Since being introduced as an ornamental plant in the 19th century, Japanese Knotweed has spread rapidly as a wild plant in Britain and Europe. The lack of natural predators has enabled it to smother native plants, reducing the bio diversity of delicate habitats. The rapid growth habit and strong root system can cause severe damage to buildings, roads, drains and other structures.
Legislation obliges landowners to prevent the spread of the weed on to neighbouring property, and classifies cut or dug material as controlled waste that can only be disposed of at specified landfill sites.
The law also prohibits construction and development on infested land.
1. Spray Programmes
The most cost-effective method of control is through regular applications of systemic herbicide to the growing plant, which eventually exhausts the perennial roots. Control by this method may take many years, and so this method is only suitable for reclaiming land such as parks, amenity sites and gardens where the soil will not be disturbed.
2. Stem Injection
A newly developed technique involving the direct injection of herbicide in to the base of the plant. This is more time-consuming and costly per application than overall sprays, but has the advantage of leaving neighbouring plant life unaffected. This method is more suitable for smaller stands of the weed, and for environmentally sensitive sites. Control can also be achieved more quickly than by conventional spraying.
3. Excavation and Burial
An option on development sites, this involves complete excavation of the plant material and roots, to a depth of up to 5 metres, and burial in a sealed and bunded receptacle area on the site itself. This method has the advantage of relative speed, allowing infested sites to be reclaimed for development in a matter of a few weeks, subject to Environment Agency approval. The cost is much higher than for spraying or injection. Followed by a programme of site monitoring and remedial spraying where necessary, excavation can lead to total long-term control.
4. Excavation and Removal
This method also involves excavation, but on sites where burial is not practical, the only option is removal of the excavated material to landfill. Because of the large volume of material to be moved and the high cost of disposing of the waste, this is by far the most expensive option, although it is also the fastest.