Japanese knotweed risk for developers

Most developers are well aware of the need to check a potential site for pollution and contamination in terms of chemicals and industrial waste. However, many are less savvy about plant life, even though certain species such as ragwort, himalayan balsam, giant hogweed and Japanese knotweed can cause havoc, particularly within the construction industry.

Japanese knotweed spreads by underground roots which can work their way through most materials, including concrete. As it spreads, it grows directly through paths, roads, foundations and drainage systems, causing significant structural damage and failure. It is classified as ‘controlled waste’ and as such it must be disposed of at a licensed landfill site. If the weed has been treated chemically, it then constitutes hazardous waste.

Land where knotweed is found becomes commercially blighted and can take a number of years to be cleared effectively. Chemicals rarely kill these plants immediately – they will usually have to be sprayed over several seasons. Fuller information about the handling and disposal of knotweed can be found on the Environment Agency’s website.

The presence of knotweed on a site imposes legal liabilities on the owner. While the environment agency has no powers to force owners to remove the plant from their land, they can become liable to neighbours for illegal nuisance. Property owners have a duty of care to neighbours to not allow anything to escape from their land that might detract from the enjoyment of adjoining property. If the knotweed escapes on to a neighbour’s land, it could well lead to court action to claim expenses and compensation.

Buyers should instruct their surveyors to check whether there is knotweed on a potential site or on any adjoining land such as railway embankments or derelict allotments. Developers should be cautious before importing any topsoil for landscaping to ensure that knotweed is not inadvertently introduced to a development site. If knotweed is discovered on a site, competent and experienced contractors should be employed to oversee its treatment and removal.

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