Karen Harper, LISI Manager
By now, we should all be aware of the potential impacts that Japanese knotweed (Fallopia japonica), an invasive non-native species, can have on local biodiversity, on built structures and on our local amenity. It is important to know how to identify it throughout the year. This task becomes more difficult in winter when it has lost its distinctive leaves and flowers.
Why do we need to identify it in winter?
Knotweed rhizomes stay dormant during the winter and can easily be spread if the soil and rhizomes are disturbed or moved. This can then lead to unexpected new infestations coming up in the spring/summer.
How do you manage this?
Unfortunately, you cannot treat Japanese knotweed until it starts to grow again in spring and summer. To reduce the risk of its spreading, it is best not to disturb any site where knotweed is present. As the rhizomes can be present up to seven meters away from the visible stems, it is best to leave a seven meter buffer if you need to disturb the soil in a Japanese knotweed infested area.
How to identify Japanese knotweed in winter
During winter, all that you will be able to see on site will be orange/brown coloured woody stems with the fallen material causing a dense layer at the base of the plant. The stems can retain their ‘zigzag’ appearance which is a good way to identify the plant at any time of year. It also retains quite a distinct growth form with the base of the individual stems being dense and the upper vegetation falling outwards.
(click images to enlarge)
Photograph shows upper vegetation falling outwards, with dense fallen plant matter at the base.
Photograph shows zigzag growth pattern of Japanese knotweed stems.
What to do next
Report any Japanese knotweed to LISI and GiGL using the online form or email enquiries@londonisi.org.uk. Now is also a great time to start planning management for spring and summer when treatment can be effective. Identification sheets and treatment information will soon be available on the LISI website. A great source of guidance is the Environment Agency’s ‘The knotweed code of practice’.
Karen Harper moved to London from Australia a year and a half ago to take up the post of LISI manager. Karen has recently received a lot of calls about Japanese knotweed in neighbours’ gardens. She advices the best way to address this is to introduce both yourself and the plant to your neighbour. Most people don’t know it’s in their garden and are keen to manage it when they realise the problems that it can cause.