened base and lacking hairs on the underside (Booy et al., 2015). See also Fallopia x bohemica and Fallopia sachalinensis.


Lowers biodiversity by crowding out native plants. Riparian habitats invaded by knotweeds have lower invertebrate abundance, species richness and biomass, and lower plant species richness compared to uninvaded sites, which is likely to impact on amphibians, reptiles, birds and mammals that use riparian habitats (Gerber et al., 2014). Has been shown to exert allelopathic effects (releases chemicals into the soil that inhibit the growth of other plants) in common riparian plants such as nettles (Urtica dioica) (Moravcová et al., 2011). Presence of F. japonica leaf litter in streams has also been shown to have effects on the species composition of streams (Lecerf et al., 2007).


Mires, bogs & fens; Heath, scrubland & tundra; Woodland, forest and other wooded land; Regularly or recently cultivated agricultural, horticultural or domestic habitat; Inland unvegetated or sparsely vegetated habitats; Constructed, industrial or other artificial habitats; Miscellaneous


All plants occurring in the British Isles thought to be male sterile (Bailey & Connolly, 2002) and reproduction is almost entirely asexual with very little viable seed produced (0% to <2%) (Tiébré et al., 2007). Spreads through rhizomes and vegetatively locally, vegetative growth can occur from fragments as small as 2cm (60% regeneration) (Sasik & Elias, 2006). May reproduced sexually by hybridising with other Fallopia spp.: F. sachalinensis to produce F. x bohemica; F. baldschuanica to produce F. connollyana; and may backcross with F. baldschuanica to produce viable seeds (Bailey, 2001; Tiébré et al., 2007).

Pathway and vector description

Extremely popular garden plant in the 19th century, it was awarded a gold medal in 1847 by the Society of Agriculture & Horticulture at Utrecht as "the most interesting new ornamental plant of the year" (Siebold 1848 in Bailey & Connolly, 2000) and has been commercially available in Europe since 1848 (Bailey & Connolly, 2000). Introduced into the wild in Ireland in the 1900s (Reynolds, 2002), it was still widely available in the 1930s and sold in garden centres up until the 1980s (Bailey & Connolly, 2000). Spread to its current range has been through planting and garden discards due to its vigorous growth (Reynolds, 2002), with the plant then spreading vegetatively from those areas. More complete history of introduction to the British Isles in Bailey & Connolly (2000).

Mechanism of impact

Competition, Bio-fouling

Broad environment


Habitat description

Found in a wide variety of habitats though particular established and persistent on urban waste ground, roadsides and river banks (Preston et al., 2004; Reynolds, 2002).

Species group


Native region

Temperate Asia

Similar species

 Fallopia x bohemica, Fallopia sachalensis 


World distribution(GBIF)

Irish distribution

Established - Widespread & Common.

Native distribution

Native to China, Korea and Japan (Bailey & Connolly, 2000).

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2022

The following map is interactive. If you would prefer to view it full screen then click here.

How can you help

Seek advice on control.

­          Do NOT cut and discard any part of the plant on the ground, this could cause it to grow and spread

­          Do NOT mow, strim or hedge-cut Japanese knotweed. This could cause it to spread and grow from broken plant fragments.

­          Do NOT dig it out of the ground and break-up the rhizome system unless it is part of controlled deep excavation works

­          Do not compost cut knotweeds as they may grow from this.

Report sightings to the National Biodiversity Data Centre: Once the sighting has been verified, it will be made available online through Biodiversity Maps for all to access.  

Further information

Download answers to Frequently Asked Questions on Japanese knotweed in Ireland:

Invasive Species Specialist Group (ISSG) of the SSC (Species Survival Commission) of the IUCN (International Union for Conservation Nature) consider this species one of its 100 Worst Invaders globally.

Delivering Alien Invasive Species In Europe (DAISIE) project list this species one of the 100 Worst Invaders in Europe.



Bailey, J. P., & Conolly, A. P. (2000). Prize-winners to pariahs-a history of Japanese knotweed sl (Polygonaceae) in the British Isles. Watsonia, 23(1), 93-110.

Bailey, J. P. (2001). Fallopia x conollyana the railway-yard knotweed. Watsonia, 23(4), 539-542. Bailey, J. P., Bímová, K., & Mandák, B. (2009). Asexual spread versus sexual reproduction and evolution in Japanese Knotweed sl sets the stage for the “Battle of the Clones”. Biological Invasions, 11(5), 1189-1203.

Booy, O., Wade, M. & Roy, H. (2015) A Field Guide to Invasive Plants & Animals in Britain. Bloomsbury.

Gerber, E., Krebs, C., Murrell, C., Moretti, M., Rocklin, R., & Schaffner, U. (2008). Exotic invasive knotweeds (Fallopia spp.) negatively affect native plant and invertebrate assemblages in European riparian habitats. Biological Conservation, 141(3), 646-654.

Lecerf, A., Patfield, D., Boiché, A., Riipinen, M. P., Chauvet, E., & Dobson, M. (2007). Stream ecosystems respond to riparian invasion by Japanese knotweed (Fallopia japonica). Canadian Journal of Fisheries and Aquatic Sciences, 64(9), 1273-1283.

Moravcová, L., Pyšek, P., Jarošik, V., & Zákravský, P. (2011). Potential phytotoxic and shading effects of invasive Fallopia (Polygonaceae) taxa on the germination of native dominant species. NeoBiota, 9, 31-47.

Preston, C.D., Pearman, D. A. & Dines, T. D. (2002). New atlas of the British and Irish flora. An atlas of the vascular plants of Britain, Ireland, the Isle of Man and the Channel Islands, Oxford University Press.

Reynolds, S.C.P. (2002) A catalogue of alien plants in Ireland. National Botanic Gardens. Glasnevin, Dublin.

Sasik, R. & Elias Jnr, P. (2006). Rhizome regeneration of Fallopia japonica (Japanese knotweed) (Houtt.) Ronse Decr. I. Regeneration rate and size of regenerated plants. Folia oecol, 33: 57–63.

Siebold, P. F. (1848). Extrait du catalogue et du prix-courant des plantes du Japon et des Indes-Orientales et Occidentales Neerlandaises. Jaarboek van de Koninklijke Nederlandsche Maatschappij tot Aanmoediging van den Tuinbouw, pp. 38-49.

Stace, C. (1997). New Flora of the British Isles 2nd Edition. Cambridge University Press, Cambridge.

Tiébré, M. S., Vanderhoeven, S., Saad, L., & Mahy, G. (2007). Hybridization and sexual reproduction in the invasive alien Fallopia (Polygonaceae) complex in Belgium. Annals of Botany, 99(1), 193-203.

Global Invasive Species Database

CABI - Datasheet

DAISIE Factsheet