Combined Methods

Excavation, herbicide treatment and the installation of membranes can be combined in different ways to produce more cost-effective, sustainable and quicker results. Below is an outline of the combinations most commonly used by Wreford but this is not an exhaustive list. The maturity of the knotweed infestation, its location, the considerations of the site, the budget available and, on development schemes, the schedule of the construction project will all be factors in deciding which treatment options we feel are most feasible for each individual contract.

Mechanical Disturbance
Combining controlled mechanical disturbance of knotweed stands with a standard herbicide programme can shorten the overall length of such a programme. By breaking up some of the rhizome structure, the quantity of nutrients reaching the knotweed stems is reduced and the knotweed will respond to this operation by producing new growth from virtually every separate piece of the newly fragmented rhizome. This enables more herbicide to reach a greater percentage of the rhizome and, with the knotweed being in a slightly weakened state, a higher rate of kill in the first season of treatment can be achieved. Further reductions in programme length can be obtained by combining mechanical disturbance with Wreford’s ‘advance and treat’ herbicide system.

Excavation and Relocation
On large sites, or phased developments where one section of the site will remain untouched for a suitable period of time, excavation and relocation can prove both a cost-effective and sustainable solution. Japanese knotweed is removed from areas where it impedes the schedule of the development works and relocated to a more convenient section of the site, where it can either be incorporated into a bund, which will be landscaped on completion of the treatment programme, or spread into a temporary stockpile of a maximum depth of 1m (and an optimum depth of no more than 0.5m). The knotweed will be allowed to grow within the mound or bund and will be treated by means of a structured herbicide programme. With a stockpile, generally the shallower the mound the quicker eradication will be achieved. Implementing Wreford’s ‘advance and treat’ herbicide method on a shallow stockpile can speed up the timescale still further.

Please note that the Environment Agency will not allow such treated material to be removed from site at the end of the programme unless it goes to landfill as knotweed-infested waste, so such material must be seen from the outset to be part of the finished development scheme – either as a landscaping feature or as soil to be spread across certain areas of the site. We therefore recommend that on any site where treated material is re-used in this manner, the implementation of an annual monitoring scheme is highly advisable. Wreford recommends conducting annual

Reduced Dig
There are circumstances where a full excavation is not always necessary and only a partial excavation of the knotweed is required. The remaining knotweed material is treated with residual herbicide (where appropriate) and contained within specialist root barrier membranes (in-situ capping). In some cases such operations will also involve ‘mop up’ herbicide programmes and annual monitoring.

Crown Scrape
A cross between reduced dig and mechanical disturbance, a crown scrape will see the removal of the top growth, crowns and thickest layer of rhizomes from a knotweed stand. The remaining rhizome will produce much weaker growth and will therefore be more susceptible to herbicide control. Adopting Wreford’s ‘advance and treat’ system will make the herbicide programme even more effective, usually achieving results in less than 18 months. As with all operations where knotweed material remains on site, an annual monitoring programme is highly recommended.