In November 2021, our Invasive Species Specialists carried out a full walkover survey at a site due for development in Cornwall, following our clients concerns of Japanese knotweed in the area.
The site had originally been cleared of structures back in 2011 and had since been redundant. Our team soon realised the land had become densely vegetated overtime preventing access to some areas and would require a complete site clearance in order to confirm the presence of Japanese knotweed. This would need to take place before the end of February to avoid bird-nesting season as it is advised any work carried out to trees, hedges or vegetation should not be implemented during this season under the Wildlife and Countryside Act (WCA) 1981.
Clearance work quickly commenced including the removal of vegetation and various items: several skips containing mixed waste, gas bottles and 700+ tyres. During this process, Japanese knotweed was found to be covering the southwest corner of the land, against the boundary with a neighbouring builder’s yard and adjacent railway line. It was prevalent the team ensured there was no cross contamination of Japanese knotweed during the clearance process and ensured the contaminated waste streams were separated ready for disposal. Once the site was clear, works to remove the invasive species could begin.
After careful consideration, the limitations on site would require the process of off-site removal, often referred to as ‘Dig and Dump’, this process is ideal when the species is within a development footprint. It involves the physical removal of the plant and soil through excavation, which is then disposed of at a licensed landfill. Prior to excavation, the soils were tested for any additional contaminants e.g. hydrocarbons, organic matter, asbestos etc, which can increase the disposal rates. The results showed Asbestos was discovered within the soil and would need to be managed with local partners and disposed of to provide a clean site and a one stop shop for our client.
Excavation works were now able to begin by pushing 300mm of soils from the zone of influence back towards the knotweed stand. Relatively good ground was encountered appearing free from penetrating rhizome, this allowed access to the bund of material heavily infested with rhizome. The excavation progressed further in 150mm layers, each pull with the excavator being visually inspected by the site manager and transported to a suitable stockpile location ready for haulage and off-site disposal. During the excavation an unknown steel service pipe was encountered. Our team undertook careful and safe hand excavation in order to remove the Japanese knotweed that was found growing amongst this obstruction. The excavation continued until exhaustion of the Japanese knotweed was achieved; the area being excavated to a depth of 1.5m.
The species was also found to be present within a neighbouring boundary but unfortunately could not be excavated due to permissions and access. To prevent the re-encroachment of Japanese knotweed back onto site, impacting the development stage, a membrane was installed to act as a vertical barrier. The excavation areas were then back filled with clean site won material and levelled off accordingly.
Our client was extremely happy with the results, which enabled them to quickly commence with their planned development scheme. Any plots falling within the excavation area are eligible for a 10 year warranty and monitoring of the former JK areas will take place over this duration. Particular attention will be given to the development program with visits regularly undertaken during the growing seasons.
