Japanese Knotweed is an invasive perennial plant brought to Britain from Japan between 1825 and 1841. It was originally introduced as an ornamental plant and was favoured by landscapers due to its quick rate of growth forming dense screens.
In the UK it has been prevalent in Wales for some time due to the moist climate but has since spread throughout the UK and is now so widespread that there is not a 6 sq mile area in the country where it is not found. Locally Japanese Knotweed has been found in Nailsworth, Stroud, Eastington and Bussage and is bound to be elsewhere. It is tolerant to adverse habitats such as soil acidity, heavy metal contamination and air pollution meaning it is able to thrive where other plants would fail.
The Government took action on the problem in 1981 when it was included in the Wildlife and Countryside Act 1981and it was made in offence to ‘plant or otherwise cause Japanese Knotweed to grow in the wild’. The disposal of waste containing knotweed is covered by the Environment Protection Act 1990.
The Japanese Knotweed can be identified by its bamboo like stems in a thick central stand of plants. It has oval or heart shaped leaves and produces white flowers during it vigorous growing season from May-October. This growing season is when it can be most readily identified. The flowers and leaves die back during the winter months but leave the stems standing allowing identification to be made if with more difficulty. A guide to identification and further information can be downloaded from this link.
Japanese Knotweed is a massive problem both economically and environmentally in the UK. It is estimated to cost the UK £150 million every year.
Japanese Knotweed has extensive root systems that can stretch, from a well developed stem, 2 metres down and 2 to 3 metres laterally from the plant. A new plant can spread from a fragment of root as small as 0.8 gram making it extremely difficult and labour intensive to remove.
As its growth can outstrip native plant species it can spread quickly across wide areas if not controlled. In environmental terms it provides a poor habitat for native insects, birds and mammals.
Japanese Knotweed can cause severe damage due to the strength of its root system leading to damaged patios, paths, drives, outbuildings, conservatories and other garden structures. Although much damage is aesthetic it can also make structures unstable and dangerous where retaining walls and building foundations are affected. Drains and other buried surfaces can also be damaged. All of these can be very expensive to put right and work cannot be started until the infestation has been fully cleared.
An infestation can make your house unmortgageable. Loan companies are becoming reluctant to lend on properties affected by the invasive weed. Some individual lenders will consider applications where Japanese Knotweed is found by their surveyor but will ask to see evidence of initial treatment of the problem and commitment to fund three to four year treatment programs in advance of lending. Most insurance companies policies do not cover damage caused by Japanese Knotweed and have recently become reluctant to provide cover on properties damaged by an infestation.
Eradication methods are both time consuming and expensive. There are a number of different ways in which this can be carried out.
Excavation of the plant and it roots. This may seem a simple and cheap way of removing the weed but ALL of the roots must be removed, which is time consuming and labour intensive. Secondly, all the affected soil must be disposed of correctly which mean paying a specialist contractor.
Burial on-site. This avoids the high cost of disposal but the knotweed needs to be covered with 5m or more of overburden or a root barrier installed to prevent regrowth.
Chemical treatment. Japanese Knotweed is resistant to most herbicides but can be treated over several seasons (at least three years) with glyphosphate. This is usually the most realistic option and