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Japanese Knotweed Treatments

Japanese knotweed can be treated or removed using a variety of techniques. These are discussed below and are outlined in the following video, which was produced by Groundsure to support their innovative remote sensing technology.

Chemical Treatment

24-48 months

Applications of specialised herbicides are used over the duration of two to four growing seasons. This Japanese knotweed solution is only ever carried out by fully qualified and experienced sprayers; this is often the cheapest treatment option. If a quicker solution is needed there are other methods that Phlorum can provide. The herbicide treatment period can also be reduced by using a number of clever techniques to reduce the vigour of knotweed rhizomes.

Some unscrupulous contractors suggest that knotweed can be eradicated using herbicides within a single growing season. Various sources of guidance and advice from professional bodies and regulators, such as RICS and the Environment Agency, warn of these overblown claims. There is currently no product or commercially available chemical system that will, on its own, eradicate mature knotweed growth in a single season.

Relocation and Herbicide Programme

24-48 months

On large development sites Japanese knotweed infested areas of land can be moved to specified treatment bunds or stockpile areas. This frees up areas to be developed and relocates the Japanese knotweed to more convenient areas of the site where it can be treated with an appropriate herbicide programme.

Reduced Dig and Herbicide Programme

12-24 months

This solution removes the most vigorous and hard to treat areas of knotweed plants. The remaining material can then be dealt with using a variety of soil sorting and herbicide application techniques.

In-Situ Capping

This method encapsulates some or all of the Japanese knotweed growth within or under a specialist root barrier membrane. This barrier is of very high puncture resistance and is robust enough to withstand cracks or tears from most spoil materials. This process can provide instant eradication of Japanese knotweed from many sites.

On-site burial

On-site burial is another instant Japanese knotweed eradication method. This involves the creation of a burial cell that holds the excavated knotweed waste. This technique is often best suited to areas where there are no deep groundworks or services planned – e.g. car parks or public open spaces.

Picking, Screening and Sorting

Volumes of knotweed waste can be significantly reduced by separating rhizome (root) material from affected soil. This can be achieved using a variety of picking, screening and sorting operations using a range of different resources, including picking stations and various types of mechanical screens. Different techniques are more effective depending on the volume of Japanese knotweed infested soils that are being treated. On larger sites the process can require an Environmental Permit, which Phlorum can obtain from the appropriate regulator (e.g. Environment Agency or SEPA).

Excavation and Removal

Excavation of Japanese knotweed and removal of wastes to a landfill site is a frequent option where time and space don’t allow other treatment strategies. Previous Environment Agency guidelines stated that excavation of Japanese knotweed should be undertaken within a 7 metre zone around plants and to a depth of 3 metres. However, by carefully managing the knotweed excavation process, Phlorum’s experts can reduce volumes of waste arisings to amounts significantly lower than the former Environment Agency guidance suggested (often resulting in volume savings by as much as 50-90%!).

Japanese knotweed falls under the classification of “controlled waste”. Therefore, it is essential that if it is to be removed from its site of origin, it should be disposed of at an appropriately licensed waste facility.

In order to save our clients money, Phlorum has had significant experience in obtaining landfill tax exemption for knotweed waste. However, in line with the UK Government’s push to reduce the amount of waste that is sent to landfill, the exemption scheme has recently been dropped. Notwithstanding this, there remain ways to reduce Japanese knotweed waste charges and Phlorum offers advice on how to make the most of these. Of particular interest to many clients is the land remediation tax relief scheme, where significant sums can be saved when treating Japanese knotweed contaminated sites. Further details on this scheme can be found on the information note produced by Feist Hedgethorne Chartered Accountants, which is available for download here.

Phlorum can also create packages that combine treatment options in order to save money depending on our clients’ needs.

Biological Control

In invasive species management, biological control involves the introduction of a “pest” species that will attack and control, but not necessarily kill, a target “host” species. Examples of successful biological control include the introduction of the Argentine cactus moth to control the invasive spread of prickly pear cactus in Australia and the use of the myxomatosis virus to control rabbit populations in the same country. Absolute host-specificity is key to successful biological control and this can be very difficult to identify and achieve. If an introduced pest species affects organisms other than the target host, then valuable native species and habitats can be put at risk. It is also difficult to extrapolate from laboratory and field trials to determine if an introduced pest will actually have a significant effect on the target species in the wild.

In March 2010, following a public consultation exercise, Defra approved the release of a sap sucking insect (Aphalara itadori) from Japan to biologically control Japanese knotweed in the UK. The release was initially focused on a few field trial sites, in order to gauge the effectiveness of a potential wider release. It has not yet proven to be a success. However, even if this method of control becomes useful, the best predicted scenario is that Japanese knotweed will be reduced to a background level in the wild, and biodiversity in areas affected by the plant will improve. Nevertheless, it is highly unlikely that the law will change with regard to allowing Japanese knotweed to spread and it will probably remain a significant problem on development sites and at existing commercial and residential properties.