A 20-hectare site destined to be a major housing development in Cornwall had a serious Japanese knotweed infestation. Ebsford discovered 3 areas of concern requiring treatment before the client could start any works. As an invasive non-native species, Japanese knotweed often proves a significant barrier for development, and it is paramount that the species is controlled or removed to prevent further spread and ensure the site is left free from contamination.
It quickly became apparent that another contractor had previously attempted to manage the infestation. Japanese knotweed had been aggressively treated with herbicide, limiting the amount of growth visible, this left it very difficult to identify amongst other vegetation. The previous owner had also attempted excavations and the burial of some contaminated material independently. Treatment had been ineffective, and the site was in need of an efficient and environmentally friendly solution that would completely eradicate the knotweed in its entirety, ready for development to begin.
Within the previously treated areas, viable rhizome had remained within the soil of the which meant the only way to ensure the knotweed is fully eradicated was to excavate the area. A screening programme was recommended as the best solution, ensuring all contaminated material is thoroughly processed and disposed of in the correct way.
Two of the smaller areas were targeted initially, and screening successfully completed over a period of 3 weeks, through the use of our latest screener within the Enviroscreen 2020 family. The 3rd area of Japanese knotweed was growing from the burial mound prepared by the previous owner of the site and another knotweed contractor with a view to contain the plant. The cell burial did not conform to the required standard and was poorly constructed with no attempt made to seal the membranes used. As a consequence, the entire cell had to be reshaped and reburied to achieve a clear site.
When the cell membrane was cut away Japanese knotweed growth was found beneath – the cell had been completed and covered with 300mm of soil over 3 years ago. Within 7 days, fresh growth was present in the area highlighting the ability of knotweed rhizome to survive with no light over prolonged periods.
Huge amounts of rhizome were removed during the screening process and incinerated on site to save transportation to landfill further reducing the carbon footprint of the project.
All material within the burial mound had to be realigned to move the form out of the footprint of a proposed road. The new burial cell was shaped into an existing feature and covered with the correct amount of fill to an agreed standard. A new membrane measuring 35m x 25m and weighing 750kg was installed to contain and cap the mound effectively.
Ebsford handed the site back finally free of Japanese knotweed and under warranty to give the client the reassurance that development could commence without the risk of the species returning.
