Japanese knotweed (Fallopia japonica) (JPK) was introduced into the UK in the 1800’s as an ornamental garden plant. It is now widely spread throughout the UK and on the River Spey it is known to be present in the lower river from Ballindalloch downstream.
Highly invasive in nature Japanese knotweed is damaging to native biodiversity but it can have a huge economic impact due to its considerable reputation for damaging roads, buildings and other infrastructure. Indeed it can be difficult to obtain a mortgage for a property if there is Japanese knotweed present.
Despite being incapable of setting seed in the UK Japanese knotweed has still managed to become established in almost every county in the UK. Established stands are supported by an extensive underground rhizome system. In the spring new season growth can be seen where the previous year’s plants had died back. The reason why JPK is able to spread so rapidly is that is it capable of taking root from small fragments of root or shoot; even a thumbnaill size piece is capable of taking root. Small pieces of plant can be spread by natural processes such as river erosion but often if it human intervention that assists and promotes its spread.
Left to its own devices stands of JPK will spread slowly, expanding a little each year. However inappropriate management such as strimming will only encourage new growth and if cut stalks are allowed to fall into the water they will take root further downstream. Bad practice by those responsible for maintaining riverbanks, local authorities and gardeners has had a huge bearing on the spread of JPK.
A variety of control mechanisms are available but the only practical options for control on riverbanks are chemical spraying and stem injection. It should be noted that any chemical applications within 10m of a watercourse will require a SEARS licence.
Further details about Japanese knotweed can be found on the Invasive Species Scotland website.