JAPANEESE KNOTWEED : Fallopia japonica
NATIVE RANGE: Eastern Asia
DESCRIPTION
Japanese knotweed is an upright, shrublike, herbaceous
perennial that can grow to over 10 feet in height. As with all
members of this family, the base of the stem above each joint
is surrounded by a membranous sheath. Stems of Japanese
knotweed are smooth, stout and swollen at joints where the
leaf meets the stem. Although leaf size may vary, they are
normally about 6 inches long by 3 to 4 inches wide, broadly oval to somewhat triangular and pointed at
the tip. The minute greenish-white flowers occur in attractive, branched sprays in summer and are
followed soon after by small winged fruits. Seeds are triangular, shiny, and very small, about 1/10 inch
long. Japanese knotweed is designated a noxious weed in the State of Washington.
The problem
Japanese knotweed was introduced from Japan in 1825 as an ornamental plant. The plant is not
unattractive but its rapid annual growth and relentless spread, allows it to easily overwhelm other garden
plants. Where established as a wayside weed, native plants are also aggressively over-run.
Although it does not produce seeds, it can sprout from very small sections of rhizomes and, under the
provisions made within the Wildlife and Countryside Act 1981, it is an offence to cause Japanese
knotweed to grow in the wild. Much of its spread is probably via topsoil movement or construction traffic.
New legislation (IN THE UK WHICH IS STRICTER THAT THE US)
An amendment to the Anti-social Behaviour, Crime and Policing Act 2014 includes Japanese knotweed
and other invasive non-native plants. Full details of how this will work for the homeowner are not yet
available, but here are some key points:
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It is not illegal to have Japanese knotweed in your garden
On your property, you should aim to control this plant and other invasive non-native plants such
as Himalayan balsam and giant hogweed, to prevent them becoming a problem in your
neighbourhood. If they have a "detrimental effect of a persistant or continuing nature on the quality
of life of those in the locality", the legislation could be used to enforce its control
Control can be carried out by the homeowner (see the control section below) and doesn't
require a specialist company. However, a specialist company will be skilled at control and can
dispose of the plant waste
Identification is important. Japanese knotweed can be confused with other plants including
Persicaria microcephala (e.g. P. microcephala 'Red Dragon'), Leycesteria formosa and Houttuynia
cordata
Where problems with Japanese knotweed occur in neighbouring gardens, we suggest that you
speak or correspond directly with your neighbours (who may already be taking action to
control this difficult weed). These informal steps should be taken before contacting your council to
talk about control using the legislation
HOW TO ID KNOTWEED ALL YEAR
ECOLOGICAL THREAT
Japanese knotweed spreads quickly to form dense thickets that exclude native vegetation and greatly
alter natural ecosystems. It poses a significant threat to riparian areas, where it can survive severe floods
and is able to rapidly colonize scoured shores and islands. Once established, populations are extremely
persistent. Japanese Knotweed was first introduced into Ireland by the late Victorians as an ornamental
plant for their gardens. However, outside its native habitat of Japan and Northern China the plant has
proven to be an aggressive colonizer without the checks and balances of the predators and pathogens
present in its native habitat. Japanese Knotweed poses a serious threat to:
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Biodiversity through light exclusion
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Erosion to riverbanks after winter dieback
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Access to river banks for anglers, boating, swimming or inspection
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Local tourist economies through habitat destruction.
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Foundations, tarmac, hard standings, roads, railway infrastructure, etc.
Economic Importance
Japanese knotweed is a very aggressive species (Hitchcock and Cronquist 1964) that is capable of
crowding out all other vegetation (Ahrens 1975); Hickman (1993) lists the species as a noxious weed. In
addition, the plant can create a fire hazard in the dormant season (Ahrens 1975). Japanese knotweed is
an escaped ornamental that is becoming increasingly common along stream corridors and rights-of-way in
Washington. The species forms dense stands that crowd out all other vegetation, degrading native plant
and animal habitat. This perennial plant is difficult to control because it has extremely vigorous rhizomes
that form a deep, dense mat. In addition, the plant can resprout from fragments; along streams, plant
parts may fall into the water to create new infestations downstream.
History
A native of Asia, this species was introduced to England in 1825 for use as an ornamental (Patterson
1976). Japanese knotweed was subsequently introduced to the U.S. for use in ornamental hedges and for
erosion control (Pridham and Bing 1975).
DISTRIBUTION IN THE UNITED STATES
Current distribution of Japanese knotweed includes 36 states in the
lower 48 from Maine to Wisconsin south to Louisiana, and scattered
Midwest and western states. It is not currently known to occur in
Hawaii. As its name indicates, Japanese knotweed is a native of
Japan (Hickman 1993). However, it has become naturalized in North
America, where it is found from Newfoundland and many parts of the
northeastern U.S. (Muenscher 1955), west to California (Hickman
1993) and the Pacific Northwest (Hitchcock and Cronquist 1964).
HABITAT IN THE UNITED STATES
Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures,
high salinity, and drought. It is found near water sources, such as along streams and rivers, in low-lying
areas, waste places, utility rights-of-way, and around old home sites. It can quickly become an invasive
pest in natural areas after escaping from cultivated gardens. An escaped ornamental, Japanese knotweed
is often found in waste places, neglected gardens, roadsides, and along streambanks (Muenscher 1955;
Figueroa 1989).
BACKGROUND
Japanese knotweed was probably introduced to the U.S. in the late 1800's. Also known as crimson
beauty, Mexican bamboo, Japanese fleece flower, or Reynoutria, it was first introduced as an ornamental
and has also been used for erosion control and for landscape screening. It is now found throughout the
eastern U.S., in several western states, and Alaska, which has few exotic invasive plants to date.
BIOLOGY & SPREAD
Japanese knotweed spreads primarily by vegetative means with the
help of its long, stout rhizomes. It is often transported to new sites as a
contaminant in fill dirt seeds, sometimes distributed by water, and
carried to a lesser extent by the wind. Escapees from neglected
gardens, and discarded cuttings are common routes of dispersal from
urban areas.
MANAGEMENT OPTIONS
Grubbing is effective for small initial populations or environmentally
sensitive areas where herbicides cannot be used. Using a pulaski or
similar digging tool, remove the entire plant including all roots and runners. Juvenile plants can be hand
pulled depending on soil conditions and root development. Any portions of the root system not removed
will potentially resprout. All plant parts (including mature fruit) should be bagged and disposed of in a trash
dumpster to prevent reestablishment.
Non-chemical controls
When tackling Japanese knotweed, cultural control methods pose some problems;
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Digging out is possible, but due to the depth that the rhizomes can penetrate, regrowth usually
occurs. This method also creates problems over disposal as Japanese knotweed is classed as
'controlled waste' under the Environmental Protection Act 1990. This requires disposal at
licensed landfill sites. Specialist Japanese knotweed contractors are usually licensed to safely
remove the weed from site but check first before employing their services. Alternatively, it can be
destroyed on site by allowing it to dry out before burning. On no account should Japanese
knotweed be included with normal household waste or put out in green waste collection
schemes
If digging out is attempted, remove as much root as possible, then repeatedly destroy regrowth. In
this way the energy reserves in the remaining underground parts will be gradually exhausted; a
process which may, however, take several seasons
Biological control
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A plant sucker (psyllid) is being released in the UK as a biological control for Japanese knotweed.
It is currently only being released at a handful of trial sites and is not available to gardeners.
However, if successful it will be released more widely and will become widespread in Britain over
the next five to ten years by natural spread
Chemical controls
Cut stem application
Use this method in areas where plants are established within or around non-target plants or where vines
have grown into the canopy. This treatment remains effective at low temperatures as long as the ground
is not frozen. Cut the stem about 2 inches above ground level. Immediately apply a 25% solution of
glyphosate (e.g., Roundup®, or use Rodeo® if applying in or near wetland areas) or triclopyr (e.g.,
Garlon) and water to the cross-section of the stem. A subsequent foliar application of glyphosate may be
require to control new seedlings and resprouts. \
Foliar application
Use this method to control large populations. It may be necessary to precede foliar applications with
stump treatments to reduce the risk of damaging non-target species. Apply a 2% solution of glyphosate or
triclopyr and water to thoroughly wet all foliage. Do not apply so heavily that herbicide will drip off leaves.
A 0.5% non-ionic surfactant is recommended in order to penetrate the leaf cuticle, and ambient air
temperature should be above 65 ºF.
Glyphosate
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Perhaps the most effective and simplest method for the home gardener to tackle Japanese
knotweed is with the glyphosate-based weedkiller Scotts Roundup Tree Stump & Rootkiller.
This has label recommendation for controlling Japanese knotweed, instructing it to be applied to
the cut canes. Bayer Garden Super Strength Weedkiller also has label control for this weed
Alternatively, try other tough formulations of glyphosate (e.g. Scotts Roundup Ultra, Scotts
Tumbleweed, Bayer Tough Rootkill, Doff Maxi Strength Glyphosate Weedkiller or Westland
Resolva Xtra Tough Concentrate)
Glyphosate is usually applied to the foliage and is passed within the plant to the underground parts
It is useful to cut away old stems during the previous winter to allow good access. As with other
weeds, the most effective time for spraying Japanese knotweed with glyphosate is at the flowering
stage in late summer. However, it is difficult to spray at this stage, when the weed is 2.1m (7ft) or
more high
A more practical approach is to allow Japanese knotweed to grow to about 90cm (3ft), which will
usually be reached in May, and spray then. There will be regrowth and consequently a second
application in mid-summer is useful. Check during September and if it has grown once more, spray
again before growth begins to die down in the autumn. Check again the following spring
Avoid spray coming into contact with garden plants. Glyphosate-treated knotweed will often
produce small-leaved, bushy regrowth 50-90cm (20in-3ft) in height the following spring. This is
very different in appearance to the normal plant and it is essential that this regrowth is treated
It usually takes at least three to four seasons to eradicate Japanese knotweed using glyphosate.
Professional contractors, however, will have access to more powerful weedkillers that may reduce
this period by half
Residual control
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The residual weedkiller Bayer Ground Clear Weedkiller containing
glyphosate/flufenacet/metosulam comes in a soluble sachet. It may provide a moderate check in
growth, but because of the extremely persistent rhizomes, is unlikely to eradicate the weed
Seeking help from the professionals
Some professional companies offer Japanese knotweed removal. They can usually report on risk for
mortgages with suggested treatment plans and offer insurance-backed guarantees where required. The
British Association of Landscape Industries (BALI) has a directory of members offering invasive weed
control. See our page on hiring contractors for more guidance.
Other Resources:
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https://en.wikipedia.org/wiki/Fallopia_japonica
https://www.invasivespeciesinfo.gov/plants/knotweed.shtml
http://www.newsweek.com/2014/07/11/japanese-knotweed-driving-men-murder-257257.html
(GREAT ARTICLE)
https://www.youtube.com/watch?v=ZTgoan0jLnQ