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Japanese knotweed management plan

Japanese knotweed management plan


Japanese Knotweed Rootbarrier  

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Various root barrier membranes are available which claim to prevent Japanese knotweed penetrating. A root barrier membrane is only as good as the way in which it has been laid. It is therefore essential that there is expert supervision when the root barrier membrane is supplied and installed. A root barrier membrane physically protects a structure or clean soil. It must be made of a material that is fit for purpose. It should be made of a material that can be used without damage; provided in large sizes, to minimise the need for seals; sealed securely; remain intact for at least 50 years; resist UV damage if it is exposed to sunlight.

Japanese knotweed will tend to break through holes or joins in the fabric, so it is essential that the integrity of the root barrier membrane is maintained, and there is a minimum number of seams. Ideally, root barrier membrane material should consist of a single sheet. You must ensure that root barrier membranes containing leachable chemicals do not pollute streams and groundwater. Given that Japanese knotweed rhizome may remain dormant for at least 20 years, it is important that a root barrier membrane carries a guarantee well beyond that time. Root barrier membranes are vulnerable to damage from burrowing mammals. Burying root barrier membrane cells 2m or deeper should provide some protection against smaller mammals, such as rats. If badgers and rabbits are present, you should consider deeper burial.

Vertical Barrier

Carefully using a vertical root barrier membrane can be used to prevent the horizontal growth of Japanese knotweed. This is usually used against uncontrolled infestations from neighbouring properties. Vertical root barrier membranes are also often used around the edge of horizontal membranes, as a precaution against re-growth from any residual rhizome. Vertical root barrier membranes can often be most conveniently used when reinforced by a plywood frame. If it is not known how deep the rhizome has spread, vertical root barrier membranes should be used to 3m deep as a standard. The root barrier should be installed vertically and as taut as possible.

The root barrier should line the side of the trench nearest the Japanese knotweed with back-filling to the backside only. To prevent rhizomes growing over the top, the root barrier should finish at ground level. When backfilling the excavated trench, care must be taken that any sharp stones or debris, which would be capable of puncturing the root barrier should be removed. The backfilling should be carried out with continual compaction to prevent settlement.

As backfilling takes place, the root barrier can, in some circumstances, be dragged down during consolidation. It is important, that the installer should allow for this to prevent the finished level being below what is required and cut off any excess at the surface following successful consolidation.

Where the soil contains flint, sharp stone or any other sharp object these should be removed from the face of the trench, which is to be lined with root barrier. If this is not practicable consideration should be given to lining both sides of the root barrier with plywood or some other suitable material.

Where the screen will be bisected by existing services it will be necessary to cut the root barrier and re-seal with joining tape preferably from both sides. Where the root barrier will bisect land drains, it is important that they be re-routed around the root barrier and not through the barrier.

As with all root barriers, it is important to determine the correct depth, length and position to prevent the rhizomes growing under and around the screen. Care must be taken that the position of a root barrier does not affect the stability of any tree or boundary wall and that the loss of rooting material is such that it does not cause trees to go into terminal decline. In addition the installer must take into account the varying conditions of individual sites, for example, the necessity to install drainage to prevent the build up of hydrostatic pressure on sloping ground. The installer should also ascertain whether the input of a structural engineer is required, for example, if the excavation of the trench to take the root barrier could affect existing foundations and neighbouring structures.

Horizontal Barrier

To be completed


To be completed

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Company’s Registered Office Address: 597 Etruria Rd, Basford, Stoke On Trent, Staffordshire. ST4 6HP.
Japanese Knotweed Survey © Copyright 2011

Japanese knotweed survey, management, control, eradication & land remediation relief.
Areas include Staffordshire, Cheshire, West Midlands, Manchester, Birmingham & Stoke On Trent.

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