Japanese Knotweed

August 2006

Japanese Knotweed has recently made the national press with news that the Environment Agency is to launch an approved code of practice for its Management, Destruction and Disposal.

Fallopia Japonica, more commonly known as Japanese Knotweed (JK) was first introduced to the UK from Asia during the 1800s. It has a rigorous growth potential and is able to regenerate from rhizomes buried at depths of 1 metre, penetrate asphalt 5 cm thick and damage concrete structures. It has bamboo like stems and can grow to heights of between 2-3 metres. The rhizome system from any ‘parent’ plant may spread up to 7 metres laterally and to a depth of 2 metres. Very small fragments of rhizome or fresh stems can produce viable plants in as little as 6 days. JK is a dioecious plant, which means that you need male and female plants for spread via germination to occur. However, in Europe only female plants are found and spread is via vegetative means. This can occur naturally in riparian areas or more commonly by human activity such as fly-tipping or transportation of contaminated soil. The invasive weed forms dense clumps and through competition and out-shading of native species biodiversity is reduced.

The mechanism of disturbance and spreading means that it is commonly found at sites where Victorian waste has been deposited and subsequently disturbed, along railway lines, riverbanks and roads in graveyards and at derelict sites. The BBC recently reported that the cost of its removal at the 2012 Olympic sites in East London could run into hundreds of thousands of pounds.

Across the UK the species occupies more than half of the 10 km squares used to map plant species and is only absent from the Orkney Islands. It has spread across much of mainland Europe, is found in many States in the USA and Canada and has also been reported in New Zealand.

The Wildlife and Countryside Act (1981) / Wildlife (Northern Ireland) Order (1985) made it illegal to spread JK. Its propensity for rigorous growth is a potential risk on development sites, where its presence can damage structures and impede landscape developments. Early identification of JK allows a developer to include options and costs for disposal and management so as to avoid excessive cost, potential prosecution and prevent physical damage to buildings and hard surfaces. Treatment can be a lengthy and costly process and may take many years to achieve complete eradication.

Herbicide is most effective when applied between July and September, but may take several years to be effective. Early treatment is recommended at a development site prior to any disturbance to reduce the vigour of growth. The choice of herbicide is primarily dependent upon the proximity to controlled waters and whether the soil is to be used for replanting, as only certain chemicals are permitted near water courses and some are persistent and may impact on future planting.

Extreme care is required in conjunction with treatments involving Excavation & Disposal to ensure that all equipment used on site is free of JK before leaving the site to reduce the risk of vehicles carrying knotweed containing soils on the wheels or bodies of their vehicles and that the vehicle is suitably covered or enclosed to prevent escape during transport.

Excavation is commonly carried out during the winter months and re-growth treated chemically during the spring and summer. The majority of viable rhizome exists in the upper layers of topsoil, which can be scraped into a pile by an excavator. The exposed ground is then cultivated to a depth of 50cm, to stimulate rhizome generation and the piled material re-spread over the cultivated area. This process causes the rhizome to produce a higher density of stems, which renders it more vulnerable to herbicide treatment.

Any excavated soil from sites where JK has established must be disposed of at an appropriately licensed waste management facility and not re-used in further construction or landscaping. During disposal the operator of the waste management facility (licensed landfill site) must be made aware of any consignments containing JK and must ensure that the waste is buried at a depth of at least 5 metres. Section 34 of the Environmental Protection Act (1990) places a ‘duty of care’ on all waste producers to ensure that any wastes are disposed of safely. It is possible that materials contaminated with JK will be liable for Landfill Tax exemption.

At Terragen our consultants are experienced in identifying Japanese Knotweed and incorporating its management, destruction and disposal into remediation strategies.