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Fallopia japonica (Houtt.) Ronse Decr. (Reynoutria japonica
Houtt.), Japanese Knotweed

Account Summary

Introduction, neophyte, a frequent, widespread and locally dominant garden escape or discard. Native of Japan and the Far East, widely introduced and established in Europe and N America.

12 June 1974; Hackney, P.; Lower Lough Macnean, near Belcoo.

April to October.

Growth form and preferred habitats

Of far-eastern origin (ie Taiwan & N China), F. japonica is a tall, stout, erect, clump-forming dioecious perennial first introduced to gardens in Britain and Ireland in 1825, from where it first 'escaped' into the wild around 1886. Apart from a rarer compact form (see below), it appears that all the F. japonica plants grown in European gardens derive from a single Dutch import from Japan made in the 1820s. A detailed history of the introduction and spread of F. japonica, F. sachalinensis and their hybrid F. × bohemica has been provided by Bailey & Conolly (2000). F. japonica is designated a noxious weed and it is an offence to allow it to escape into the wild.

Japanese Knotweed is now regarded by many as the most aggressive introduced herbaceous plant in the flora of Britain and Ireland. It is widespread and well established throughout these islands in lowland, man-made, disturbed or linear habitats, especially by flowing water (Grime et al. 1988; Charter 1997; Hollingsworth & Bailey 2000; J.R. Akeroyd, in: Preston et al. 2002). The Fermanagh sites closely fit this pattern of occurrence in neglected ground on wayside, waterside and woodland marginal areas, including in landed estates. It also grows on many types of artificial man-made habitats and generally appears on unmanaged, disturbed ground including beside railway tracks and in old quarries. The species is described as light-demanding and salt-tolerant (Jonsell et al. 2000). It is, however, sensitive to both frost and drought.

The overwintering organs of F. japonica lie below ground level in the form of a thick rhizome and roots. The fleshy, bamboo-like aerial parts of the plant can reach over 2.0 m in height, but they die down and disappear each year, apart from a minority of standing dead stems. Nevertheless, when introduced to new ground the plant very quickly establishes and develops a laterally creeping, deeply penetrating, thick, perennating rhizome. In spring, this underground foodstore enables very rapid growth of both root and stem, producing vigorous, spreading, dense leafy clumps. A large dense canopy develops which provides the good nutrient stocking that underpins the high competitive growth strategy that is strongly characteristic of the species (Grime 1979; Schnitzler & Muller 1998).

Reproduction

Creamy-white female flowers with sterile staminodes are borne in tassel-like panicles up to 15 cm long in the axils of upper leaves in late summer and early autumn. Male-fertile plants of var. japonica are unknown in the British Isles (Beerling et al. 1994), and thus the plant spreads entirely by vegetative means. The dispersal of Japanese Knotweed is closely associated with linear landscape features, eg waterways and railways (the latter now largely defunct in much of rural Ireland), and more recently along roadsides to which it has spread through the all too common unauthorised hedgerow dumping of rhizome-infested garden waste (ie 'fly-tipping'). The legitimate trade in transported topsoil is also known to have spread the species, some loads being contaminated with the plant. Tiny fragments of rhizome as little as 8 gm fresh weight are sufficient to allow regeneration, and under suitable conditions cut stems also can produce adventitious roots within a few days.

Hybrid Knotweed

Populations of F. japonica generally contain both male-sterile and hermaphrodite individuals, as is also the case with its much larger-leaved close relative, F. sachalinensis (Giant Knotweed). When these two knotweeds occur together they frequently hybridise, and their intermediate product (F. × bohemica) forms viable seed. Although F. sachalinensis is decidedly rare in Ireland, it is very possible that some of the many stands recorded as F. japonica really belong to this hybrid, but are mistakenly identified as the more frequent parent species.

Variation and taxonomic nomenclature

In its native area of East Asia (Japan, China, Taiwan and Korea), F. japonica is very variable and is often a pioneer colonist of bare volcanic soils. In Europe, however, very little of this variation is represented in the introduced material (Jonsell et al. 2000).

Two varieties of F. japonica are described (var. japonica and a much smaller dwarf, var. compacta), but the two have not been distinguished in Fermanagh. The dwarf form is the preferred garden subject, but it is also capable of becoming so very well established it may offer a threat to the ground it occupies.

Apart from the type race var. japonica and var. compacta there is a further very tall variant in W Norway with stems up to 2.5 m and leaves 18 cm long that are darker and thinner than the type. This variant is referred by some to Reynoutria japonica var. teminalis Honda. However, Jonsell et al. (2000) reckon that further studies are needed to confirm the identity of this form of the plant.

The taxonomic revision of this member of the Polygonaceae has also given rise to numerous changes, reflected in its naming. Fallopia japonica is also known as Reynoutria japonica Houtt., and previously it was referred to Polygonum cuspidatum Siebold & Zucc.

Fermanagh occurrence

There seems to have been just one possible record of Japanese Knotweed prior to 1974, when Meikle and co-workers recorded either this species or F. sachalinensis (Giant Knotweed) in 1953 on a roadside between Tempo and Brougher Mountain. F. japonica is now very widespread throughout the county and has been recorded in 102 tetrads, 19.3% of those in the VC.

F. sachalinesis is rare in Fermanagh and F. × bohemica has not yet been recorded, although the latter has been very rarely recorded near Lough Neagh in Co Antrim (Hackney et al. 1992). In confirmation of this, a plant of F. × bohemica was recorded at the mouth of the River Six Mile Water, Antrim town in summer 2009 (P. Hackney, pers. comm. November, 2010).

British and Irish occurrence

F. japonica is common, widespread and produces very persistent, well-established dense colonies in suitable lowland ground as described above throughout both Britain and Ireland. It appears to still be increasing (Clement & Foster 1994). Conolly (1977) first documented the history of the spread of F. japonica and three other members of the family Polygonaceae in the British Isles. He recognised three phases of invasion (also observed for many other alien plants): a pioneer stage of scattered occurrences accumulating relatively slowly, followed by rapid secondary spread from primary foci, and a final era of consolidation resulting in more or less total coverage of all suitable habitats.

As is typical of other alien introductions, Japanese Knotweed is host to relatively few herbivorous insects, and it is not attacked by either nematodes or fungi. Its distribution in the British Isles is limited by late frosts and by summer drought, neither of which figure greatly in Fermanagh or in other western areas of Britain and Ireland. On the other hand, these two limiting factors do help explain why it is essentially a lowland species.

Gilbert (1989, 1994) has suggested that Japanese Knotweed is not as serious a problem in Britain as it is often considered to be. His observations of its behaviour around Sheffield indicate that it is showing signs of adapting to British conditions. Furthermore the indigenous flora and fauna are also adapting to it. Not everyone is quite so sanguine about this vigorous competitor as Dr Gilbert is, however.

Since 2013, in Britain it is illegal to sell property without checking the ground to see if Japanese Knotweed is affecting it. Where it is, the seller is required to provide a management plan for its eradication from a professional company. Japanese Knotweed is classed as 'controlled waste' under the Environmental Protection Act 1990. This requires its disposal at licensed landfill sites.

Toxicity

Aerial parts of F. japonica are grazed by sheep, cattle and horses, but the plant contain glycosides and is unpalatable. The rhizome at least is known to have killed a goat in Britain which ate it (Cooper & Johnson 1998, p. 180).

European and world occurrence

On the European continent, F. japonica is widespread being represented in over 40% of the territory. It is particularly frequent in W & C Europe. In Scandinavia, it is mainly coastal (Baltic and N Atlantic), while southwards in Europe it peters out north of the Alps, Pyrenees and other mountain ranges (Jalas & Suominen 1979, Map 422). F. japonica is also an introduced, invasive alien in N America (including Canada) and New Zealand.

Eradication or control

Cutting and burning are useless in controlling the plant (Beerling et al. 1994). Eradication is difficult and expensive, requiring application of systemic herbicide carefully timed to coincide with the near exhaustion of stored photosynthetic reserves in the late spring or early summer. The National Rivers Authority advises that eradication by cutting can take up to ten years, but other evidence would suggest this form of extirpation (or even attempted control) is quite impossible (see Beerling et al. 1994, p. 967). The only real alternative to this is the use of chemicals (ie glyphosate or 2,4-D amine), which is permissible under licence where no runoff into watercourses is anticipated. Repeated spraying for up to four years may be required. Spraying is most effective when the plant is at the flowering stage in late summer or early autumn.

Uses

The dried rhizome of F. japonica is used in traditional Chinese and Japanese herbal medicine against a variety of complaints including dermatitis, gonorrhoea and athlete's foot (Beerling et al. 1994)! Despite what is said above regarding its toxicity, young stems are considered edible as a spring vegetable, although they have a flavour similar to extremely sour Rhubarb. The flowers are an important source of nectar for honeybees, and in NE USA it gives rise to a monofloral honey often referred to as 'Bamboo honey' (https://en.wikipedia.org/wiki/Fallopia_japonica (consulted on 16 June 2018)).

Names

The genus name 'Fallopia' was given in honour of Gabriele Fallopi (1523-1562), the Italian anatomist, after whom the Fallopian tube was also named. The alternative genus name 'Reynoutria' was given in honour of Reynoutre, a 16th century French naturalist who was an acquaintance of Lobel (Stearn 1992). The Latin specific epithet 'japonica' simply refers to the geographic origin of the species in E Asia.

English common names include Japanese Knotweed, Asian Knotweed, Fleeceflower, Himalayan Fleece Vine, Monkeyweed, Monkey Fungus, Hancock's Curse, Elephant Ears, Pea Shooters, Donkey Rhubarb, Sally Rhubarb (although it is not a rhubarb), Japanese Bamboo, American Bamboo and Mexican Bamboo (although it is not a bamboo) (https://en.wikipedia.org/wiki/Fallopia_japonica (consulted 12 June 2018)). Some of these names refer to the hollow, bamboo-like stems, others are more mysterious.

Threats

A noxious weed, threatening woodland, hedgerows and waterside vegetation. Eradication is very difficult and needs to be sustained over several years.

Fallopia sachalinensis (F. Schmidt) Ronse Decr., Giant Knotweed

Introduction, neophyte, a rare garden escape or discard.

1953; MCM & D; road between Tempo and Brougher Mountain.

April to December.

Growth form, introduction and preferred habitats

This very large rhizomatous, herbaceous perennial develops annual stems that can grow up to 3, 4 or even 5 m tall. Like F. japonica (Japanese Knotweed), it has a vigorous, thick, spreading rhizome that enables it to develop dense clonal colonies, described as "coarse thickets" by Lousley & Kent (1981), but considered less dense than F. japonica clumps by Jonsell et al. (2000). The heart-shaped leaves are of coarse texture and are the largest in the family Polygonaceae, growing up to 40 cm long and 28 cm wide. The species is rather similar to Japanese Knotweed, but is distinguished by its greater height and its leaves having a heart-shaped (not straight) base and a crenate margin. It is gynodioecious, the majority of plants being functionally female, although bisexual (hermaphrodite) plants also occur.

Like F. japonica, F. sachalinensis is a native of NE Asia that was introduced to farms and gardens in Britain and Ireland in the mid-19th century (possibly as early as around 1861) (Conolly 1977). In this case, the species came from both Sakhalin Island and N Japan. Originally young shoots of the plant were considered good forage for horses and cows, especially since it grew well in times of drought. Giant Knotweed was experimentally planted for this purpose and was said to produce 80-120 tons of green fodder per acre. You need a metric conversion as well in brackets. I have calculated this to be (rounding) 30-44 tonnes/ha but please check Later it was realised that it was not as productive nor useful as predicted, and its fodder cultivation dwindled and eventually ceased or was abandoned.

Despite its dingy white flower panicles, the plant was grown as a decorative garden subject. It was promoted by Veitch's nursery in the 1880s and 1890s, one of the most important suppliers of exotic trees and shrubs to the Victorian and Edwardian gardener well into the first quarter of the 20th century. F. sachalinensis plants also appeared in the catalogue of the Daisy Hill Nursery in Newry in 1891, offered at one shilling each (Bailey & Conolly 2000).

On account of its larger scale, growing to around twice the height of F. japonica, cultivation of F. sachalinensis was naturally avoided by most gardeners with more modest-sized plots or properties, and only the owners of estates and very large gardens would have been likely to introduce it, perhaps intending it as cover for game. In some cases it may have been planted to provide rapid growing fodder for horses and cattle, although this usage was probably confined to areas of the European continent. Otherwise it is easy to see the species being a fashionable horticultural status symbol at the end of the 19th century.

By 1896 Giant Knotweed was reported growing unplanted in the wild, on waste ground near the Lagan Canal at Lisburn, Co Antrim (H39). The subsequent spread of the species, however, has not matched that of F. japonica, most reports of it representing primary escapes or cast-outs of garden material (Conolly 1977).

The species is somewhat more shade tolerant than F. japonica and its typical habitat in Britain and Ireland consists of open areas in estate woods, on damp areas of riverbanks, lakeshores, roadsides and waste ground, where it develops large, persistent clones that die down in winter leaving a tangle of decaying branches. In its native Japan, characteristic habitats include unstable, moist soils on river banks, volcanic detritus and on basaltic lava flows. It can also appear there on stabilised scree below coastal cliffs (Conolly 1977).

Variation

Unlike its near relative F. japonica, F. sachalinensis displays considerable variation across Europe. This reflects the origin of the plant, probably collected from numerous sites in its native territory in the 19th century, and possibly its introduction to Europe as seed, as well as rhizome fragments (Bailey & Conolly 2000).

Reproduction

Both F. japonica and F. sachalinensis are gynodioecious, with female and bisexual (hermaphrodite) plants occurring. Most plants of F. japonica in Britain and Ireland are var. japonica which only has female plants. The dwarf form, F. japonica var. compacta, has both male and female plants here. Most plants of F. sachalinensis are female, but male plants are not regarded as all that rare (Sell & Murrell 2018). The inflorescence of F. sachalinensis is shorter and on stouter branches than in F. japonica and the flowers are greenish white, the 8 cm panicles drooping. Giant Knotweed plants occasionally bear mature-looking fruits and may set viable seed. Despite this there are no definite reports of establishment and naturalisation of F. sachalinensis from seedlings in the wild anywhere in these islands (Conolly 1977; Hollingsworth & Bailey 2000).

Evidence of genetic diversity in clones examined along rivers in England and Scotland, however, does point to the possibility of seed reproduction and dispersal occurring, at least in some areas of SE England where the species is most prevalent. The pattern of spread along riverbanks, however, could also be achieved by floating plant fragments re-rooting and establishing.

Despite the vigour, size and shading capacity of the annual shoots, and the potential for both sexual and asexual reproduction, F. sachalinensis does not appear to be nearly as invasive as the smaller, much more widespread F. japonica, although reproduction of the latter is entirely vegetative. Analysis of the dates of published records of F. sachalinensis in non-garden sites clearly shows that it does not spread rapidly in these islands (Conolly 1977; Bailey & Conolly 2000).

It thus appears that the reproduction and spread of F. sachalinensis may well be limited, at least in part, by climatic conditions. Perhaps our winters are too mild and wet for seeds to chill sufficiently, break dormancy and germinate in spring. Instead, the seeds may simply die, rot and disappear. Maps of the species ex-garden occurrence in Britain and Ireland very probably simply reflect the incidence of independent primary garden escapes near habitation, or discards established on open, disturbed or waste ground (Conolly 1977; Preston et al. 2002).

Hybrid Knotweed

Populations of F. sachalinensis, like those of its smaller relative F. japonica, generally contain both male-sterile and hermaphrodite individuals. When these two knotweeds occur together they frequently hybridise, and their intermediate product (F. × bohemica (Chrtek & Chrtková) J.P. Bailey (Bohemian Knotweed)) forms viable seed in Britain, though reportedly it does not do so in Scandinavia (Jonsell et al. 2000). The hybrids are described as "variably fertile" and back-crossing can occur (Stace et al. 2015). While F. sachalinensis is decidedly scarce in Ireland, it is very possible that some of the very many stands recorded as F. japonica, really belong to the hybrid form, but are mistakenly identified as the more frequent parent species.

Fermanagh occurrence

In Fermanagh, there are records of this very large rhizomatous perennial from a total of just ten tetrads, eight with post-1975 dates. As the distribution map indicates, its ex-garden Fermanagh distribution is centred on the Irvinestown-Ballinamallard area NW of Enniskillen and it is chiefly represented on the Necarne estate and at Riversdale Forest. The latter is now a conifer plantation, but previously it was a subsidiary portion of the nearby Castle Archdale estate. Either of the two estates where the species persists might have been the primary point of Giant Knotweed's local introduction.

The first Fermanagh record, as listed above, dates from 1953 and was made by Meikle and his co-workers, except that they remarked that they might have mistaken it for F. japonica (Japanese Knotweed), and that it needed checking! The site and habitat given for their find, on the roadside between Tempo and Brougher Mountain, certainly suggests to us that F. japonica is by far more likely the correct identification of their plant. Therefore the first definite record for F. sachalinensis is Paul Hackney's find in Enniskillen Town in September 1974, a voucher for which exists in BEL. Subsequent records are as follows: Glen Lodge, 3 km N of Ederny, 1986, D. McNeill; by riverside track through conifer plantation, Riversdale Forest, Ballycassidy, 27 December 1987 and 8 July 2000, RHN; old railway ground to NE of Ballinamallard, 1991, I. McNeill; Belleisle, Upper Lough Erne, 27 June 1992, RHN; Necarne estate near Irvinestown, 19 September 1993 and 3 July 1995, RHN; S. of Drumsloe, 12 April 1995, RHN; Jamestown House, Magharacross, 2000, I. McNeill.

So far, there are no records of the hybrid, F. × bohemica from Fermanagh, but it grows in similar damp wayside habitats as Japanese Knotweed, and it is equally invasive (Stace et al. 2015).

British and Irish occurrence

Giant Knotweed had escaped from cultivation and was naturalised in the north of Ireland by 1896 and in Britain by 1903. It has subsequently spread and become widespread in both islands, but it is not as frequent, abundant, or as rapidly invasive as its smaller relative, F. japonica. Nevertheless, in Britain it stretches from S Cornwall to Shetland. The frequency of F. sachalinensis is greatest in the south of England and around the larger conurbations, its presence thinning noticeably northwards from Liverpool. In Ireland, while extending from SW Kerry (H1) to N Antrim (H39), it is much more thinly spread than in Britain and the distribution displays a slight northern and western tendency.

In 1981, the British parliament passed the Wildlife and Countryside Act which proscribed Giant Knotweed and Japanese Knotweed. It is now an offence to introduce these species into the wild anywhere in the country.

European and world occurrence

F. sachalinensis is quite widely naturalised in NW and C Europe, although it is nothing like as frequent or omnipresent as F. japonica (Jalas & Suominen 1979, Map 423). The species has been introduced and become naturalised in N America, and it is also present to a lesser extent in S Africa, India, Australia and New Zealand (https://www.cabi.org/isc/datasheet/107744 (consulted on 18 June 2018)).

Control or eradication

Both Knotweed species are well known to be remarkable difficult to eradicate. Once a clone has outgrown the space the gardener notionally allotted it, attempts to limit further spread, and to reduce or eliminate the problem it becomes, would naturally tend to involve uprooting and discarding of portions of the plant. However, digging, cutting and burning have proved useless in controlling the plant. The thick and extremely tough ramifying rhizome of F. sachalinensis can easily survive even the most severe physical mistreatment, and even a very tiny weight of rhizome tissue can regenerate the plant after its upheaval, transport and refuse deposition. Real, effective, professional eradication is difficult and expensive, requiring application of systemic herbicide carefully timed to coincide with the near exhaustion of stored photosynthetic reserves in the late spring or early summer. Eradication often requires repeated spraying over several growing seasons.

Names

The genus name 'Fallopia' was given in honour of Gabriele Fallopi (1523-1562), the Italian anatomist, after whom the Fallopian tube was also named. The alternative genus name 'Reynoutria' was given in honour of Reynoutre, a 16th century French naturalist who was an acquaintance of Lobel (Stearn 1992). The Latin specific epithet 'sachalinensis' simply refers to the geographic origin of the species in E Asia.

Threats

Although this is a very large, dominant patch-forming alien, it has not proved as vigorous and mobile as F. japonica. Since it remains rare and is confined to demesnes, one could not yet call it invasive in Fermanagh. Nevertheless, it is a noxious weed that is a potential threat to woodland and waterside vegetation, and since it produces a vigorous, fertile, invasive hybrid with F. japonica, it is all the more threatening to native vegetation. Eradication is very difficult, involves spraying with expensive herbicides, and to be effective it must be sustained over several years.