Fallopia japonica; of which the distinct British clone is best known as Fallopia japonica var. Japonica. It is a member of the Polygonaceae family. Earlier names for Fallopia japonica were Reynoutria japonica, Polygonum multiflorum and Polygonum reynoutria. Earlier Names for Fallopia japonica var. Japonica were Polygonum cuspidatum and Polygonum sieboldii.
Japanese knotweed, Mexican bamboo, [Japanese] fleece-flower, Japanese bamboo, wild rhubarb, huzhang (Chinese), Sally rhubarb, donkey rhubarb, gypsy rhubarb, Hancock's curse, crimson beauty and itadori (in Japanese = "strong plant").
Japanese Knotweed is native to Eastern Asia. In Japan, it is found on the islands of Hokkaido, Honshu, Kyushu and Shikoku. Its range also extends to Northern China, the island of Taiwan (formerly Formosa) and also on the mainland of Korea.
It is disputed as to when the plant was first brought to the West and to the United Kingdom. The plant was first registered as Reynoutria japonica in 1777 by a Dutchman named Houttuyn. Its introduction to the West is likely to have begun circa 1823 when Philippe von Siebold is said to have brought the plant to his nursery in Leiden, Holland. Von Siebold sent the flower to the Royal Botanical Gardens Kew in August 1850 and was introduced to the gardens. It was planted at the Royal Botanical Gardens Edinburgh in 1854. The spread of the plant and its rhizome across the country can be attributed to a number of factors:
• The commercial sale of the plant, beginning in the 19th century, as at the time it was regarded as a fashionable ornamental plant.
• Traveling along watercourses
• Transportation by and on vehicles such as cars, trains and lorries. Some areas of vehicles, such as tires, are susceptible to picking up pieces of the plant and rhizome
• Urban development: the transportation of soil and materials for the construction of roads and buildings distributed the rhizome around many modern urban areas.
Japanese Knotweed is a hardy, herbaceous, rhizomatous perennial. Plants are fully dioecious, with all individuals of the UK clone being functionally female. Rhizomes measure up to 10cm in diameter, bearing nodes at 1-2cm spacings, and extend generally up to 7 metres from the parent plant (though distances of 20 metres have been recorded). The rhizome penetrates downward to a depth of 2 metres or more. Stems can reach 3 metres high and are stout, hollow and bamboo-like with erect bases that eventually branch. Stems are sometimes red-brown, but often green and are glabrous to adpressed-pilose, with thinly membranous sheath. Leaves are broadly ovate with the bases truncated and the tips abruptly cuspidate or acuminate. The leaves measure 5-12cm wide and 5-15cm long, petioles 1-3cm long and stipules 8-15mm long. The female flower is greenish-white and drooping. Flowers are 2.5-3mm across in dense, branched, axillary panicles, which are 5-9cm long. Fruits are 3-4mm long; achanes acutely trigonous and are shiny black/brown.
The following describes the growth process of Japanese knotweed:
The buds around the crown and the rhizome's many nodes thrust shoots at such speed so as to smother other species present, using energy from a mature rhizome's huge reserve of stored food. Shoot extension can exceed 8cm daily. As the stems develop, broad leaves are produced in an alternating pattern on the stems. This arrangement allows the leaves to be placed where they can receive the maximum sunlight to fuel the photosynthetic factory. Biochemical switches do not allow all the nodes to produce shoots, so that maximum profit of energy receipts over expenditure are achieved by not wasting it on shoots which will be shaded by neighbours.
With all leaves fully expanded, the phloem system increasingly carries the starches, made in the leaves, back to the rhizomes, both to fuel current extension, and to provide the stored energy for explosive growth the following spring.
Plant rhizome systems may extend at several metres per year. Shoots develop from the tips and nodes, capable of breaking the surface as new stems, from depths of greater than one metre.
In the late summer, fully-grown stems produce sprays of flowers, which, in the case of the UK clone, are all female from female plants. Where plants are pollinated, which is by bees and other insects, the pollen-parent will usually be the related Giant Knotweed, or male-fertile specimens of hybrid origin. Whether or nor pollination is achieved, the Knotweed flowers are a good source of late nectar for many flying insects. Few seeds are produced from these hybrid unions, and seedling survival is uncommon.
In Japan, and other countries where pure-strain seed does develop, high volumes of seed are produced about two weeks after flowering. Germination and seedling survival rates are low, and seeds are dormant when shed. This dormancy is broken by a combination of environmental factors, which, when achieved, give the best chance of establishment for the fragile and frost-tender seedlings. Seed dormancy can be prolonged, with germination not occurring for more than one year.
All remaining moisture is withdrawn from the stems as autumn progresses, and all above ground parts die back to a forest of brown, persistent stems.
In preparation for the winter dormancy stage, the rhizomes and the clusters of dormant buds and rhizomatous material reside at the base of each dead stem. These structures, from which the rhizomes grow, are known as crowns.
In the absence of seeds, dispersal is achieved by the transport of portions of rhizome to fresh sites. Water flow and human activity are the main vectors. So tenacious is the rhizome that portions weighing no more than 0.7g residing 300mm below the surface can give rise to new plants. Even internodal tissue, not furnished with growth-bud primordia, can develop and initiate shoots. Stem material is also very able to make roots and shoots if detached from the parent plant.
Japanese Knotweed has spread with vigour throughout the United Kingdom in a relatively short amount of time. This is somewhat due to the wide variety of conditions in which the plant can survive. Some of its native habitats in Japan include riverine gravels, highway margins and well-fertilised pastureland. However, in areas of volcanic activity, the plant can thrive in soil with pH levels of less than 4 and is often the first plant life to grow after an eruption. In mountainous areas, its hardiness allows it to withstand months of frost, surviving temperatures of -35°C.
The UK strand of the plant has developed to exaggerate its normal adaptability. It now grows at a similar pace in areas with varying pH levels, nutrients, and organic matter. The ready availability of water and light are probably the key environmental factors governing its speed and extent of growth. Its pH tolerance lies between 3 and 8.5, although these extremes are seen only as rare examples. Further competitive advantage is present in its ability to grow on sites with little available nitrogen and high levels of metallic and other contaminants. Saline habitats are likewise tolerated. The ideal growing conditions would involve all of the following: sunshine, fertile soil, moisture, a sheltered position and no competition.
The spread of Japanese Knotweed throughout the United Kingdom has not been restricted by geographical area; it has been sighted from Cornwall to the Scottish Highlands, as well as Wales and Northern Ireland. Both rural and urban areas are affected. Watercourses are frequently populated, whether rural or urban, in part due to the presence of water for growth as well as rhizome distribution. Recent research suggests there being a negative correlation between the economic wealth of an area and the presence of Japanese Knotweed i.e. poorer areas have proportionately more Knotweed than wealthier areas. The weed can inhabit contaminated land, development sites, and landfills sites. There are now strict regulations for its introduction to the landfill sites as it is classed as controlled waste.
Keeping Japanese knotweed in control on a site can usually be achieved given adequate labour, persistence, and a good understanding of techniques.
Eradicating knotweed is a different matter. The word implies that every last 0.1 grams of every stem or rhizome has been killed by herbicides, or without question found, removed to landfill and buried according to the requirements of law. If this last, minimum sized propagule, has not been removed or killed, the infestation has the potential to start all over again.
If these problems were not sufficient, the probability that some parts of the rhizome system can, following an assault by herbicides, play dead for twenty years or more, only to wake like a sleeping dragon, further emphasizes the difficulty of the weed manager's task.
Methods of containment or management require much repetition of labour. Small patches may be controlled by pulling or mowing. Mowing conducted every four weeks during the growing season will serve to slow or halt the spread of the rhizomes. Increasing the mowing frequency to every two weeks has been found to eliminate small infestations if continued for three growing seasons. Similarly, pulling up shoots in July or August, continued for three years, has likewise proved effective in some instances. Pulled material should be spread to dry and burned at the earliest opportunity. As with all techniques which interfere with Knotweed, care must be studiously exercised to ensure that no material is carried to fresh sites on equipment etc., is blown by the wind or enters a watercourse.
Formerly used as a stock feed, knotweed is palatable to grazing animals, including cattle, sheep, goats, horses and donkeys. It is possible to achieve some measure of containment by introducing grazing animals early in the growing season whilst the shoots are palatable. Later introductions will require pre-cutting of the knotweed to encourage tender shoots to grow.
Biological techniques have not yet been introduced for knotweed control in any part of the world. It is, however, well known that the multiple effects of pest and disease organisms (in the midst of many other factors) help to keep native-range knotweed in reasonable check. It would certainly seem reasonable to hope that our mono-clonal Japanese Knotweed might be strongly susceptible to attack, in the same way as the mono-clonal English Elm. The same might not however apply as readily to the increasingly frequent "Bohemica" hybrid, which has several genotypes. An initial project phase, led by CABI Bioscience and Leicester University, ended in 2000. This project, which included cooperation from scientists from Kyushu University, identified various pathogens and pests with a view to testing efficacy and the all important host specificity of each promising species. 2003 saw the start of a four year programme, which involves CABI and the group of organisations known as the "Knotweed Alliance". This project has resulted three survey visits to Japan covering the whole Knotweed growing season. Amongst the species of promising herbivorous insects found (which will receive thorough testing in secure quarantine) are a stem-boring weevil of the genus Lixus, and a sap-sucking plant louse or Psyllid, of the genus Aphalara. Also showing promise is a leaf-spot fungus of the genus Microsphaerella. All three species are knotweed-specific in Japan, though this will be subjected to the most rigorous scientific scrutiny under extended quarantine conditions. The Alliance is in close contact with Japanese scientists in the hope of finding further potential biological control agents.
The current realistic pathways to eradication are by digging out and dumping the Knotweed and by chemical control methods.
In brief, digging must be carried out to a depth of 3 metres throughout the infested area, and to the same depth to an extent of 7 metres beyond the emergence of the last shoots peripheral to the colony. All the knotweed material may be burnt on site to reduce viability and burial on site is allowable if within the aims of the management programme. On-site burial should be to a depth of 10 metres.
Burning in the open, prior to burial, can be permitted under an exemption from Schedule 30 of the Waste Management Regulations, 1994. All knotweed and Knotweed contaminated soil is classed as "controlled waste" under the Environmental Protection Act (1990) Duty of Care Regulations, 1991. Due regard must also be given to the requirements of the Wildlife and Countryside Act, 1981. What this legislation amounts to is making it illegal to allow knotweed, in the process of being controlled, from spreading either from where buried or stored, or during transportation.
Where transported to landfill, the required burial depth is a minimum of 5 metres, but at least 5 metres above the base of the landfill. No burial must take place within less than 7 metres of the periphery of the site, or within the same distance of drains or bunds.
If herbicides have been used as part of the control programme, their nature and soil persistence must be notified to the manager of the disposal facility.
The main method of effective control is by chemical herbicides. The aim of the weed manager is to apply these in such a manner that the plants absorb a sufficient quantity to be translocated throughout the rhizome mass and ensure plant death.
The main herbicides in the UK, for use against Japanese knotweed, are Picloram, 2,4-D, Glyphosate and Triclopyr. Selection is based on location and future requirements as to land use.
Is persistent in the soil and damages all broad leaved plants for a substantial period, It has little effect on grasses. It may not be used in or near water. It is the herbicide of choice where replanting is not envisaged, or where a grass sward only is to be maintained.
Also spares grasses, and may be used on or near water. Its soil persistence varies, though replanting with broad-leaved subjects is usually possible in twelve months from the last application.
Is a total herbicide, which harms all kinds of green plant tissue. It has no soil persistence in practical terms, so may be used where site replanting is envisaged, irrespective of type.
Is another selective herbicide, affecting broad leaved species, but not grasses. It has some residual action in the soil, and is of variable persistence. It is not recommended for use in areas where replanting, other than with grasses, is a requirement. It may not be used on or near water.
A now little used herbicide, Ammonium Sulphamate was tested by researchers R. Scott and R.H. Marrs during a two year trial from 1982. It has the disadvantage of being expensive, because of high application rates, which must be balanced against its low toxicity and rapid degradation in the soil to Ammonium Sulphate. This common fertilizer is rapidly broken down to plant available nitrates, which could help in swift re-vegetation of effectively treated sites.
Translocated herbicides are found, in general, to be absorbed most efficiently when plants are near the flowering stage, but not entering senescence. It will be understood that this is difficult to achieve with this weed because of over 2 metre growth and greatly disrupted access found at flowering time. The accepted compromise seems to be to spray when plants are in full growth and about 1 metre tall.
A common and useful pre-treatment of infested sites is to winter dig with excavators to a depth of 50cm, then to evenly re-spread the crowns, soil and rhizomes back over the site, achieving a level surface. This results in stimulation of a large number of growth buds, which would normally have remained dormant. The increased shoot density and leaf canopy area produced by this method renders the plants more susceptible to herbicide, by increasing the amount which can be absorbed and translocated. back to top