What Architects ought to know about Japanese knotweed

Japanese knotweed is the UK’s most aggressive and destructive invasive plant. Early identification, management and removal is essential on development sites to prevent the spread of the plant. We asked Nic Seal, director of Environet UK Ltd, a leading knotweed specialist firm to explain.

What is Japanese knotweed?

Japanese knotweed, the country’s most invasive weed, was imported into the UK around 1820. It grows very quickly and has now spread to every county of the UK. It is causing significant problems for owners and occupiers of affected land, and in particular for developers and contractors on development land. The plant has an extensive root system that spreads horizontally and to great depth. If the root system is not killed or removed in its entirety, it will continue to grow, infesting more land and exacerbating the problems it causes.

How is Japanese knotweed identified?

Early identification of the plant is essential to prevent disturbance of ground within infested areas. Ground disturbance results in rapid spread of knotweed to other areas on or off site, which has significant legal ramifications.

Japanese knotweed is usually fairly distinctive as it is a tall (2-3m high) plant. However, being perennial its appearance changes with the seasons, starting with new shoots in spring, growing to its full height by July. It flowers in August or September before dying back in November leaving brittle leafless canes in winter, making it more difficult to identify.

The appearance of the plant can be substantially altered by application of certain herbicides, making the identification of Japanese knotweed even more difficult to the untrained eye. Where treatments have not been completely effective the knotweed re-growth returns with stunted plants, with small leaves and spindly stems. Environet provides a free identification service from emailed photos (environetuk.com)

Developing a site with Japanese knotweed

If a developer has a site infested with knotweed, then complete eradication of the knotweed should be the goal. This is not easy, but tried and tested methods do exist, and when undertaken correctly can provide the desired result. However developers should be wary, there are many pitfalls. The old adage ‘you get what you pay for’ applies to the knotweed eradication market.

The legislation

If a developer causes knotweed to be spread off site, they could find themselves at the wrong end of criminal proceedings under either the Wildlife and Countryside Act 1981, or the Environmental Protection Act 1990 ‘duty of care’. Offences under these Acts, whether intentional or not, can result in significant fines and, in extreme cases, custodial sentences.


Eradication methods

A number of eradication methods exist which suit different circumstances and which involve differing costs and levels of effectiveness.

Herbicide treatment

If the ground is not going to be disturbed (which is rarely the case on a development site) and some limited regrowth is tolerable, then in-situ herbicide treatment will be the most cost effective method. Herbicide treatment is typically applied either as a foliar spray or by stem injection. There is concern within the Japanese knotweed eradication industry regarding the effectiveness of stem injection as an eradication technique. Discussion has centred on whether stem injection simply kills the above ground part of the plant, leaving the rhizome system alive but in a state of temporary dormancy. Application by foliar spray is tried and tested, and if done correctly can kill knotweed. However it is not possible to verify with 100% certainty that all of the rhizome system is dead. Because of potential dormancy one cannot conclude knotweed is dead simply from the lack of growth at the surface.

Dormancy and the risk of accidental contamination

Japanese knotweed has a particular characteristic known in the industry as ‘dormancy’ – the plant can be shocked into a state of temporary dormancy by herbicide application. It may look as if the problem has been resolved at the surface, when in fact viable root rhizome remains in the ground, ready to resurface when the conditions are favourable to the plant. As Japanese knotweed root extends 2m deep, and as the rhizomes spread laterally into areas which may appear unaffected, the risk is clear on a site being redeveloped. Ground disturbance will fragment, spread and re-activate the hidden rhizomes to other areas of the site, significantly increasing the remediation cost. And if the knotweed is allowed to spread off-site then it’s possible there would be a breach environmental legislation.

Hence, the best advice is, do not rely solely on herbicide treatment to eradicate knotweed from development sites.

Physical removal

A quicker and more reliable method is physical removal of the rhizome system. Historically, ‘Dig & Dump’ was seen as the solution, where all infested soil was excavated, loaded into large lorries and taken to landfill. This is extremely costly, both to the client’s budget and the environment. Better and less expensive solutions are now used.

A method developed by Environet called Xtract™ avoids the need to consign vast quantities of infested soil to landfill, and does not rely upon the use of herbicide. Infested soil is excavated and processed on site using purpose-designed and patented technology to separate and remove the knotweed rhizome from the soil. The offending rhizome is removed from site, and the processed soil re-used on site.

On larger development sites, another option may be available where space permits. Infested soils are excavated from construction-critical areas and stockpiled elsewhere on site for subsequent herbicide treatment over one or more growing seasons. This method can be cost effective but does require considerable space for the stockpile.

On-site burial is a method referred to in the Environment Agency Knotweed Code of Practice, whereby the infested soils are buried on site at depth within a membrane-lined cell. This method relies upon the membrane containing the knotweed and limits future use of that part of the site.

“Bearing in mind knotweed rhizome can remain dormant for up to 20 years, on-site burial is not a method we would recommend, far better methods exist,” Nic Seal comments.

The best solution will be dependent upon site conditions, programme and cost.

“We’re happy to provide free advice to professional advisors on the most appropriate method, and provide quotations for the work. When choosing a contactor ensure they have the experience, skills and financial resources to complete the work, and make sure they are able to provide insurance backed guarantees,” Seal concludes.

For more information please contact Environet on 01932 868 700 or visit www.environetuk.com