Identifying Other Types of Knotweed in the UK

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.

In a few locations, male giant knotweed has been found growing alongside Japanese knotweed, which can lead to cross pollination and the production of the extremely hardy hybrid species known as ‘bohemica’.

Giant knotweed is also known as Sakhalin knotweed, Polygonum sachalinense and Fallopia sachalinensis.

Giant knotweed, as the name suggests, can grow taller than Japanese knotweed and has considerably larger leaves. It can reach up to 4-5 metres in height but mature stands can also be found at lower heights of 3-4m. The leaves grow alternately on the stem (as opposed to opposite each other), creating the distinctive ‘zig-zag’ shape found on most knotweeds. The main stems are hollow and bamboo-like and subsidiary stems mature to a greenish-brown colour. The leaf stems themselves can be pinkish-red but are most commonly green.

The leaves are an elongated heart shape and can grow up to 40 cm long and up to 27cm wide, though are more commonly found between 15-35cm in length. They are pointed at the tip and somewhat crinkled in appearance. Fine white hairs, known as trichomes, can be found on the undersides. Flowers, appearing in late summer or early autumn, are usually green-white in colour and hang off the plant in dense clusters called panicles. The internal colour of giant knotweed rhizomes tends to be creamier rather than the distinctive orange of Japanese knotweed.

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.

Experiments have revealed a clear link between altitude and growth habit with this species, leading some to hypothesise that dwarf knotweed is actually an alpine variation of Japanese knotweed. It is often the earliest coloniser of volcanic lava and ash fields in the Far East, capable of growing over 2,400m above sea level on Mount Fuji in Japan. It possesses a high tolerance to sulphur dioxide.

Dwarf Japanese knotweed is also known as ‘Compacta’, Compact Japanese knotweed, Fallopia japonica var. ‘Compacta’, Polygonum Reynoutria, Polygonum compactum and Polygonum pictum.

Dwarf Japanese knotweed only reaches around 70cm-1m in height. Shoots emerge in late spring and stems carry the familiar reddish colouring (occasionally speckled) of its larger cousin. They also have the same ‘zig-zag’ shape, as the leaves are placed alternately along the stem.

The leaves bear a similarity to Japanese knotweed, but with quite distinct differences. They are darker green and much more variable in shape - ranging from slightly heart-shaped through to shield-shape, though some can also be found tapering to the leaf stem. The leaves have crinkled edges, a leathery texture and are pointed at the tip. They have a tendency to curl up into a concave form and only grow up to around 11 cm long and up to 10 cm wide. The leaves can have reddish veins and edges when immature. Flowers appear in late summer and are initially pale pink or white in colour, maturing to a dark pink or red. They are tightly packed and form along protruding spikes.

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.

Himalayan knotweed is also known as Polygonum polystachyum, Polygonum wallichii, Persicaria polystachya, Reynoutria polystachya and Aconogonum polystachyum.

Because of the shape and grouping of the leaves, this plant is often mistaken for non-related species, such as Himalayan balsam (Impatiens glandulifera), and related species such as pale persicaria (Persicaria lapathifolia). Consequently, it can often escape detection as a knotweed. The leaves are mid-green in colour and variable in shape. They are elongated and slender, tapering to a point at the tip, but the leaf bases can range from heart-shaped to tapered where they meet the leaf stem. Leaves can vary from 10-20 cm long and are usually slightly crinkled along the edges. Short hairs (trichomes) can be found on the undersides, veins and leaf edges.

Himalayan knotweed grows quickly to a height of up to 1.8m-2m. Stems are not bamboo-like and the leaves do not grow alternate off the stems. The stems do zig-zag however, with the bends being caused by node points where several leaf stems can be found branching off from it. At these branching points can be found brown sheaths that persist on the plant throughout the growing season. The predominantly green stems are covered in fine, white hairs. At the point where the leaf stems branch off from the main stem, they are often pink-red in colour. Small, five-petalled flowers bloom between mid-summer and late autumn. They are white (sometimes pale pink) with yellow centres. They grow in upright, loose clusters around 20-35 cm long on the ends of the stems.

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.

Experts believe that the current distribution of this species is greater than can be explained by simple hybridisation. It therefore seems feasible that ‘bohemica’ was sold commercially by nurseries in the nineteenth and twentieth centuries as “Japanese knotweed”, making its sale almost undetectable to modern researchers. Reynoutria japonica x bohemica went largely unnoticed as a separate species until it was formally recognised in 1983 in Náchod in north-eastern Bohemia (Czech Republic) and reported in a Czechoslovakian wildlife magazine. Both male and female flowers exist on ‘bohemica’ plants in the UK. It has shown itself to be more prone to dormancy following herbicide treatment than Japanese knotweed. Observations have shown ‘bonsai’ regrowth to be rare following treatment and emerging growth following a period of dormancy is more likely to be at full strength.

This hybrid species is also known as Bohemian knotweed, hybrid knotweed, Fallopia japonica x bohemica, Fallopia japonica var. japonica x Fallopia sachalinensis and Polygonum x bohemicum.

‘Bohemica’ grows to an average height of 2.5m-3m, though taller heights have been recorded (up to 4m). Stems tend to be a little sturdier than Japanese knotweed, with new growth in established stands often extremely bamboo-like in appearance from the moment they appear. Stems are usually green with reddish-purple speckles. Alternate leaves produce the familiar ‘zig-zag’ in the stems. The leaves can grow up to 25 cm long and up to 18 cm wide. Being a hybrid species, leaf shape can vary greatly, though are more commonly pointed at the tip, textured, crinkled around the edges and heart-shaped. They tend to be darker green than Japanese knotweed and in immature growth the veins are quite distinctly dark red. Short, pointed, white hairs (trichomes) can be observed on the veins on the undersides of the leaves, particularly in the early part of the growing season.

Flowers appear in late summer or early autumn, persisting on the plant until early winter. They are creamy-white and grow in densely packed clusters. Flowering ‘bohemica’ is often found in a male form, with flower clusters upright rather than hanging down off the stem.

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