Reynoutria japonica, synonyms Fallopia japonica and Polygonum cuspidatum, is a species of herbaceous 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, China and Korea. In North America and Europe, the species has successfully established itself in numerous habitats; it is classified as a pest and invasive species in several countries. The plant is popular with beekeepers and its young stems are edible, making it an increasingly popular foraged vegetable with a flavour described as lemony rhubarb.
Description The flowers are small, cream or white, produced in erect racemes long in late summer and early autumn. 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 are broad oval with a truncated base, long and broad, with an entire margin. 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. Also they may posses two symmetrical nonparallel yellowish stripes around 1-2 cm in diameter.
Mature R.japonica forms , dense thickets. Leaves shoot from the stem nodes alternately in a zigzag pattern. Plants that are immature or affected by mowing and other restrictions have much thinner and shorter stems than mature stands, and are not hollow. The below-ground rhizome network acts as a long-lived storage system; it is often concentrated in the upper metre of the soil profile, but may extend well beyond the visible stand.
Similar species Japanese knotweed may be confused with other plants suspected of being knotweed owing to the similar appearance of leaves and stems. Dogwood, lilac, houttuynia (Houttuynia cordata), ornamental bistorts such as red bistort (Persicaria amplexicaulis), lesser knotweed (Koenigia campanulata), Himalayan balsam (Impatiens glandulifera), broad-leaved dock (Rumex obtusifolius), bindweed (Convolvulus arvensis), bamboo, Himalayan honeysuckle (Leycesteria formosa), and Russian vine (Fallopia baldschuanica) have been suspected of being Reynoutria japonica.
Distribution and habitat It is native to East Asia in Japan, China and Korea. In North America and Europe, the species has successfully established itself in numerous habitats, and is classified as a pest and invasive species in several countries. Spread in invaded ranges is mainly vegetative, through rhizome pieces, stem fragments and soil contaminated with plant material; waterways, construction activity and garden waste can move these propagules over longer distances. Many introduced populations are dominated by male-sterile clonal plants, but seed set, hybridisation with giant knotweed and the production of viable seed by Bohemian knotweed have also been reported.
As an 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 can crowd out other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States. The species' competitive effect is produced by rapid shoot growth, dense canopy cover, and persistent leaf and stem litter that can suppress the germination and growth of other plants beneath stands. Dense monospecific stands can reduce native plant assemblages and modify ecosystem functioning, including nutrient cycling. 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 can survive temperatures of and can extend horizontally and deep, making removal by excavation extremely difficult. The plant is also resilient in response to cutting, vigorously resprouting from the roots.Gover, Art, Jon Johnson, and Larry Kuhns. "Managing Japanese knotweed and giant knotweed on roadsides." Roadside Vegetation Management Factsheet 5 (2005)
Impact
Japanese knotweed has a strong and extensive root system that can exploit cracks and weaknesses in hard surfaces such as concrete, asphalt, and brick. As the plant grows, it can damage vulnerable paving, walls, drains, retaining structures and architectural sites, and dense growth can impede access and maintenance. Recent scientific literature describes the threat of property damage as partly a perceived risk, while noting that this perception has nevertheless affected mortgage lending and housing valuation.
Japanese knotweed shades out other vegetation, grows over buildings and other structures, encourages fire, and damages paved surfaces.
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, 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 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 plant 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. The hybrid with giant knotweed, Fallopia × bohemica or Reynoutria × bohemica, can produce viable seed and has been considered potentially more invasive and harder to control, although evidence for different responses to control methods remains inconclusive.
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, flood 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 in residential areas 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.
In the United Kingdom, evidence or suspected evidence of knotweed can affect saleability and lending decisions because surveyors, lenders and owners treat infestation as a property-risk and management issue. The species is expensive to remove. It cost £70 million to eradicate knotweed from 10 acres of the London 2012 Olympic Games velodrome and aquatic centre. Defra'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 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. Its diet is highly specific to Japanese knotweed and shows good potential for its control.Richard H. Shaw, Sarah Bryner and Rob Tanner. "The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union". UK Biological Control. Volume 49, Issue 2, May 2009, Pages 105-113. 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, superseding the Wildlife and Countryside Act 1981. This act states that is an offence to spread, intentionally or unintentionally, Japanese knotweed (or other invasive species).
In Northern Ireland, it has been recorded in Counties Down, Antrim and Londonderry. The earliest record is in 1872.Hackney, P. (Ed) 1992. Stewart & Corry's Flora of the North-east of Ireland Third edition. Institute of Irish Studies The Queen's University of Belfast
In the Republic of Ireland, it has been recorded from Howth Head, County Dublin at Doldrum Bay.Dhuill, E.Ni and Smyth, N. 2021. Invasive non-native and alien garden escape plants species on the southern cliffs of Howth Head, County Dublin (H21) Irish Naturalists' Journal, 37: 2 pp 102-108
United Kingdom lending controversy In the United Kingdom, Japanese knotweed has received significant 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 [knotweed] being 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 lender 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 41 of the 50 states in the US, excluding Alabama, Arizona, Florida, Hawaii, Nevada, New Mexico, North Dakota, Texas, and Wyoming,
Canada