F. japonica is an extremely invasive weed despite its lack of extensive sexual reproduction in most of its introduced range. It is included on various lists of invasive weeds and is one of the 100 worst invasive species as identified by the IUCN. It is a potential contaminant of soil, and its ability to tolerate a remarkable range of soil types and climates means that it has the potential to spread much further than it has to date. It has gained a fearsome reputation for breaking through hard structures in the built environment and being almost impossible to eradicate once it has taken hold and is often recognized as one of the most pernicious weeds in any recipient country.

Fallopia japonica was independently classified as Reynoutria japonica by Houttuyn in 1777 and as Polygonum cuspidatum by Siebold in 1846. It was not until the early part of the twentieth century that these were discovered to be the same plant (Bailey, 1990), which is generally referred to as Polygonum cuspidatum by Japanese and American authors but, following Meissner's 1856 classification, as Fallopia japonica in Europe (Bailey, 1990). Galasso et al. (2009) proposed transfer back to Reynoutria based on rbcL plastidial sequence analysis and Reynoutria japonica Houtt. is cited as the preferred name in The Plant List (2013).

The two most common introduced varieties are var. japonica and var. compacta and it is the former that is the main problematic weed. The closely related Fallopia sachalinensis can hybridize with F. japonica to form Fallopia x bohemica, first described in 1983, which is proving to be more problematic than F. japonica var. japonica in the UK.

The common name in English is Japanese knotweed. The Japanese common name 'itadori' has a literal meaning 'take away pain'. Other common names used in the UK include: 'Hancock's curse', believed to be named after a plant supplier in Cornwall, UK; 'German sausage' referring to the characteristic flecking on the round stems; and 'pea-shooter plant', coined by children who used the cut stems to blow pellets at each other.

The plant is a vigorous growing herbaceous perennial with annual tubular, glabrous stems that ascend from an erect base. These stems are light green often with reddish flecks, branched and reach up to 3 m in height (Beerling et al., 1994). Where introduced, F. japonica is generally taller than in its native range in Japan (Holzner and Numata, 1982), where it is recorded as being 0.3-1.5 m tall (Makino, 1997). Stems arise from strong rhizomes to form a dense thicket. Rhizomes are thick and woody when old, and have been recorded as spreading 5-7 m laterally (Pridham et al., 1966). The rhizome has ring-like structures at about 2 to 4 cm intervals which are reduced leaf scales, whilst on the underside are adventitious roots travelling into the soil. The rhizome snaps like a carrot when fresh to reveal a yellow/orange colour. The main aerial shoots emerge from the large bulbous rhizome crown about 30 cm x 30 cm across. This acts as a carbohydrate store in the winter months when it represents the complete live biomass of the plant. Spreading out from this central region are a number of radial penetrating rhizomes that twist together to form a sizeable and considerable penetrating force. The leaves are 5-12 cm x 5-8 cm, broadly ovate, cuspidate at the tip and truncate at the base. At the base of each leaf petiole is located a small gland that functions as an extra-floral nectary. The flowers are off-white and borne in ochreate clusters of 3 to 6 on terminal and axillary panicles, with the main axis up to 10 cm long and with slender branches 5-9 cm long (Lousley and Kent, 1981). Sepals 5, the outer 3-keeled; stamens 8, included within a perianth in male-sterile plants, filaments 0.4 mm, anthers small, flat, empty 0.3 mm, styles 3, distinct, stigma fimbriate, exceeding the perianth; perianth greatly enlarged in fruit and conspicuously winged, completely enclosing the trigonous achene. Achenes (or nuts) 2-4 mm long, 2 mm wide, dark brown and glossy, mean weight 1.6 mg. Inflorescences initially erect but drooping at maturity. Male fertile plants are not known from the introduced range.

It is likely that the plant has spread further than can be deduced from the literature because of under reporting.

F. japonica is native to Japan, China, Taiwan and the Korean peninsula. The most likely date of its introduction to Europe was 1849, at the nursery of Philip Von Siebold, who later sent it to the Royal Botanical Gardens at Kew, UK, in 1850 (Conolly, 1977). That was also the first year that F. japonica var. japonica was made available to the public by Von Siebold as an ornamental, and later promoted as a potential source of forage. F. japonica was sent to the Royal Botanical Gardens at Edinburgh, UK in 1854, where it was then further distributed across the UK and most likely into the USA also. It had certainly become naturalized in the UK by the late 1880s, since it was reported as growing in abundance on cinder tips near Glamorgan, Wales, and had appeared on most patches of cultivated ground in Oldham, Lancashire (Storrie, 1886; Walters, 1887). It was intentionally introduced as an ornamental into the Czech Republic as early as 1892 (Pysek and Prach, 1993). Early in the 1900s the number of reports of naturalizations increased rapidly. These establishments were most likely to have been escapes from gardens as it was a popular exotic plant whose rapid growth made an ideal natural screen for the privy house in the garden (which lead it to be called the ‘outhouse plant’ in the USA). Introduction and spread in other countries followed a similar exponential pattern as that in the UK.

F. japonica is now a well-known invasive species in most potential recipient countries, so is no longer sought after as an ornamental though some varieties are still for sale and otainable through the Internet. However, the risk of introduction of rhizome material as a contaminant of soil and compost remains high in those countries where the plant is well established.

F. japonica has been able to spread without the aid of sexual reproduction and the resulting seed. Thus it is remarkable that it has managed to spread as far and wide as it has through passive means, i.e along river corridors and through human assistance, intentional or accidental. In the areas where it is introduced it is still on its expansive phase after the usual lag phase.

Amphibians have been shown to have reduced foraging success in knotweed patches (Maerz et al., 2005) and any native species forced to compete with knotweed, i.e riparian plants, are likely to suffer consequences, as demonstrated by Gerber et al. (2008).

F. sachalinensis, or giant knotweed, a closely related species which is not normally as much of a problem weed as F. japonica, is similar in many respects but is generally a much larger plant; 4-5 m tall and with much larger leaves, 20-40 cm long. Another distinguishing characteristic is at the base of the leaf, which in F. sachalinensis is rounded acuminate forming a heart shape. The hybrid between F. japonica and F. sachalinensis is called Fallopia x bohemica and is very similar to F. japonica, though it can be distinguished from its parents by having an intermediate leaf base shape similar in size to F. japonica. The closely related Polygonum polystachyum, or Himalayan knotweed, can be distinguished from F. japonica by its slightly hairy stems and longer, more slender leaf shape. It grows up to 1.8 m tall and can cause localized problems itself in similar habitats to F. japonica.

In its native range of Japan, Taiwan and Korea F. japonica is found growing in sunny places on hills, high mountains and along road verges and ditches. Other typical habitats are gravel riversides and managed pastures, where high levels of nitrogen fertilizer are applied (Child and Wade, 2000). In its introduced range, the plant can be found mainly as a riparian weed as well as an invader of man-made environments such as spoil heaps, derelict land, road and railway verges and gardens. There is a clear association with disturbed sites and urban areas thanks to its use as a horticultural plant as well as on transport corridors where vegetation management and snow ploughing can exacerbate its spread. It is found primarily in open sites, and its growth and abundance are depressed in shady sites (Beerling, 1991; Seiger, 1993) and it is consequently unable to successfully dominate in forests.

F. japonica in Japan is attacked by a suite of natural enemies, both arthropod and fungal, not present in its native range. To date, 186 species of arthropod and around 40 species of fungus have been recorded from the plant in its native range of Japan (Shaw et al., 2009). As a result of this attack it is not able to compete with local flora as effectively as it does in the introduced range and does not normally reach the same massive size. Of these natural enemies, some exert significant damage such as the sawfly Allantus luctifer and the beetle Gallerucida nigromaculata, which was described as having potential as a biocontrol agent by Zwoelfer (1973) but this chrysomelid is now thought to be G. bifasciata Motchulski and not adequately specific. Recent research by CABI has shown that the psyllid Aphalara itadori is a highly specific sap sucker and was released in England in 2010 as a biocontrol agent (Shaw et al., 2009).  A Mycosphaerella leafspot has also undergone significant safety testing and appears to be highly specific as well. In its introduced European range, F. japonica is attacked by the green dock beetle Gastrophysaviridula, but this is only when its normal Rumex host has been consumed and beetle populations are elevated.

The estimated annual control costs for one county council in Wales, UK, in 1994 was £300,000 (approximately US $600,000). The budget needed to control the 64 ha knotweed infestation in the City and County of Swansea was estimated to be £5.79 million in 1998 (Shaw et al., 2001). To control F. japonica on a national scale in the UK would cost an extrapolated £1.56 billion (approximately US $3 billion) were it to be attempted, as reported by the UK Department of Environment, Food and Rural Affairs in its recent non-native species policy review. A more recent review put the cost of Japanese knotweed to the GB economy at £166 million per year (Williams et al., 2011). An accepted estimate of control costs is £10,000 per hectare for a 3-year spraying regime, with two sprays per year, but this is probably an underestimate if revegetation costs are taken into account. Its presence can add around 10% to the costs of a development project, especially if soil is considered contaminated and subject to additional removal fees. Indeed, a spraying programme on a development site is estimated to be £27.19 per m² (approximately US $54 per m²), and including finance costs this almost doubles to £50.88 per m² (approximately $100 per m²) if soil has to be removed and clean soil imported and compacted (Child and Wade, 2000).  The worst case scenario for a 1m² stand of knotweed in a development site has been put at £46,000 (M Wade, RPS Group, UK, personal communication) because of the cost of soil removal to landfill, the associated landfill tax as well as the best practice of using a geo-textile membrane to prevent reinvasion. Recently in the UK, mortgage lenders have been refusing to lend against properties with Japanese knotweed present and this has resulted in the devaluation of properties which has been highly publicised in the press.

F. japonica infestations are often a sign of poverty in development regions of Wales and Cornwall in the UK, a factor compounded by the extra cost of development associated with F. japonica infestations. Stands become litter traps, which become evident in winter once the leaves fall. In addition, the plant can create a fire hazard in the dormant season (Ahrens, 1975).

Owing to its rapid rate of growth, F. japonica has been considered as an energy source (Bernik and Zver, 2006), although in early studies it was not found to be economically viable (Callahan et al., 1984). It is unlikely that any biofuel material would be harvested from the wild. 

Economic Value

Owing to its rapid growth rate, F. japonica has also been considered as a source of biofuel, although it was not found to be economically viable (Callahan et al., 1984).

Social Benefit

F. japonica is a commonly used food source in certain areas of Japan (R Shaw, CABI, UK, personal communication, 2008), and is reported as tasting like rhubarb or asparagus.

F. japonica is not without its uses, and in its native range is believed to have medicinal properties, not surprising when the Japanese name 'itadori' means "take away pain". It is used in Japan and China as a traditional medicine for ailments such as schistosomiasis, hyperlipemia, gonorrhoea, dermatitis and athlete's foot, where it is known as hu zhang, hu chang, tiger cane, kojo-kon and hadori-kon. The roots of F. japonica and F. sachalinensis contain relatively high levels of resveratrol, an anti-cancer drug, and are the source for most of the resveratrol sold in nutritional supplements. This extract has shown anti-tumour effects in mice (Kimura and Okuda, 2001). F. japonica is reported as having other therapeutic properties, with extracts appearing to have antipyretic and analgesic activities on mice and rats. It protects the gastric membrane against stress ulcers and inhibits gastric secretion with no effect on blood pressure. However, the drug depressed the activity of the central nervous system in mice. Leaf extracts from the closely related giant knotweed, F. sachalinensis, have been shown to inhibit the performance of common fungal pathogens of crops (Paik, 1989; Herger and Klinghauf, 1990).

Environmental Services

Where it is introduced F. japonica is claimed to be of value to bees and invertebrates as it flowers later than most native plants. The true benefit as a bee forage has not been evaluated, but since F. japonica plants in the UK do not produce pollen it could only serve as a late nectar source. As is often the case with invasive weeds, apiarists consider F. japonica to be of value to bees and invertebrates, with an increase of 45 kg in hive weight in 5 days being reported from a knotweed stand (Andros, 2000).

Knotweed has been used to stabilise riverbanks and other steep slopes, and the microclimate under its canopy has been likened to that of oak woodland (Gilbert, 1992).

The UK Environment Agency have produced a Code of Practice, and the Cornwall and Devon Knotweed Forum have produced an excellent guide which has advice on identifying the plant in the field at various stages of the season. as have the British Columbia Ministry of Forest and Range.

Prevention

F. japonica appears on the UK Wildlife and Countryside Act (1981) and as such it is illegal to cause the plant to grow in the wild. It is listed as a noxious weed in many states and provinces of North America and appears on many weed lists around the world.

SPS measures

Vehicles should be inspected when moving from infected sites to new ones.

Rapid response

It is possible to eradicate knotweed if a new infestation of rhizome is spotted quickly and the resultant plants pulled or treated before roots have become well established. 

Public awareness

The success of knotweed management is greatly improved if the public buy in to the process of prevention and control. There are many examples of knotweed information material around the world from t-shirts to mugs. The media are impressed by the concrete-cracking ability of the weed and it often features in invasive reviews. The Public Consultation carried out for the release of the biocontrol agent in the UK in 2009 also raised awareness thanks to the extensive media coverage generated at the time.

Eradication

Although present in Australia for around 100 years and naturalized a number of times in New South Wales, Tasmania and Victoria F. japonica appears to have so far achieved only very limited spread. Based on the information presently available, eradication of this species from Australia appears both feasible and highly desirable (Ainsworth et al., 2002). See Control.

Control

Due to a large and persistent rhizome system knotweed is highly resistant to control efforts (Ainsworth et al., 2002). The effectiveness of control and eradication interventions has recently been reviewed thoroughly by Kabat et al. (2006), who included 65 articles in their meta-analysis. Six categories of intervention were included, none of which could eradicate Japanese knotweed or its hybrid in the short term. Cutting treatments alone were not found to result in significant decreases in knotweed abundance. However, statistically-significant reductions in abundance can be achieved by short term application of a) glyphosate, b) imazapyr, c) imazapyr + glyphosate, d) cutting followed by filling stems with glyphosate, and e) cutting followed by spraying with glyphosate (Kabat et al., 2006). However, these authors were still unable to conclude long term efficacy for any control measure.

Cultural control and sanitary measures

There is little cultural control that is appropriate for F. japonica, although goats and cattle will graze newly emergent shoots in the spring.

Physical/mechanical control

Mechanical control is difficult but continual mowing will reduce the resources of the extensive rhizome system if carried out throughout the growing season. Glasshouse trials have shown that repeated cutting at least every 4 weeks and at least 7 weeks prior to senescence can be effective (Seiger and Merchant, 1997). Pulling up plants complete with root systems can eliminate small stands and is appropriate for local eradication in sensitive areas, but only if carried out continually over a number of years (Baker, 1988). Digging up roots, however, is even more challenging since they can extend to a depth of 2 m, and 7 m away from the crown, and despite the best efforts, it normally results in an increased stem density. This may be useful for integrated control. Stem injection is becoming more widely practiced and can be highly effective on small patches though concerns exist over the possibility of exceeding the maximum permissible dose per hectare.

Biological control

F. japonica is an ideal candidate for biological control since it has been introduced without any of the suite of natural enemies that keep it in check in its native range. It was identified as one of the best targets for biological control in the UK, with the likelihood of success being high (Shaw, 2003). It also scores highly in a review of targets for Europe (Sheppard et al., 2006). A programme has been underway, on behalf of UK and North American sponsors, since May 2003 with two candidate agents, namely a Mycosphaerella leafspot and the psyllid Aphalara itadori. Both of these agents have undergone extensive host range testing but only the psyllid has been subjected to full assessment and as a result was licensed for release in England in 2010 and a five-year monitoring programme is underway. Parallel research is also underway in Canada by AAFC Lethbridge and in the USA at Oregon State University. Given the difficulty faced by property developers, there would appear to be a market for a mycoherbicide, although registration costs are hindering this approach.

Chemical control

The use of chemicals to control F. japonica will depend on the intended goal and the restrictions in place for the environment invaded. For example, chemicals that are permitted on or near water are normally restricted as will be the potential for full control. Child and Wade (2000) recommended five herbicides for F. japonica control, to be applied as foliar sprays. Triclopyr and imazapyr can be applied to young, actively growing shoots when grasslands need to be protected; glyphosate is suitable during active growth periods when leaves are fully expanded, although larger plants may need to be sprayed using a telescopic/long lance sprayer; picloram can also be used as a soil drench due to its persistence, but not where planting is required within 2 years; and 2,4-D amine is used during the active growing period and as a selective translocated herbicide to be used in grassland, amenity areas and forest situations, although this may depend on which formulation is used in which country. Of the five herbicides, only glyphosate and 2,4-D amine can be used near water. In general, cutting and removing dead stems at the end of the season prior to a spraying regime the following season is advisable to aid access. F. japonica is a very resilient plant and unless extremely toxic chemicals are appropriate, repeated well-timed applications should be anticipated, and follow up spot treatments of any regrowth will often be required. 

Stem injection of various herbicides is a relatively modern phenomenon and can produce very good results in some conditions but concerns remain over the amount of chemical that is actually applied per hectare exceeding statutory maxima. Hagen and Dunwiddie (2008) discovered that using glyphosate, through the injection method results in the short-term dieback of injected stems. However, drawbacks to its use in certain scenarios should be considered when developing an integrated management plan for knotweed control.

IPM programmes

Using a combination of mechanical and chemical techniques can be effective, such as cutting and a follow up spray of new growth, but it is necessary to apply the chemical more than once a season (de Waal, 1995). There are two basic methods: to cut plants to 5 cm height and immediately apply a 25% solution of glyphosate or triclopyr to the cut stems; or cut or mow infestations when the plants reach the early bud stage in the late spring or summer and treat the regrowth in the autumn with glyphosate or triclopyr. If deep digging is used to effectively increase the above ground:below ground biomass ratio, then subsequent chemical application can reduce the time required to achieve effective control (Child et al., 1998). Another herbicide strategy is an integrated strategy with mowing or cutting. Integration of traditional management techniques with the psyllid (see above, under Biological Control) are yet to be assessed.

Control by utilization

Owing to its rapid rate of growth, F. japonica has been considered as an energy source (Bernik and Zver, 2006), although in early studies it was not found to be economically viable (Callahan et al., 1984). However, more modern bioengergy production methods may change this.

Monitoring and Surveillance

There are various GIS surveys on-going in the UK, the first being in Swansea, followed by Cornwall and Devon. These have provided a useful resource to planning authorities as well as national bodies.

Mitigation

Rapid eradication of newly-established F. japonica is possible but only if the rhizome has not become too extensive.

Ecosystem Restoration

Knotweed’s ability to hyper-accumulate heavy metals, including copper, zinc and cadmium, more effectively than other angiosperm species has been proven in Japan (Nishizono et al., 1989) and Croatia (Hulina and Dumija, 1999).

Considerably more work is required to investigate the reproductive strategy in North America since there is increasing evidence of seed set and seed germination. Unfortunately, this means that this region does not have the luxury of dealing with a clonal target weed. The same goes for parts of continental Europe. The sharing of unpublished experience with knotweed management would save a lot of wasted effort trialling techniques that have already been assessed in similar ecoclimatic areas.