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Other knotweeds

There are several species of knotweed in the UK in addition to Japanese knotweed. Four of the most prevalent are giant knotweed (Reynoutria sachalinensis, formally Fallopia sachalinensis), Himalayan knotweed (Persicaria wallichii), dwarf Japanese knotweed (Reynoutria japonica var. ‘Compacta’, formally Fallopia japonica var. ‘Compacta’) and the less problematic lesser knotweed (Persicaria campanulata). Also there are several hybrid (or cross breed) species. Most are either rare in the UK or short-lived (or both). Fallopia x conollyana is a hybrid produced by the interaction between Russian Vine (Fallopia baldschuanica, a.k.a. ‘Mile a Minute’) and Japanese knotweed, though it is rare and lacks any of the invasiveness of its parent plants. The most significant knotweed hybrid is Reynoutria japonica x bohemica (formally Fallopia japonica x bohemica and also known as ‘Bohemian knotweed’ or simply ‘Hybrid knotweed’), formed by the interaction between Japanese and either giant or dwarf knotweeds. This form of knotweed is strong, increasing in number and has shown more resistance to herbicides than Japanese knotweed. As such it presents the greatest issue amongst the hybrids.


Giant knotweed

Giant knotweed (Reynoutria sachalinensis) originates from Sakhalin Island, Honshu (in the north of Japan), Korea and the Kurile Islands. The exact date for its introduction to Britain is unknown, but is believed to be sometime during the late 1860s. While closely related to Japanese knotweed, it is less widely distributed outside of the Far East. Like most knotweeds it prefers moist, sunny locations and appears to be less able to adapt to shade than Japanese knotweed. Unlike Japanese knotweed, it occurs in Europe and the UK in both male and female forms and so produces viable seed. It has been discovered in Britain in female, male and hermaphrodite (i.e. male and female flowers on the same plant) forms. Giant knotweed in Europe appears to possess a greater genetic diversity than Japanese knotweed. It is possible this is due to multiple introductions from different source areas, or possibly the difference between plants grown from seed and plants grown from vegetative regeneration.


Dwarf Japanese knotweed

Dwarf Japanese knotweed (Reynoutria japonica var. ‘Compacta’) is a smaller variety of Japanese knotweed and is often mistaken for its larger cousin. It is rarely naturalised in Europe and, even when it is, it tends to remain fairly localised. As a species it was championed by Gertrude Jekyll in the late 1800s, who planted it in many of her ornamental garden schemes. Whilst not particularly widespread, dwarf knotweed does exist in in the UK with both male and female flowers. Reproduction from seed is commonplace, both in Europe and in its native range. Consequently, the main issue this knotweed presents is its ability to form hybrids with other species, particularly Japanese knotweed and giant knotweed.


Himalayan knotweed

Himalayan knotweed (Persicaria wallichii) is a native of the Himalayan area of south central Asia. It is one of the least common knotweeds in the UK, but is found in greater concentrations in areas such as the south-west of England and appears to be becoming more widespread elsewhere. In its native habitat, it can be found growing in wet meadows and marshes and can grow in altitudes in excess of 2,500m. Its predilection for moist soils creates its principal environmental impact. It has a tendency to survive flooding events and takes advantage of such events to quickly colonise scoured shores when the flood waters recede. This enables it to completely dominate the area before anything else can re-establish itself. It is present in Europe and the UK in both male and female forms, as the flowers are actually hermaphrodite (i.e. contain both male and female parts). This gives it the ability to hybridise with other knotweeds and similar species.


Bohemica (Hybrid knotweed)

‘Bohemica’ (Reynoutria japonica x bohemica) is a hybrid knotweed, formed by the interaction of giant knotweed and Japanese knotweed. A smaller form of ‘bohemica’ can also be formed by giant knotweed and dwarf Japanese knotweed. ‘Bohemica’ is the most successful of the knotweed hybrids and occurrences of this plant are continually increasing in Britain and parts of Europe. It appears to be much more vigorous and persistent than either Japanese knotweed or giant knotweed and it has shown a greater initial resistance to herbicides.

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