Japanese knotweed or Fallopia japonica is a perennial plant native to Japan, China and Korea. There are several other closely related species to Japanese knotweed including giant knotweed and Russian vine which have successfully established themselves in Europe and in North America. The plant is listed by the World Conservation Unit and one of the world’s worst invasive species, the root system can damage concrete foundations, buildings, roads, paving, retaining walls and flood defences. The Environment Agency has described Japanese knotweed as “Indisputably the UK’s most aggressive, destructive and invasive plant”. Japanese knotweed was brought to Europe in the mid 19th century by a German botanist who had found the plant growing on the side of volcanoes. Lauded originally for its beauty and potential for animal feed the plant was introduced to the Royal Botanical Gardens at Kew in 1850, by 1854 the plant was sold commercially by nurseries throughout the UK.
Japanese knotweed is now so prevalent that there is not a single six-square mile patch where the species has not been found, with the exception of Orkney Japanese knotweed is a nationwide problem. It was not until 1981 that the Wildlife and Countryside Act made it an offence to ‘plant or otherwise cause Japanese knotweed to grow in the wild’.
How to Identify Japanese Knotweed
Though not closely related, Japanese knotweed grows hollow stems which give the appearance of bamboo; stems can reach up to three or four metres and feature board oval shaped leaves. In late summer and early autumn the plant goes into flower, featuring small racemes which are often a cream or white hue. Leaves are arranged in a zig-zag pattern along the stem with mature plants entirely hollow. During the winter the leaves will fall away leaving behind innocuous brown stems which have the appearance of dead shrubbery. However, below the surface Japanese knotweed forms a complex root system often several metres deep. The root or rhizome can be identified as a dark leathery brown bark like material which when snapped features and orange body beneath the bark.
How can Japanese Knotweed Affect Residential Property?
Over the last twenty years Japanese knotweed has caused problems because of the damaging effects of this invasive plant. Often such concerns are born from misunderstanding, overaction and inaccuracies in the media. Below is a synopsis of those areas commonly affected by Japanese knotweed:
Drains and Underground Services: Knotweed will often seek out water sources and cracked drains can be at risk of blockage from knotweed roots.
Paths & Driveways: Knotweed cannot grown through concrete but can penetrate movement joints between existing paved or tarmacked areas.
Walls: Dense stands of knotweed have the capacity to undermine retaining or boundary walls and cause sudden collapse.
Gardens: With no natural predator knotweed can easily overrun existing plants and vegetation if left untreated.
Eradicating Japanese Knotweed
The Environmental Protection Act 1990 contains a number of provisions about the removal of Knotweed as it has been designated as ‘controlled waste’, disposing of ‘contaminated’ soil can often be a laborious and expensive process given the amount of soil that often needs to be removed. The volume of such removal can extend to up to 3 metres below ground level results in large volumes of waste soil, at up to £50 per tonne the removal of material can end up being prohibitively expensive.
Japanese Knotweed can be excavated and subsequently buried on site, it is recommended that any burial of less than 5 metres of overburden should also feature a root barrier membrane to prevent continued growth. Root barriers can also be installed vertically to prevent lateral cross contamination from site to site; knotweed however can grow very vigorously and as such root barriers are often best employed alongside other methods of treatment.
Biological control involves the introduction of a species of insect that will attack the species and effectively reduce the presence of knotweed by grazing on the leaves and root system.
Chemical control is the most common application and often the most economical. Specialist herbicides are introduced to the plant over a number of seasons although this method can take several years to complete effectively. In densely packed residential environments careful chemical control is often the only safe treatment method available in dealing with Japanese Knotweed.
Will the Presence of Knotweed Impact the Value of a Property?
By quantifying the cost of treatment and any associated repairs the impact of Japanese Knotweed can be monetised and subsequently be reflected in the valuation like any other defect. In circumstances where knotweed is present but has not caused any damage it could simply be the cost of the treatment that is reflected in the value and therefore may not affect the value of the property as a whole.
For more information see: http://www.rics.org/Global/Japanese_Knotweed_and_residential_property_1st_edition_PGguidance_2012.pdf