Taxonomy

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.

Synonyms: Reynoutria japonica, Polygonum cuspidatum.

Distribution

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 (2km x 2km 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.

Ecology

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.

Status

Origin: both species were introduced from Asia in the 19th century, and the hybrid is therefore treated as non-native, too.

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.

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• Fallopia japonica
• Fallopia sachalinensis
• Fallopia x bohemica