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
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.
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.
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.