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Fallopia japonica, F. sachalinensis & F. x bohemica


The taxonomy of the Japanese Knotweeds is not entirely straightforward. Bailey & Conolly (2000) explain how true Japanese Knotweed, Fallopia japonica (Houtt.) Ronse Decraene, and Giant Knotweed, F. sachalinensis (F. Schmidt. ex Maxim.) Ronse Decraene, hybridise freely to form a fertile hybrid (F. x bohemica).

Synonyms: Reynoutria japonica, Polygonum cuspidatum.

Chromosome No.: 2n = 88 (Stace 2010) for F. Japonica.



Chromosome No.: 2n = 44, 66 (Stace 2010) for F. x bohemica 


Fallopia japonica

Fallopia japonica

Photography: A.J.Lockton

Chromosome No.: 2n = 44 (Stace (2010) for F. sachalinensis


Fallopia japonica is believed to have been introduced to Britain in 1825 (Akeroyd, in: Preston, Pearman & Dines 2002), and it subsequently spread from gardens throughout the British Isles, mostly in urban areas and on riversides and waste ground. It is now found almost everywhere, but usually in small, isolated stands. David Pearman, in an article in The Plantsman, calculated that it occurs in just 2 or 3 patches per tetrad (2 km x 2 km square). Although the range of F. japonica includes the whole of the British Isles, it is rare in mountain regions and in the east. F. sachalinensis has a similar range, but is much less common, and is more concentrated in the south-east of England.

BSBI Hectad Map 

Click on the map to view full-size on the BSBI Maps Scheme website.

Fallopia japonica

Fallopia sachalinensis

Fallopia x bohemica


  • Rarity: all three taxa are now common and spreading in Britain and Ireland.
  • Threat: not threatened; all three taxa are increasing in Britain and Ireland.
  • Conservation: the most interesting thing about F. japonica and its relatives is their status. They are what Bailey & Conolly (op. cit.) described as pariahs - aliens that must be destroyed. Plants can establish from tiny root fragments dispersed in topsoil or garden waste, making it almost impossible to eradicate or control. The Defra website gives the facts. It is illegal under schedule 9 of the Wildlife & Countryside Act to introduce it into the wild, but there is nothing wrong with having it grow in a private garden. It is not a notifiable weed, and plants are not considered ‘hazardous’ waste, but ‘controlled’ waste, which means that anyone disposing of it needs a licence to do so. The reason for the concern about this plant is primarily that it can grow through tarmac, and thus cause damage to new developments. It is not entirely apparent whether some of the more alarmist claims that can be found on the internet are supported by evidence: for example, it is sometimes stated that Japanese Knotweed can damage concrete structures. Whether it is worse than, say, trees is this regard I do not know. Defra (ibid.) also claims that Japanese Knotweed harms the native flora. This seems questionable, and no references ever seem to be attached to such statements. Ford (2004) eradicated a stand of F. japonica but found that bluebells had been thriving under it in Cornwall. Kabat, Stewart & Pullin (2006) conducted a systematic review, but their remit was restricted to the efficacy of control, and did not encompass what one might assume would be the prerequisite of the need for such control.
  • Fallopia sachalinensis

    Fallopia sachalinensis

    Photograph: A.J.Lockton


    Plants in Japan suffer damage from a range of invertebrate pests which attack rhizomes, stems and leaves. No such pests have been found on British plants and no doubt this has aided its spread (Bailey & Connolly 2000). Man-made habitats such as waste ground, railway lines and urban sites provide disturbed, and often drought-stressed niches (e.g. cinder tips, clinker of railway lines, asphalt, quarries, rubble, wasteland) similar to conditions occupied in its native habitat on volcanic rocks.

    Further Work 

    There is a clear need for scientific investigations into the true effect of this, and other invasive aliens, on the native flora. HM Treasury recently contacted the BSBI for an assessment of the extent of land in the UK that is occupied by F. japonica. We calculated that, as it is recorded in some 2,500 10 km squares, and if there was up to 1 ha of it per square, that there might be between 1,000 and 3,000 ha in total. But is this estimate even close to the truth?

    • Bailey, J.P. & Conolly, A.P. 2000. Prize-winners to pariahs - A history of Japanese Knotweed s.l. (Polygonaceae) in the British Isles. Watsonia 23, 93-110.
    • Ford S. 2004. Cut and inject herbicide control of Japanese Knotweed Fallopia japonica at Rocky Valley, Cornwall, England. Conservation Evidence, 1, 1-2.
    • Kabat T.J., Stewart G.B. & Pullin A.S. 2006. Are Japanese knotweed (Fallopia japonica) control and eradication interventions effective? Systematic Review No. 21. Centre for Evidence-Based Conservation, Birmingham, UK.
    Lockton, A.J. (date accessed) Species account: Fallopia japonica & F. sachalinensis. Botanical Society of the British Isles,