Japanese knotweed growth across a lawn

Japanese knotweed on a development site

Japanese knotweed can make a site economically unviable to develop. 

Recently we were commissioned to assess a residential development site that had been undergoing a knotweed herbicide treatment programme by another Japanese knotweed specialist for 7 years, to check whether the knotweed was eradicated prior to site purchase. Our client was looking to build fourteen houses on the site. 

Herbicide treatment as a knotweed removal solution is never recommended on development sites where the ground is to be disturbed, as it is likely that viable rhizome remains in the ground, in a dormant state. Once development work begins, and soil is disturbed, further growth is triggered, often spreading the original infestation further. 

During the site survey, a number of bonsai shoots of knotweed were noted, indicating that live viable rhizome remained in the ground. The company treating the knotweed agreed that eradication had not been completed, but were reluctant to do anything other than keep treating the knotweed, however ineffective, with herbicide, stating that was all they were contracted to do. The problem being, no time limit was specified to achieve eradication in their contract, in effect causing an unquantifiable delay to any construction work starting on site.

Environet recommended an intrusive investigation to determine both the area and depth of viable rhizome, and to determine how effective the 7 years of spraying had been. It wasn’t a great surprise to find bright orange healthy rhizome just under the surface penetrating into the ground to a depth of around 1.5m. 

Physical removal was therefore the only viable option. Cost estimates of between £250,000 for Xtract™ and £500,000 for “dig & dump” made the site uneconomic to develop, so the purchase did not proceed. 

This debacle illustrates the importance of checking out the knotweed liabilities prior to purchase, making sure the right remediation company and method is employed from the outset, and that validated insurance backed guarantees are in existence that can be assigned to the new owner.  

Take a look at our FAQ for further information on the legal, technical and financial issues to consider when developing a site with knotweed.

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