This case study reviews an invasive non-native species (INNS) remediation contract completed on a large complicated development site in Wembley, London. The works involved the treatment and removal of the notorious plants, Japanese knotweed (Reynoutria japonica) and Giant hogweed (Heracleum mantegazzianum)
These plants are listed within Section 14, Schedule 9 Part II of the Wildlife & Countryside Act 1981. Once their presence is recorded a control strategy is recommended, especially where there is proposed change of land use, such as development works.
Invasive Species Survey
Our survey to investigate the site was complicated as the INNS had undergone previous chemical treatment, disguising their true extent. Thankfully, the previous contractors were PCA members, so there were accurate records and plans of the plants’ locations for our reference.
Even following chemical treatment, the ground remains contaminated by the knotweed rhizome and Giant hogweed seeds and it requires removal to allow for free development.
Removal of trees and vegetation from some areas of the site was required to provide better access and to enable a full investigation. We provided a watching brief to oversee the tree surgeon’s activity to ensure that green waste was not inadvertently mixed, dragged and chipped with invasive weeds, further contaminating other areas of the site.
Our survey recorded active and chemically treated areas of Japanese knotweed to be 163m2, with Giant hogweed totalling 9172, both of which were spread over wide, multiple areas of the site.
INNS Remediation Evaluation
For every site we become involved in, we evaluate the most appropriate INNS remedial methods to deploy. This involves cross-referencing our survey with proposed site plans and/or site changes and assessing how and when remedial works are undertaken. A particular consideration for this site was the proximity of the INNS to the River Brent. Three issues were identified that influenced our proposals:
- Tree stumps and tree roots close to the River Brent wall needed to be left in situ, as removing them could affect the structural integrity of the river wall
- Full depth excavation of knotweed rhizome along the river had the potential to destabilise the wall
- Plant could not access within 25m of some affected areas adjacent to the riverbank. This limitation would need to be overcome
A remedial method was needed that provided a practical solution to removing the INNS (Japanese knotweed and Giant hogweed) given these constraints. INNS in other areas of the site could be more easily accessed and fully removed using mechanical excavation. The survey also identified INNS on a small parcel of land which was inaccessible to machinery due to a copse of trees that were to be retained. As there were no planned ground disturbance works, we recommended continuing with a Herbicide Treatment Programme for long-term control.
INNS Remediation Works
We implemented remediation methods that worked to the identified constraints. Specialist long-reach 38-tonne tracked excavators were used to provide the extended reach required to excavate down by the river wall. INNS impacted ground was removed away from the wall to an engineering depth deemed safe to maintain wall integrity.
In working on the sloped riverbank and near the river wall, our personnel worked safely in teams, wearing life jackets and using lanyards with harnesses fixed to a mobile block at the top of the bank. The teams worked in pairs in accordance with our pre-determined rescue plan for which they had received formal training. This enabled meticulous working around retained tree stumps adjacent to the river wall.
Where engineering specification required a reduced depth excavation, horizontal and vertical root barriers were installed to cap remaining contaminated soils, with root barrier sealed back to the river wall.
All remaining knotweed contaminated ground was carefully excavated to remove the full depth and lateral spread of the plant’s underground rhizome. This method provided an approved (PCA, Knotweed Code of Practice) instant eradication method for the plant and free unimpeded development use of the area on completion. The Giant hogweed contaminated ground was excavated to remove the plant's roots, tap-root and surrounding soils containing the plant's seeds.
Herbicide treatment was undertaken by a team of two people for safe working practices on slopes and near water, and when working with Giant hogweed, due to the plant’s toxicity. Herbicide was applied using knapsack sprayers with standard and long-reach lance applicators.
Excavation works ran to schedule and were completed over 5 weeks and to the customer’s satisfaction. In total, over 930 tonnes of waste materials were removed from site as controlled waste, carried by registered waste carriers, and disposed of at a specially licensed landfill site. All waste tracking records were provided to the client along with the completion of a work validation report, detailing and recording all the remedial works undertaken, including pictures, and completed works site plans. Also referred to as Invasive Weed Management Plans or Japanese Knotweed Management Plans, these reports are retained by the principal contractor who normally includes them within the development’s operations and maintenance manuals.
An ongoing 3-year Herbicide Treatment Programme is now in place for the infestations that could not be excavated. The excavated areas of the site are under surveillance monitoring, with further treatment to be undertaken in the unlikely event of any recurrence of the growth. Following two-years' successful monitoring we are providing a 10-year Insurance Backed Guarantee.
About Japanese Knotweed Ltd
Japanese Knotweed Ltd is the UK's trusted Japanese knotweed treatment and remediation company, with a proven track record in working successfully for local authorities, developers, construction companies and private landowners.
Our highly knowledgeable and skilled staff provide a comprehensive service and are committed to providing the highest level of customer care.
We provide surveys and knotweed management plans, including for long term chemical treatment, guarantees and immediate excavation options. Our remedial methods are tailored to each site and provide sensible and efficient eradication solutions to meet all requirements.