A plant which can damage hard surfaces such as tarmac, grow through the floors of houses, occasionally even through the foundations, and by its vigorous growth can exclude almost all of our native species!

If you think this sounds like the return of the Triffids, you would be wrong enter Japanese Knotweed. Within one week in May 2003 the Tuffin Feraby and Taylor building consultancy team in Bristol surveyed seven separate commercial buildings throughout the UK. They found Japanese Knotweed growing at five of those sites, as far apart as South Wales and Glasgow.

Identifying the problem
Japanese Knotweed (scientific name Fallopia Japonica) was introduced from Asia to Europe, including the UK, in the mid-nineteenth century, primarily as an ornamental plant. In fact, it is a highly invasive perennial weed, which grows to a height of up to 3m, and comprises Bamboo-like stems with a purple speckled appearance. Dense areas of tall canes grow during summer and then die back in the autumn. Creamy white flowers appear late in the season. Once present at a site, Japanese Knotweed grows quickly if left unchecked to cover large areas. During later autumn/winter the canes lose their leaves and turn dark brown. The dead canes remain standing and can then take up to three years to decompose. This dead material often forms a dense organic layer, which suppresses competition from our native flora and garden plants.

In the UK the reproduction of Japanese Knotweed is mainly through vegetative regeneration of rhizomes and fresh stems. The rhizome is a root-like underground stem, which can reach a depth of 3m and extend up to 7m away from the parent plant. Sections of rhizomes as small as 0.7g can grow into new plants! In the same way that poppies appear on sites, Japanese Knotweed thrives on disturbance. Fragments of the plant can be dispersed on high water flows resulting in its growth on the banks of waterways where it chokes the riverbank. Fly tipping and transportation of soil containing rhizome fragments are now seen as a major cause of spread, particularly in urban areas.

Keeping Control
Japanese Knotweed is not an easy plant to control. Unfortunately, the extensive underground rhizome system sustains the plant even when growth above ground is removed. Therefore, the aim of any control programme must be to target the rhizomes. One way of achieving this is the use of chemical treatments, for example, herbicides such as glyphosate. The use of such chemicals can require consents from the Environment Agency, particularly in areas where there may be a risk of run-off to water courses, or where there is adjacent sensitive vegetation. Treatment can take several years before the plant is completely eradicated and, during this period, continuous monitoring is necessary to check that no new shoots appear. In controlling Japanese Knotweed you need to remember that it regenerates vegetatively. Very small fragments of rhizome and fresh stem material are able to produce viable shoots and roots. It is now an offence in the UK to cause this plant to grow in the wild under the 1981 Wildlife and Countryside Act, and any waste material, such as that arising from cutting, mowing or excavation, needs to be disposed of in accordance with the Environmental Protection Act 1990 (Duty of Care) Regulations. Another method of control is grazing, as the plant is palatable to sheep, goats, cattle and horses. However, this will not eradicate the plant and it will continue to grow once grazing ceases.

On a recent development site on which Tuffin Ferraby and Taylor have been involved, advice from the Local Authority was to treat and remove the stems. Indeed, this work was undertaken by a contractor appointed by the Local Authority under their supervision. Following the removal of the plant above ground, the area was grassed over and a maintenance regime put in place involving cutting the grass at least 16 times per year. The costs associated with eradicating or just managing Japanese Knotweed are considerable. We have heard of costs of up to £60/m2 for its removal from development sites. Bearing in mind that the mere presence of the plant will adversely affect land values, it is important that, as building surveyors, we are aware of and can identify the problem.

When acting for either a tenant or landlord the implications of eradicating or treating Knotweed need to be considered, as should its potential to escape from one site to another. The biggest problem in the UK to date has been in the Swansea area in South Wales. The Welsh Development Agency have produced a number of publications, including a model specification and tender documents that also contain useful information regarding Japanese Knotweed. As for the future, control tests are currently under way involving the importation of the plants natural enemies from Asia. This might involve insects or pathogens that will attack the plant, while not endangering other adjacent species or crops native to the UK. Such methods of biological control have worked in the past, and the Environment Agency is currently undertaking tests with an imported Japanese beetle.

Japanese knotweed was first introduced as an ornamental plant, but has since plagued the environment; removal is costly and time consuming. Now a team has identified natural predators from its native home that could also control it in the UK. The plans have been submitted to the government for approval. If the proposal gets the go-ahead, it will be the first time that bio control - the use of a natural enemy to control another pest - will be used in Europe to fight a weed. James MacFarlane, vegetation adviser for Cornwall County Council, said: "The weather patterns have ended up with us seeing a lot of knotweed spread in a number of areas."

In Japan, the original home of knotweed, the plant is common but it does not rage out of control like in the UK. Dick Shaw, the lead researcher on the project, from Cabi, a not-for-profit agricultural research organisation, said: "In 2000, we went out to Japan to see whether the plant had any natural enemies that it had lost when it came here. "We found that it had a lot: there were 186 species of plant-eating insects and about 40 species of fungi." The team then began to test the predators to find those that only had an appetite for Japanese knotweed - and not any other plants. The researchers tempted them with plant species that were very closely related to knotweed, less closely related species that belonged to the same tribe or family, and important UK plants such as apples and wheat. A leaf sport fungus only has an appetite for knotweed. Those that attacked any plants other than Japanese knotweed were ruled out, explained Dr Shaw. Eventually, the list was whittled down to two: a sap-sucking psyllid insect (Aphalara itadori) and a leaf spot fungus from the genus Mycosphaerella. Dr Shaw told the BBC: "We have done some efficacy trials here in the lab and they are showing a significant impact."

Successful bio controls do not eradicate the target weed - this would mean wiping out their only food source and effectively making themselves extinct - but they do bring them under control. Dr Shaw said: "The psyllids are having an effect on the plants' height and the treated knotweeds produce tiny curled leaves rather than big light absorbing leaves, which means that less resources will get to their root system. "The nymphs are literally sucking the life out of the plant," he added.

Japanese knotweed, like many other non-native, invasive plants, was first introduced to the UK for horticulture. Its 3-4m-tall (10-13ft) stems, ornamental leaves and clusters of white flowers made it an attractive option for gardens. Simon Ford, a nature conservation adviser from the National Trust, said: "Japanese knotweed first came into the UK in 1840 through Wales - but it was only about 10 to 15 years later that people started to see it as a threat." The plants grow incredibly quickly - some have been shown to grow four metres in just four months, they can spread at a tremendous rate, and they can quickly obliterate any other vegetation growing nearby. Mr Ford said: "By the time they realised this it was already far too late."

Today, Japanese knotweed is prevalent throughout the UK. It has not only caused great damage to plant biodiversity but it is also causing problems for hard structures, including buildings, paving stones and flood defence structures. It has been estimated that to remove all knotweed from the UK would cost several billion pounds. Cabi scientists believe that natural control could offer a much simpler solution. The project has cost about 600,000 over five years. Dr Shaw said: "In comparison to the current control methods, if it works, this will be significantly less costly." The team has submitted its proposals to the government. The Department for Environment, Food and Rural Affairs (Defra) along with independent peer reviewers will assess the work before it is submitted for a public consultation. Dr Shaw said: "We do not know if this is going to be the silver bullet for knotweed, but if we were able to just stop the thing spreading at the rate it currently is, or make it easier to kill, I will be very happy." The research project has been paid for by a consortium of UK sponsors including Defra, the Environment Agency, the Welsh Assembly Government, Network Rail, South West Regional Development Agency and British Waterways, coordinated by Cornwall County Council.

Download an identification leaflet on Japanese Knotweed. (Adobe PDF)