Sunday, 27 September 2009

Japanese Knotweed - set a thief to catch a thief?

We can give great thanks to the Victorian era “plant hunters” who brought back many of the exotic species which we now take for granted as part of the colour and structure of our gardens - but we can also curse them for some of their introductions – notably Rhododendron ponticum and especially Fallopia japonica (“Japanese Knotweed”).
What’s the problem?
Japanese knotweed is native to Japan, China and Korea, where it is part of the natural ecology with its own controlling pests, and was brought here around 1825 for decorative effect. Lacking any ecological balances here, it has been so successful that it is now widespread throughout the UK (with the exception of the Orkneys) and threatens the indiginous plants in large parts of Wales and South-West England. In addition to its effects on flora, and the implications this has for bio-diversity and food chains, it has adverse impact on riverbanks, leading to erosion and increasing flood risks, and on fish stocks within rivers; it can damage asphalt surfaces, building foundations, retaining walls and drains; it can create safety issues by obscuring railway signals and road signs and create trip hazards in paving.
For these reasons it was made illegal to plant, propagate or otherwise spread it in the UK by the Wildlife and Countryside Act 1981. It is also classed as "controlled waste" in Britain under the Environmental Protection Act 1990, so it may not be disposed of along with other “green” or household waste. This also applies to any soil contaminated with parts of the plant – adding extra costs to development and regeneration schemes. To give a sense of scale to the problem a DEFRA review in 2004 gave an estimate of £1.5bn for the cost of eradicating it – mainly with chemical herbicides, which are themselves considered “unsustainable”. In addition to our problems it is classified as a highly invasive weed throughout Europe and the USA and is listed as one of the 100 worst invasive species by the World Conservation Union.
How can it be recognised?
Recognition is fairly easy – it has hollow purple-speckled stems with raised nodes which look similar to bamboo canes and which carry broadly-oval grass-green leaves about 8-12cm (3-5”) long by 5-10cm (2-4”) wide alternately along the zig-zag stems. The leaves have a smooth (“entire”) edge and a “cut-off” base. Spikes (“racemes”) of creamy-white flowers, about 5-15cm (2-6”) are produced in late summer – these are initially erect, but tend to droop as they fade; they are not (yet!) fertile in the UK so the plant does not spread by seeding. The hollow stems become brown and brittle but remain after the rest of the plant dies back in late autumn.
It spreads by rhizomes, which can reach 7m in length and 3m deep, to form dense colonies that crowd out other herbaceous species. It is often found on roadsides and wasteland where it can be extremely difficult for site developers as the depth and spread of rhizomes make it virtually impossible to excavate and it re-grows strongly from any remaining pieces or after being cut down. Any plant material, or contaminated soil, must be disposed to properly licensed landfills.
How can it be controlled?
Currently the most effective control is through (repeated) treatments of translocated (“systemic”) Glyphosate-based herbicide. This is a non-specific product and will therefore kill any actively growing plants which absorb the spray through their leaves - so if the stand of Japanese knotweed is within other desirable plantings (as in the photos) it will be very difficult to remove. In a domestic situation, some success may be had through the use of a glyphosate gel, painted directly onto the knotweed leaves, covering as much of the plant as possible whilst avoiding contact with other plants – though this will be an extremely time consuming “labour of love”.
Biological Control
There is some hope now that biological controls might prove effective, of which there are 2 promising candidates – a leaf spot fungus, Mycosphaerella polygoni-cuspidati, and a sap-sucking insect, Aphalara itadori. The latter is a naturally occurring control in eastern Asia, but is not currently found in Europe, and is therefore prohibited under the same act which bans spread of the knotweed plant, i.e. The Wildlife and Countryside Act 1981, and is also restricted under the Plant Health Order 2005. DEFRA and the Welsh Assembly Government are currently asking for views from people/organisations with an interest in the impact of Japanese knotweed on the natural and built environment - including the horticulture industry, landscape managers/contractors and people interested in the control of invasive species - on the release of this insect as a control agent. If approved, this would be the first use of a non-native insect to control a plant species in the UK and Europe. The consultation input is required by end of October – further information on this can be found at and at
General information about Japanese Knotweed can also be found at and at Devon’s excellent site

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