Best practice dictates that, wherever possible, Japanese Knotweed and other invasive plants should be treated in their original locations. Codes of Practice dictate that, where this is not possible, the next best preferred solutions are a range of on-site treatment methods, of which deep burial is often the simplest one. When considering the deep burial option, it should be kept in mind that Japanese Knotweed, in particular, has very extensive underground root systems. Typically, its root network can extend up to 7m horizontally in all directions from the plants themselves, and can reach a depth of 3m. In the case of sites which have been subject to previous importation of fill material, the root system can extend even deeper. For the burial method to be successful, all risk material has to be identified and removed, so even for small plant infestations, the volume of material to be excavated and buried can be very substantial.
Prior to excavation, the invasive plant material should be treated with a non-persistent herbicide and left in situ for the herbicide's prescribed "active" period. The burial location should be as close as possible to the infestation, but if material has to be moved across a site for disposal, biosecurity measures should be deployed. These would include, controlled haul routes and work areas, temporary root barrier protection measures, vehicle wash down facilities, and screening and monitoring protocols.
The infested material must be buried to a depth of at least 5m, covered by a proprietary root barrier membrane, and then infilled above with 5m of inert fill or soil. The location should be carefully selected and mapped, and should be where there will be no risk of future disturbance from subsequent development or landscaping, services installation, or erosion. If site conditions dictate that deep burial is not feasible then one of the other on-site treatment options should be considered, which include , underground cells, temporary bunds, root barrier systems, and soil screening