Japanese Knotweed has recently made national headlines, as Network Rail were found liable in the Court of Appeal, for damage from untreated Japanese Knotweed spreading into neighbouring gardens. Japanese Knotweed is one of Britain’s most invasive plants and the prevention of its spread is a legal obligation for landowners under the Wildlife and Countryside Act 1981. It is difficult and expensive to manage but non-intervention is not really an option. Early treatment of a new colony is quicker, cheaper and preferable to leaving it to become established.
Japanese knotweed was introduced to the UK by the Victorians in the mid 1800s. The Royal Botanic Gardens at Kew have a record of receiving a plant specimen, sent unsolicited by a keen botanist overseas, on 9 August 1850. The earliest record of a nurseryman selling the plant was in 1854, when neighbouring Kingston nurserymen Messrs Jackson and Son started trading in it , hopefully not relatives of our Matt Jackson! Surrey’s famous garden designer Gertrude Jekyll even recommended the planting of a dwarf version in some of her designs, although she later rescinded, saying it should be “planted with caution.”
Well caution was perhaps an understatement, as the roots of Japanese Knotweed can spread up to 7 metres a year, and in severe cases can cause structural damage to houses, and regularly growing through tarmac. As a result most home conveyancing questionnaires specifically ask whether Japanese Knotweed is present, and if it is the usual requirement is for a Japanese Knotweed Management Plan to deal with it. This is something we are often asked to provide, either for mortgage companies or for a condition of planning consent for a new development.
We have been treating Japanese Knotweed for over twenty years, previously working through Wildlife Woodlands. The first question to be certain about is, is it Japanese Knotweed? Click here for a link to a good identification guide. In the past we have found Himalayan Balsam, Dogwood and even Broad-leaved Dock mistaken for Japanese Knotweed! If you do have Japanese Knotweed then early treatment is important and can save increased costs later. Treatment in situ is usually possible, providing you have time to treat over more than one season.
The most commonly used herbicides for treatment are glyphosate and triclopyr; picloram, once popular, is now withdrawn for use in the United Kingdom. In most situations our own preference is to treat with glyphosate in September, with the addition of a surfactant to help spread the herbicide and increase absorption. In September the plant is about to draw nutrients back into its root system, as it dies back for winter, and this helps absorption of the herbicide. Control is usually improved if the herbicide is applied to both the topside and the underside of the leaves. Repeat treatments are nearly always required, as the rhizome system is so extensive. Several year treatment is the norm, as in the second year onwards there is a much reduced leaf area and less herbicide is absorbed as a result.
Stem injection into the hollow stems is possible in sensitive locations e.g. adjacent to watercourses or on nature reserves, but even than some follow-up spray treatment can be required. We actually worked on some early trials for stem injection in north Cornwall; however the system is slow and expensive and not something we would normally recommend.
For developers requiring instant eradication then burial on site or to a licensed tip (if you can find one that still accepts Japanese Knotweed) is sometimes the only way forward. But burial requires use of a root barrier membrane and covering with a minimum of 5 metres of inert material or topsoil. A better solution, if room is to place the contaminated soil onto a single membrane, and spray regrowth on the bund.
INNSA (Invasive Non-native Species Specialists Association) maintain a list of national contractors, who are well placed to deal with large jobs. More locally Cornwall Council maintain a register of people specially trained to deal with Japanese Knotweed, which includes myself. The Environment Agency produced a very good Knotweed Code of Practice, although this has been withdrawn in 2016, as government wished to concentrate on regulation rather than providing advice.
But if you just have a small area of Japanese Knotweed in your garden, don’t panic, but get it treated as soon as possible. And make sure your neighbours do so as well, as it will spread back in again if some is left untreated. Plan ahead and treat early and you will keep your costs and liabilities down. Money is better spent on a small amount of herbicide than lawyers in court!