Once the area has been treated with the herbicide, you can cultivate the top 0.25m of the ground. By doing this you will stimulate the rhizomes to produce more growth. This can then be treated again using the same procedures as outlined in the Herbicide treatment section of the website. By doing this you are able to encourage the plant to exert more energy re-growing which, in turn, gives you a larger leaf area to spray. These leaves will absorb the weedkiller and carry it down into the rhizome. This can reduce the amount of time required on site spraying the Japanese Knotweed.
Use of controlled burning can help reduce the Japanese knotweed. By burning the stem, rhizome and crown material this can achieve a number of outcomes – there is less material to dispose of either by burial or removal from site and it reduces the chance of re-growth. We do not rely on this method as the sole way of getting rid of the plant as the Japanese knotweed is particularly resilient and is a first coloniser of volcanic molten fields.
As over 90% of the rhizome can be located in the top 0.25m of soil, sometimes it is cost effective to remove this entire layer from the site. When the Japanese knotweed starts re-growing this is then treated with a herbicide treatment as per directions in the Japanese knotweed management plan. This helps to minimise what is left on site that is viable to grow back. The rhizome that is left would only be able to sustain little growth and is easy to kill off with the herbicide treatment.
This is where the Japanese knotweed is excavated and then piled up on top of a membrane on another part of the site. This gives time for us to treat the Japanese knotweed meaning that the Japanese knotweed can be dealt with whilst other building work is carried out on other parts of the site.
The soil is mounded up – ideally to a maximum height of 0.5m. There is a restriction on height as if the soil is mounded too high then the rhizome will become dormant. What we want is to encourage further growth so that it can be treated with a systemic herbicide. If the contaminated soil is being put in an area that has not been previously been infected by the Japanese knotweed then you must use a suitable membrane or root barrier to stop further contamination of the site.
Stopping Japanese knotweed encroaching from neighbouring land
This can be achieved by digging vertical trenches, usually the depth of 3m, and then laying a root barrier membrane. To support the membrane a plywood frame can be built. The vertical root barrier membrane should be guaranteed for 50 years and will act as a suitable barrier to the rhizomes. We would, however, recommend backfilling the trenches with building sand to ensure that no stones, glass or other debris are present that could damage the membrane.
If services such as pipes and conduits need to be constructed in an area that is contaminated with Japanese knotweed, it can be more effective to protect the services or the building’s foundation than to protect the whole site. It is very important that any soil contained by the root barrier must be free from Japanese knotweed. The surrounding infestation can then be treated as per the Japanese knotweed management plan.