Fallopia japonica (Japanese knotweed)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Plant Trade
- Impact Summary
- Economic Impact
- Environmental Impact
- Threatened Species
- Social Impact
- Risk and Impact Factors
- Uses List
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Gaps in Knowledge/Research Needs
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Fallopia japonica Houtt. Ronse Decr.
Preferred Common Name
- Japanese knotweed
Other Scientific Names
- Pleuropterus cuspidatus (Sieb. & Zucc.) Mildenke
- Pleuropterus zuccarinii (Small) Small
- Polygonum cuspidatum Sieb. & Zucc.
- Polygonum reynoutria Makino
- Polygonum seiboldii Vriese
- Polygonum zaccharini Small
- Reynoutria japonica Houtt.
International Common Names
- French: renouée du Japon
Local Common Names
- China: huzhang
- Czech Republic: kridlatka japonska
- Denmark: Japansk-pileurt
- Estonia: vooljas kirburohi; vooljas pargitatar
- Finland: Japanintatar; sieboldintatar
- Germany: Japan-knöterich
- Ireland: glúineach bhiorach; glúineach sheapanach
- Japan: itadori; itamidori
- Netherlands: duizendknoop, Japanse
- New Zealand: Asiatic knotweed
- Poland: rdest ostrokonczysty; rdestowiec ostrokonczysty
- Sweden: parkslide
- UK: donkey rhubarb; German sausage; gypsy rhubarb; Hancock's curse; Ladir Tir; pea-shooter plant; Pysen saethwr; Sally rhubarb
- USA: elephant-ear bamboo; fleece flower; Japanese bamboo; Japanese fleece flower; Mexican bamboo; wild rhubarb
- POLCU (Fallopia japonica)
Summary of InvasivenessTop of page
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.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Polygonales
- Family: Polygonaceae
- Genus: Fallopia
- Species: Fallopia japonica
Notes on Taxonomy and NomenclatureTop of page
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.
DescriptionTop of page
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.
Plant TypeTop of page
DistributionTop of page
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 30 Jun 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Bosnia and Herzegovina||Present|
|Croatia||Present||Introduced||Original citation: Carnet (2003)|
|Luxembourg||Present||Introduced||Original citation: Van Rompaey & Delvosalle, 1979|
|Norway||Present, Widespread||Introduced||Invasive||Original citation: Fremstad, 1997|
|United Kingdom||Present, Widespread||Introduced||1850||Invasive|
|-Newfoundland and Labrador||Present||Introduced||Invasive|
|-Prince Edward Island||Present||Introduced||Invasive|
|United States||Present, Widespread||Introduced|
|-Arkansas||Present, Few occurrences||Introduced|
|-North Carolina||Present, Widespread||Introduced||Invasive|
|-Oregon||Present, Widespread||Introduced||Invasive||Original citation: Seiger, 1997|
|-Pennsylvania||Present, Widespread||Introduced||Invasive||Original citation: Seiger, 1997|
|-New South Wales||Present||Introduced||Invasive|
History of Introduction and SpreadTop of page
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.
IntroductionsTop of page
|Introduced to||Introduced from||Year||Reason||Introduced by||Established in wild through||References||Notes|
|Natural reproduction||Continuous restocking|
|Canada||Japan||1901||Ornamental purposes (pathway cause)||Yes||No||Barney (2006)||Herbarium sheets ex Chilliwack (Fraser Valley Regional District), British Columbia; Longueuil (Champlain County), Quebec; and Niagara Falls, Ontario|
|Czech Republic||Japan||1892||Ornamental purposes (pathway cause)||Yes||No||Pysek and Prach (1993)|
|UK||Japan||1825||Ornamental purposes (pathway cause)||Yes||No||Synge (1956)||Naturalized by 1880s|
|USA||Japan||1873||Ornamental purposes (pathway cause)||Yes||No||Barney (2006)||Herbarium sheet, New York|
Risk of IntroductionTop of page
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.
HabitatTop of page
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.
Habitat ListTop of page
|Terrestrial||Managed||Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Urban / peri-urban areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Buildings||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Riverbanks||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Rocky areas / lava flows||Present, no further details||Harmful (pest or invasive)|
|Littoral||Coastal areas||Present, no further details||Harmful (pest or invasive)|
|Littoral||Coastal dunes||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page
Biology and EcologyTop of page
The most important aspect of F. japonica in its introduced range is that it has spread, historically, solely by vegetative means and from a very small number of initial introductions. Thus much of the invasive F. japonica in the world may be clonal, as is the case in the UK (Hollingsworth and Bailey, 2000). However, recent research in the USA has shown that wild F. japonica can produce large quantities of viable seeds, and seedlings have been found in the field (Forman and Kesseli, 2003). Furthermore, the increased use of more powerful molecular tools is revealing a wider genetic diversity of knotweed populations (Bzdega, 2012).
As introduced plants there is inevitably less genetic diversity in F. japonica abroad, at least in terms of the parental species. This is balanced by an extraordinary burst of hybridization involving species and cytotypes not normally sympatric in their indigenous regions (Bailey, 2003). Hybridization and relative chromosome numbers are important in differentiating F. japonica varieties and related species. The chromosome number of F. japonica var. japonica is 2n=88; F. japonica var. compacta is 2n=44; F. sachalinensis is 2n=44; and F. baldshuanica is 2n=20. The hybrid between F. japonica var. compacta and F. japonica var. japonica can produce plants with 2n=44 chromosomes. These tetraploid plants are very rare, although they are able to interbreed with either of their parents. The most commonly observed hybrid is between F. japonica var. japonica and F. baldshuanica, a commonly planted and invasive climber called Russian vine. Fortunately the seed from this hybrid very rarely survives in the wild and possesses none of the aggressive attributes of either of its parents (Bailey, 1988). The cross between F. japonica var. japonica and F. sachalinensis is known as F. x bohemica and has 2n=66. These hexaploid plants are reasonably common but only partly fertile, and any pollen produced usually contains between 30 and 66 chromosomes. If a pollen grain with 66 chromosomes were to pollinate a F. sachalinensis flower in Europe, a fertile octoploid F. x bohemica would be produced. Such plants would be able to cross-pollinate the all-female F. japonica and potentially be a replacement for the absent male F. japonica, allowing F. japonica to reproduce by seed again.
In its native range F. japonica spreads both by seed and vegetatively. The small winged seeds enable the plant to colonize recently exposed land, such as that resulting from recent volcanic activity. F. japonica is functionally dioecious, but in the UK and the USA the plants are female with male sterile flowers. Therefore the primary regeneration strategy is asexual and spread in the introduced range is solely by root and stem fragments, often along waterways and by humans. However, recent studies in Belgium revealed extensive sexual reproduction by hybridization, and that a small percentage of seeds may be dispersed outside the maternal clone (>16 m), allowing the formation of genetically differentiated individuals (Tiébré et al., 2007). This is also supported by observations in North America (Forman and Kesseli, 2003) where it is now thought that hybridization and seed germination is becoming increasingly common (J Bailey, Dept. of Botany, Leicester University, UK, personal communication, 2008).
Physiology and Phenology
Vegetative spread is normally through tiny pieces of rhizome, stems and even internodal sections of stem capable of establishing roots (Locandro, 1978; Palmer, 1990), even in water (Figueroa, 1989). Rhizome fragments weighing as little as 0.7 g are capable of regenerating into a new plant (Brock and Wade, 1992), whilst rhizome pieces with a mean weight of 4.39 g generated shoots 70% of the time in controlled greenhouse experiments, giving a conservative estimate that a 1 m² stand could produce 238 new shoots (Brock and Wade, 1992).
F. japonica is capable of colonizing land within 20 years of volcanic activity, where it is often the sole pioneer species and is reported to be replaced by other herbaceous species after 50 years or so (Yoshioka, 1974). It is often found in association with Miscanthus sinensis grassland on active volcanic fumaroles, and stands often give way to grass species from the centre after die-back (Adachi et al., 1996).
F. japonica requires high light environments and competes effectively for light in such situations. F. japonica can survive very harsh conditions with a pH range of 3.0-8.5 (Beerling et al., 1994), and an ability to survive extreme heavy metal and salt pollution and areas with low available nitrogen. It is generally associated with regions of higher precipitation in the UK (Conolly, 1977). However, Locandro (1973) reported it growing on xeric as well as hydric sites in the USA. It grows from sea level in its native and introduced ranges up to altitudes of 2400 m in Japan (Maruta, 1983), and to 2400-3800 m in Taiwan.
ClimateTop of page
|C - Temperate/Mesothermal climate||Preferred||Average temp. of coldest month > 0°C and < 18°C, mean warmest month > 10°C|
|Cf - Warm temperate climate, wet all year||Preferred||Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year|
|Cs - Warm temperate climate with dry summer||Tolerated||Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers|
|Cw - Warm temperate climate with dry winter||Tolerated||Warm temperate climate with dry winter (Warm average temp. > 10°C, Cold average temp. > 0°C, dry winters)|
|D - Continental/Microthermal climate||Preferred||Continental/Microthermal climate (Average temp. of coldest month < 0°C, mean warmest month > 10°C)|
|Df - Continental climate, wet all year||Preferred||Continental climate, wet all year (Warm average temp. > 10°C, coldest month < 0°C, wet all year)|
|Ds - Continental climate with dry summer||Tolerated||Continental climate with dry summer (Warm average temp. > 10°C, coldest month < 0°C, dry summers)|
|Dw - Continental climate with dry winter||Tolerated||Continental climate with dry winter (Warm average temp. > 10°C, coldest month < 0°C, dry winters)|
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-17||0|
|Mean annual temperature (ºC)||5||17|
|Mean maximum temperature of hottest month (ºC)||14||32|
|Mean minimum temperature of coldest month (ºC)||-7||4|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Mean annual rainfall||580||2200||mm; lower/upper limits|
Soil TolerancesTop of page
- seasonally waterlogged
- very acid
Special soil tolerances
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Aecidium polygoni cuspidati||Pathogen||Plants|Leaves||to genus|
|Aphalara itadori||Herbivore||Plants|Leaves; Plants|Stems||to species||Grevstad et al. (2013); Shaw et al. (2009)||UK|
|Gallerucida nigromaculata||Herbivore||Plants|Leaves||not specific|
|Lixus impressiventris||Herbivore||Plants|Stems||to genus|
|Machiatella itadori||Herbivore||Plants|Leaves||not specific|
|Mycosphaerella polygoni-cuspidati||Pathogen||Plants|Leaves||to species|
|Puccinia polygoni-amphibii var. torariae||Pathogen||Plants|Leaves||not specific|
Notes on Natural EnemiesTop of page
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.
Means of Movement and DispersalTop of page
Natural Dispersal (Non-Biotic)
Flooding events can facilitate the spread of F. japonica, as whole plants and/or stem parts can be dislodged and transported to new areas downstream, where they can establish easily.
Vector Transmission (Biotic)
There are no reports of animals disseminating propagules in the introduced range, though means of seed dispersal in the native range has not been investigated. It is possible that hooved animals could redistribute small pieces of rhizome in much the same way as vehicle tyres can.
Accidental dissemination is probably the most common pathway for the establishment of populations, often as a result of inappropriate control measures such as flail-mowing on a riverbank. Contamination of imported growing medium, and failure to kill rhizomes by adequate heat treatment or composting is another common means of accidental introduction by gardeners. Contaminated soil imported to development sites or for use in trench filling causes new introductions, as well as allowing the spread of previously contained infestations.
Some gardeners still consider F. japonica to be an attractive ornamental plant and may therefore plant it in ignorance. It was also promoted in the past for soil stabilization. There are reports of knotweed rhizome being planted as a means of preventing building development and in malicious attacks.
Pathway CausesTop of page
|Botanical gardens and zoos||Original cause||Yes|
|Breeding and propagation||cultivars still sold||Yes|
|Cut flower trade||stems used||Yes|
|Escape from confinement or garden escape||most common||Yes|
|Flooding and other natural disasters||common||Yes|
|Garden waste disposal||common||Yes|
|Habitat restoration and improvement||has been used for this||Yes|
|Horticulture||varieties still for sale||Yes|
|Landscape improvement||as contaminant of topsoil||Yes|
Pathway VectorsTop of page
|Debris and waste associated with human activities||Excavations and topsoil||Yes|
|Floating vegetation and debris||During flood events or after flail mowing||Yes|
|Land vehicles||Small fragments of rhizome||Yes|
|Machinery and equipment||Rhizomes can be moved on tracks of earth movers||Yes|
|Internet plant sales|
|Soil, sand and gravel||Topsoil movement||Yes|
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Growing medium accompanying plants||weeds/roots; weeds/stems|
|True seeds (inc. grain)||weeds/seeds|
|Plant parts not known to carry the pest in trade/transport|
|Fruits (inc. pods)|
|Stems (above ground)/Shoots/Trunks/Branches|
Impact SummaryTop of page
Economic ImpactTop of page
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.
Environmental ImpactTop of page
Impact on Habitats
Apart from the obvious biodiversity impact, F. japonica also damages the environment through an increased risk of flooding and its impact. In times of flood, dense stands can impede water flow and exacerbate flooding. Also, dead stems can be swept away and cause blockages downstream. In addition, rapidly growing F. japonica can actually disrupt the integrity of flood defence structures.
Impact on Biodiversity
As is often the case with invasive species, the impact that F. japonica has on biodiversity is often referred to but little studied. A riverbank that used to support a wide range of native species but now supports a monoclonal stand of F. japonica certainly has less biodiversity. Its early emergence and great height combine to shade out other vegetation and prohibit regeneration of other species (Sukopp and Sukopp, 1988). Thus it reduces species diversity and damages wildlife habitat (Palmer, 1990; Scott and Mars, 1984). Dead F. japonica stems can persist for 2-3 years producing large quantities of debris and slowly decomposing litter which also leads to a reduced floristic diversity (Child and Wade, 2000).
A recent European study by Gerber et al. (2008) showed that habitats invaded by knotweeds support lower numbers of plant species, lower overall abundance and morphospecies richness of invertebrates, compared to native grassland-dominated and bush-dominated habitats. Total invertebrate abundance and morphospecies richness in Fallopia-invaded riparian habitats was correlated with native plant species richness. This suggested a link between the replacement of native plant species by exotic Fallopia species and the reduction in overall invertebrate abundance and morphospecies richness. Moreover, the biomass of invertebrates sampled in the grassland and bush-dominated habitats was almost twice as high as that in Fallopia-invaded habitats. Large-scale invasion by exotic Fallopia species is therefore likely to seriously affect biodiversity and reduce the quality of riparian ecosystems for amphibians, reptiles, birds and mammals, whose diets are largely composed of arthropods.
A knock-on effect can be observed further up the food chain, as knotweed-invaded sites appear to be less suitable habitats for foraging frogs, probably due to reduced invertebrate populations (Maerz et al., 2005). This is supported by Gerber et al. (2008) who demonstrated the negative effects on native plant and invertebrate assemblages in European riparian habitats.
Threatened SpeciesTop of page
|Threatened Species||Conservation Status||Where Threatened||Mechanism||References||Notes|
|Astragalus robbinsii var. jesupii (Jesup's milk-vetch)||USA ESA listing as endangered species||New Hampshire; Vermont||Competition - monopolizing resources||US Fish and Wildlife Service (2008)|
|Astragalus tricarinatus (triple-ribbed milk-vetch)||NatureServe; USA ESA listing as endangered species||California||Competition - monopolizing resources||US Fish and Wildlife Service (2008)|
Social ImpactTop of page
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).
Risk and Impact FactorsTop of page
- Proved invasive outside its native range
- Has a broad native range
- Abundant in its native range
- Highly adaptable to different environments
- Is a habitat generalist
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Pioneering in disturbed areas
- Long lived
- Fast growing
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Reproduces asexually
- Altered trophic level
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Increases vulnerability to invasions
- Infrastructure damage
- Modification of hydrology
- Modification of nutrient regime
- Modification of successional patterns
- Monoculture formation
- Negatively impacts livelihoods
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Threat to/ loss of native species
- Competition - monopolizing resources
- Competition - shading
- Rapid growth
- Difficult to identify/detect as a commodity contaminant
- Difficult/costly to control
UsesTop of page
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.
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).
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).
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).
Uses ListTop of page
Animal feed, fodder, forage
- Fodder/animal feed
- Erosion control or dune stabilization
- Botanical garden/zoo
- Source of medicine/pharmaceutical
- Cut flower
Detection and InspectionTop of page
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.
Similarities to Other Species/ConditionsTop of page
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.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
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.
Vehicles should be inspected when moving from infected sites to new ones.
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.
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.
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.
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.
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.
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.
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.
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.
Rapid eradication of newly-established F. japonica is possible but only if the rhizome has not become too extensive.
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).
Gaps in Knowledge/Research NeedsTop of page
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.
ReferencesTop of page
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OrganizationsTop of page
UK: Environment Agency, National Customer Contact Centre PO Box 544, Rotherham S60 1BY, http://www.environment-agency.gov.uk/
ContributorsTop of page
19/07/13 Updated by:
Dick Shaw, CABI-UK, Egham, UK
08/08/2008 Updated by:
Dick Shaw, CABI-UK, Egham, UK
Distribution MapsTop of page
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