Fallopia japonica
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''Reynoutria japonica'', synonyms ''Fallopia japonica'' and ''Polygonum cuspidatum'', is a species of
herbaceous Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition of t ...
perennial plant in the knotweed and buckwheat family Polygonaceae. Common names include Japanese knotweed and Asian knotweed. It is native to East Asia in
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
and Korea. In
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
and Europe, the species has successfully established itself in numerous habitats, and is classified as a pest and
invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
in several countries. Japanese knotweed has hollow stems with distinct raised nodes that give it the appearance of bamboo, though it is not related. While stems may reach a maximum height of each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down. The
leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
are broad oval with a truncated base, long and broad, with an entire margin. The flowers are small, cream or white, produced in erect
raceme A raceme ( or ) or racemoid is an unbranched, indeterminate type of inflorescence bearing flowers having short floral stalks along the shoots that bear the flowers. The oldest flowers grow close to the base and new flowers are produced as the s ...
s long in late summer and early autumn. Related species include giant knotweed (''
Reynoutria sachalinensis ''Reynoutria sachalinensis'' (giant knotweed or Sakhalin knotweed Japanese オオイタドリ ''ooitadori'', Russian Горец сахалинский, Гречиха сахалинская; syns. ''Polygonum sachalinense'', ''Fallopia sachalinen ...
'', syns. ''Fallopia sachalinensis'', ''Polygonum sachalinense'') and Russian vine (''
Fallopia baldschuanica ''Fallopia baldschuanica'' ( syn. ''Polygonum baldschuanicum'') is an Asian species of flowering plant in the knotweed family known by several common names, including Russian-vine, Bukhara fleeceflower, Chinese fleecevine, mile-a-minute and sil ...
'', syn. ''Polygonum baldschuanicum'').


Names

Common names for Japanese knotweed include fleeceflower, Himalayan fleece vine, billyweed, monkeyweed, monkey fungus, elephant ears, pea shooters, donkey rhubarb, American bamboo, and Mexican bamboo, among many others, depending on country and location. In Japanese, the name is . The kanji expression is from the Chinese meaning 'tiger stick'. One interpretation of the Japanese name is that it comes from 'remove pain' (alluding to its painkilling use), though there are other etymological explanations offered.


Identification

Identification of Japanese knotweed is not always easy. Many other plants are suspected of being knotweed, due often to the similar appearance of leaves and stems. Dogwood,
lilac ''Syringa'' is a genus of 12 currently recognized species of flowering plant, flowering woody plants in the olive family or Oleaceae called lilacs. These lilacs are native to woodland and scrub from southeastern Europe to eastern Asia, and wid ...
,
houttuynia ''Houttuynia'' is a genus of two species in the Saururaceae native to Southeast Asia. One species, '' H. cordata'', is widely cultivated as a culinary herb. The genus was originally described in 1783 by Carl Peter Thunberg when he formally d ...
(''Houttuynia cordata''), ornamental
bistort Bistort is a common name for several plants and may refer to: *''Bistorta'', a genus recognized by some sources including the species: **'' Bistorta bistortoides'', native to North America **''Bistorta officinalis ''Bistorta officinalis'' (Syno ...
s such as red bistort (''
Persicaria amplexicaulis ''Bistorta amplexicaulis'' ( synonym ''Persicaria amplexicaulis''), the red bistort or mountain fleece, is a species of flowering plant in the buckwheat family Polygonaceae, native to China, the Himalayas, and Pakistan. It is a damp-loving herba ...
)'', lesser knotweed (''
Koenigia campanulata ''Koenigia campanulata'' is a species of flowering plant in the family Polygonaceae. It is native to China (Guizhou, west Hubei, Sichuan, Yunnan and Tibet), Bhutan, north Myanmar, Nepal and Sikkim Sikkim (; ) is a state in Northeastern India ...
''), Himalayan balsam ('' Impatiens glandulifera''), broad-leaved dock (''
Rumex obtusifolius ''Rumex obtusifolius'', commonly known as bitter dock, broad-leaved dock, bluntleaf dock, dock leaf, dockens or butter dock, is a perennial plant in the family Polygonaceae. It is native to Europe, but is found on all temperate continents. It i ...
''), bindweed ('' Convolvulus arvensis''), bamboo, Himalayan honeysuckle ('' Leycesteria formosa''), and Russian vine (''
Fallopia baldschuanica ''Fallopia baldschuanica'' ( syn. ''Polygonum baldschuanicum'') is an Asian species of flowering plant in the knotweed family known by several common names, including Russian-vine, Bukhara fleeceflower, Chinese fleecevine, mile-a-minute and sil ...
'') have been suspected of being ''Reynoutria japonica''. New leaves of ''Reynoutria japonica'' are dark red and long; young leaves are green and rolled back with dark red veins; leaves are green and shaped like a heart flattened at the base, or a shield, and are usually around long. Mature ''R.japonica'' forms , dense thickets. The stems look somewhat like bamboo, with rings and purple speckles. Leaves shoot from the stem nodes alternately in a zigzag pattern. Mature, healthy stems are hollow and not woody. Plants that are immature or affected by mowing and other restrictions have much thinner and shorter stems than mature stands, and are not hollow.


Uses

Japanese knotweed flowers are valued by some beekeepers as an important source of nectar for honeybees, at a time of year when little else is flowering. Japanese knotweed yields a monofloral honey, usually called ''bamboo honey'' by northeastern U.S. beekeepers, like a mild-flavoured version of buckwheat honey (a related plant also in the Polygonaceae). The young stems are edible as a spring vegetable, with a flavour similar to
rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of ''Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick rhizo ...
. In some locations, semi-cultivating Japanese knotweed for food has been used as a means of controlling knotweed populations that invade sensitive wetland areas and drive out the native vegetation. It is eaten in Japan as ''
sansai is a Japanese word literally meaning "mountain vegetables", originally referring to vegetables that grew naturally, were foraged in the wild, and not grown and harvested from fields. However, in modern times, the distinction is somewhat blur ...
'' or wild foraged vegetable. It is used in traditional Chinese and
Japanese medicine , often known simply as , is the study of traditional Chinese medicine in Japan following its introduction, beginning in the 7th century. It was adapted and modified to suit Japanese culture and traditions. Traditional Japanese medicine use ...
to treat various disorders through the actions of
resveratrol Resveratrol (3,5,4′-trihydroxy-''trans''-stilbene) is a stilbenoid, a type of natural phenol, and a phytoalexin produced by several plants in response to injury or when the plant is under attack by pathogens, such as bacteria or fungi. Sources ...
, although there is no high-quality evidence from clinical research for any medical efficacy.
Extract An extract is a substance made by extracting a part of a raw material, often by using a solvent such as ethanol, oil or water. Extracts may be sold as tinctures, absolutes or in powder form. The aromatic principles of many spices, nuts, h ...
s of resveratrol from ''R. japonica'' roots are higher in content than those from stems or leaves, and have highest levels at the end of the growing season. Ground-feeding songbirds and gamebirds also eat the seeds.


In Japan

It grows widely throughout Japan and is foraged as a wild edible vegetable (''
sansai is a Japanese word literally meaning "mountain vegetables", originally referring to vegetables that grew naturally, were foraged in the wild, and not grown and harvested from fields. However, in modern times, the distinction is somewhat blur ...
''), though not in sufficient quantities to be included in statistics. They are called by such regional names as ''tonkiba'' ( Yamagata), ''itazuiko'' ( Nagano, Mie), ''itazura'' (
Gifu is a city located in the south-central portion of Gifu Prefecture, Japan, and serves as the prefectural capital. The city has played an important role in Japan's history because of its location in the middle of the country. During the Sengoku ...
,
Toyama Toyama may refer to: Places and organizations * Toyama Prefecture, a prefecture of Japan located in the Hokuriku region on the main Honshu island * Toyama, Toyama, the capital city of Toyama Prefecture * Toyama Station, the main station of Toyama, ...
, Nara, Wakayama,
Kagawa may refer to: * , the smallest prefecture of Japan by area, located on the island of Shikoku * , a district in Kagawa Prefecture * , a town located in Kagawa District * , train station in Chigasaki, Kanagawa Prefecture * Kagawa (surname) Kagawa (w ...
), ''gonpachi'' (
Shizuoka Shizuoka can refer to: * Shizuoka Prefecture, a Japanese prefecture * Shizuoka (city), the capital city of Shizuoka Prefecture * Shizuoka Airport * Shizuoka Domain, the name from 1868 to 1871 for Sunpu Domain, a predecessor of Shizuoka Prefecture ...
, Nara, Mie, Wakayama), ''sashi'' ( Akita, Yamagata), ''jajappo'' (
Shimane is a Prefectures of Japan, prefecture of Japan located in the Chūgoku region of Honshu. Shimane Prefecture is the List of Japanese prefectures by population, second-least populous prefecture of Japan at 665,205 (February 1, 2021) and has a geo ...
, Tottori, Okayama), ''sukanpo'' (many areas). Young leaves and shoots, which look like asparagus, are used. They are extremely sour; the fibrous outer skin must be peeled, soaked in water for half a day raw or after parboiling, before being cooked. Places in Shikoku such as central parts of
Kagawa Prefecture is a prefecture of Japan located on the island of Shikoku. Kagawa Prefecture has a population of 949,358 (as of 2020) and is the smallest prefecture by geographic area at . Kagawa Prefecture borders Ehime Prefecture to the southwest and Tok ...
pickle the peeled young shoots by weighting them down in salt mixed with 10%
nigari Bittern (pl. bitterns), or nigari, is the salt solution formed when halite (table salt) precipitates from seawater or brines. Bitterns contain magnesium, calcium, and potassium ions as well as chloride, sulfate, iodide, and other ions. Bittern ...
(magnesium chloride). People in Kochi also rub these cleaned shoots with coarse salt-nigari blend. It is said (though no authority is cited) that the magnesium of the ''nigari'' binds with the oxalic acid thus mitigating its hazard.


Invasive species

This species is listed by the World Conservation Union as one of the world's worst invasive species. It is a frequent colonizer of temperate riparian ecosystems, roadsides, and waste places. It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States. The success of the species has been partially attributed to its tolerance of a very wide range of conditions; including drought, different soil types, variable soil pH, and high salinity. Its
rhizomes In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...
can survive temperatures of and can extend horizontally and deep, making removal by excavation extremely difficult. The plant is also resilient to cutting, vigorously resprouting from the roots.


Impact

The invasive root system and strong growth can damage concrete foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites. It can also reduce the capacity of channels in flood defences to carry water. Japanese knotweed shades out other vegetation, overgrows buildings and other structures, encourages fire, and damages paved surfaces.


Control

Japanese knotweed has a large underground network of roots (rhizomes). To eradicate the plant the roots need to be killed. All above-ground portions of the plant need to be controlled repeatedly for several years in order to weaken and kill the entire patch. Picking the right herbicide is essential, as it must travel through the plant and into the root system below. But also more ecologically-friendly means are being tested as an alternative to chemical treatments.


Chemical

The abundance of the plant can be significantly reduced by applying glyphosate, imazapyr, a combination of both, or by cutting all visible stalks and filling the stems with glyphosate. However, these methods have not been proven to provide reliable long-term results in completely eliminating the treated population.


Mechanical

Digging up the rhizomes is a common solution where the land is to be developed, as this is quicker than the use of herbicides, but safe disposal of the plant material without spreading it is difficult; knotweed is classed as controlled waste in the UK, and disposal is regulated by law. Digging up the roots is also very labour-intensive and not always effective. The roots can extend up to deep, and leaving only a few centimetres (inches) of root behind will result in the plant quickly growing back. Covering the affected patch of ground with a non-translucent material can be an effective follow-up strategy. However, the trimmed stems of the plant can be razor sharp and are able to pierce through most materials. Covering with non-flexible materials such as concrete slabs has to be done meticulously and without leaving even the smallest splits. The slightest opening can be enough for the plant to grow back. Soil steam sterilization involves injecting steam into contaminated soil in order to kill subterranean plant parts. Trials in Haida Gwaii, British Columbia, using sea water sprayed on the foliage, have not demonstrated promising results.


Biological

Research has been carried out on a '' Mycosphaerella'' leaf spot fungus, which devastates knotweed in its native Japan. This research has been relatively slow due to the complex life cycle of the fungus. Following earlier studies, imported Japanese knotweed psyllid insects ''
Aphalara itadori ''Aphalara itadori'', the Japanese knotweed psyllid, is a species of psyllid from Japan which feeds on Japanese knotweed (''Reynoutria japonica''). It has been licensed by the UK Government for the biological control of Japanese knotweed in Engl ...
'', whose only food source is Japanese knotweed, were released at a number of sites in Britain in a study running from 1 April 2010 to 31 March 2014. In 2012, results suggested that establishment and population growth were likely, after the insects overwintered successfully. In 2020 Amsterdam imported and released 5,000 Japanese ''Aphalara itadori'' leaf fleas, exempting them from a strict ban on the introduction of alien species, as one of the measures to contain the knotweed. The psyllids suck up sap from the plant, potentially killing young shoots and slowing or even stopping growth. It was hoped that the psyllid would hibernate over winter and establish themselves in 2021. Anecdotal reports of effective control describe the use of goats to eat the plant parts above ground followed by the use of pigs to root out and eat the underground parts of the plant.


Impact of the plant outside its area of origin


New Zealand

Japanese knotweed is classed as an unwanted organism in New Zealand and is established in some parts of the country.


Europe

European adventurer Philipp Franz von Siebold transported Japanese knotweed from a Japanese volcano to Leiden in the Netherlands. By 1850, a specimen from this plant was donated by Von Siebold to the Royal Botanic Gardens, Kew. It was favoured by gardeners because it looked like bamboo and grew everywhere.
Ann Conolly Ann P. Conolly (1917–2010) was a British botanist and teacher who contributed to quaternary botany and conducted important early work on the history and spread of Japanese Knotweed in the UK . Academic career She attended the University of Ca ...
provided the first authoritative work on the history and distribution of the plant in the UK and Europe in the 1970s. According to ''The Daily Telegraph'', the weed has travelled rapidly, aided by rail and water networks. In the UK, Japanese knotweed is a single female clone. However, it is able to readily hybridise with related species. In the UK, Japanese knotweed is established in the wild in many parts of the country and creates problems due to the impact on biodiversity, flooding management and damage to property. It is an offence under section 14(2) of the Wildlife and Countryside Act 1981 to "plant or otherwise cause to grow in the wild" any plant listed in Schedule nine, Part II to the Act, which includes Japanese knotweed. As of 2014, householders and landlords within towns, who do not control the plant in their gardens, can receive an on-the-spot fine or be prosecuted. It is also classed as "controlled waste" in Britain under part 2 of the Environmental Protection Act 1990. This requires disposal at licensed landfill sites. Some home owners in the United Kingdom are unable to sell their homes if there is any evidence of knotweed on the property. The species is expensive to remove. According to the UK government, the cost of controlling knotweed had hit £1.25 billion in 2014. It cost £70 million to eradicate knotweed from 10 acres of the London 2012 Olympic Games velodrome and aquatic centre.
Defra DEFRA may refer to: * Deficit Reduction Act of 1984, United States law * Department for Environment, Food and Rural Affairs, United Kingdom government department {{Disambiguation ...
's Review of Non-native Species Policy states that a national eradication programme would be prohibitively expensive at £1.56 billion. The Centre for Ecology and Hydrology has been using
citizen science Citizen science (CS) (similar to community science, crowd science, crowd-sourced science, civic science, participatory monitoring, or volunteer monitoring) is scientific research conducted with participation from the public (who are sometimes re ...
to develop a system that gives a knotweed risk rating throughout Britain. The decision was taken on 9 March 2010 in the UK to release into the wild a Japanese psyllid insect, ''
Aphalara itadori ''Aphalara itadori'', the Japanese knotweed psyllid, is a species of psyllid from Japan which feeds on Japanese knotweed (''Reynoutria japonica''). It has been licensed by the UK Government for the biological control of Japanese knotweed in Engl ...
''. Its diet is highly specific to Japanese knotweed and shows good potential for its control. Controlled release trials began in South Wales in 2016. In Scotland, the Wildlife and Natural Environment (Scotland) Act 2011 came into force in July 2012 that superseded the Wildlife and Countryside Act 1981. This act states that is an offence to spread intentionally or unintentionally Japanese knotweed (or other non-native invasive species). In Northern Ireland it has been recorded from Counties
Down Down most often refers to: * Down, the relative direction opposed to up * Down (gridiron football), in American/Canadian football, a period when one play takes place * Down feather, a soft bird feather used in bedding and clothing * Downland, a ty ...
, Antrim and Londonderry. The earliest record is in 1872. In the Republic of Ireland, it has been recorded from Howth Head, Co. Dublin at Doldrum Bay.


Lending controversy

In the United Kingdom, Japanese knotweed has received a lot of attention in the press as a result of very restrictive lending policies by banks and other mortgage companies. Several lenders have refused mortgage applications on the basis of the plant being discovered in the garden or neighbouring gardens. The Royal Institution of Chartered Surveyors published a report in 2012 in response to lenders refusing to lend "despite notweedbeing treatable and rarely causing severe damage to the property". In response to this guidance, several lenders have relaxed their criteria in relation to discovery of the plant. As recently as 2012, the policy at the Woolwich (part of Barclays plc) was "if Japanese knotweed is found on or near the property then a case will be declined due to the invasive nature of the plant." Their criteria have since been relaxed to a category-based system depending on whether the plant is discovered on a neighbouring property (categories 1 and 2) or the property itself (categories 3 and 4) incorporating proximity to the property curtilage and the main buildings. Even in a worst-case scenario (category 4), where the plant is "within 7 metres (23 feet) of the main building, habitable spaces, conservatory and/or garage and any permanent outbuilding, either within the curtilage of the property or on neighbouring land; and/or is causing serious damage to permanent outbuildings, associated structures, drains, paths, boundary walls and fences" Woolwich lending criteria now specify that this property may be acceptable if "remedial treatment by a Property Care Association (PCA) registered firm has been satisfactorily completed. Treatment must be covered by a minimum 10-year insurance-backed guarantee, which is property specific and transferable to subsequent owners and any mortgagee in possession." Santander have relaxed their attitude in a similar fashion. In 2022 the Royal Institution of Chartered Surveyors produced updated guidance for assessing knotweed. Since the 2012 RICS report much more information on the risks from Japanese knotweed have been gained. One important change is that the distance of a plant from a property to be considered a problem has been reduced to 3 metres (10 feet). Property Care Association chief executive Steve Hodgson, whose trade body has set up a task force to deal with the issue, said: "Japanese knotweed is not 'house cancer' and could be dealt with in the same way qualified contractors dealt with faulty wiring or damp."


United States

The weed can be found in 42 of the 50 United States.


Canada

According to Gail Wallin, executive director of the Invasive Species Council of B.C., and co-chair of the Canadian Council on Invasive Species, by 2015 it was found in all provinces in Canada except Manitoba and Saskatchewan. In Vancouver the aggressive plant went under "four lanes of highway and have popped up on the other side." At Mission Point Park in Davis Bay, British Columbia municipal crews attempted to eradicate it by digging out the plant to a depth of about three metres (10 feet) with an excavator. It grew back twice as large the next year. To avoid an epidemic as in the United Kingdom, some provinces in Canada are pushing for relaxation of provincial limits on the use of herbicides close to waterways so knotweed can be aggressively managed with strong chemicals. In spite of its status as an invasive species it is still sometimes sold or swapped in Canada as an edible "false bamboo." Bohemian knotweed, a hybrid between Japanese and giant knotweed that produces huge quantities of viable seeds, now accounts for about 80 per cent of knotweed infestations in British Columbia.


See also

* ''
Persicaria capitata ''Persicaria capitata'', the pink-headed persicaria, pinkhead smartweed, pink knotweed, Japanese knotweed, or pink bubble persicaria, is an Asian species of plants in the genus ''Persicaria'' within the buckwheat family. It is native to Asia (Ch ...
'' for another plant species called Japanese knotweed. *
Chinese knotweed Chinese knotweed is a common name for several plants and may refer to: *''Koenigia weyrichii'', native to east Asia *'' Persicaria chinensis'', widespread across China, Japan, India, Indonesia, Malaysia, and Vietnam *''Reynoutria multiflora ''R ...


References


External links


Photo of herbarium specimen at Missouri Botanical Garden, collected in Missouri in 1994

Species Profile - Japanese knotweed (''Fallopia japonica'')
National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Japanese knotweed.
Best management practice
A variety of ways to control knotweed (under "knotweed")(USA)


American Journal of Botany - Sexual Reproduction in the Invasive Species ''Fallopia japonica''

Japanese Knotweed Alliance
(UK)
Recipes
from
"Wildman" Steve Brill Steve "Wildman" Brill (born March 10, 1949) is an American forager, naturalist, environmental educator and author. He gained notoriety in 1986, when he was arrested in New York City's Central Park for eating a dandelion. Education Brill was ...

Time lapse video of knotweed growth
BBC
Insect that fights Japanese knotweed to be released
BBC News 2010-03-09
How close is Japanese Knotweed to my home?
BBC News 11 August 2017
UK location map
at planttracker.org.uk *
The alien shrub that can't be stopped
– BBC Future {{Authority control Flora of Eastern Asia Garden plants of Asia Medicinal plants Polygonoideae Stem vegetables Plants described in 1777 Flora of China