• Japanese knotweed (Fallopia japonica)

Japanese knotweed (Fallopia japonica)

Japanese Knotweed in an invasive plant which originated in East Asia and is known as ‘the most pernicious weed in Britian’. It is a rhizomatous perennial that can colonise most habitats such as river banks, woodlands, grasslands and coastal areas. It does not set seed, but rather spreads via its roots and rhizomes, which can grow through the smallest cracks in engineered surfaces and building structures and break them apart. In addition, the plant can regenerate from any small piece of knotweed material, from root, rhizome, crown or stalk, so it can easily be spread over a site if the plants are disturbed.

The danger from Japanese knotweed is to built structures and surfaces, and consequently land values. There is no danger to the health or safety of humans or animals. It can grow through walls, tarmac and concrete and causes developers major problems on brownfield sites and causes damage to river and canal banks.

It is notoriously difficult to eradicate. Treated Japanese knotweed can seem to have been dealt with effectively, with no above ground growth, but the rhizome can remain dormant in the soil for some years and start to grow again if it is disturbed.

Excavation and disposal is the Environment Agency’s least preferred option as it involves moving contaminated waste and uses up valuable landfill space. Although this method has the advantage of removing Japanese knotweed contamination rapidly, it is only effective if soil is excavated until no more rhizomes are noticed and then to at least another half metre vertically and laterally. This means that a considerable volume of soil needs to be removed.  This can be costly.

Excavation and burial on site is another option and is cheaper than removal to a licensed landfill but is unnecessary if there is space and time to treat the plants over a longer term. If the contaminated soil is to be buried on site, it needs to be encased in a root-proof membrane and buried ideally under 5m of uncontaminated soil or 2m of soil and a full concrete slab.

Treatment with herbicide is the most cost effective option and can be combined with excavation and removal to another area of site for ongoing treatment. In fact, disturbing the plants encourages growth and so renders the herbicide more effective as there will be more leaf area to absorb the toxin. The Environment Agency advises that 3 to 5 years should be allowed for eradication of Japanese knotweed, but also points out that the plants can appear dead with no re-growth yet shoot again if they are disturbed, for example by excavation for a development. If the contaminated soil is to be excavated and moved, it is important that care is taken not to spread Japanese knotweed material over other parts of the site.