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Japanese knotweed

Victorian plant hunters, in particular Philip von Siebold, were responsible for bringing many plants back to Britain from all over the world in the 1800s. Several of them have since had an uneasy relationship with gardens and gardeners and more so with conservationists trying to protect the natural world but Japanese Knotweed must top the bill of noxious introductions. Outside its natural environment of Japan and China and parts of Korea and Taiwan it has rapidly become a challenging problem. As an herbaceous perennial it was introduced as an ornamental plant for the increasingly popular culture of suburban gardens. This stout, shrub-like plant forms large dense clumps over a metre in height. It reproduces by seed and by large rhizomes which may grow up to 5m in length underground and they are incredibly tough. It has the ability to destabilise building foundations by growing through walls, paths, fences and flood defences; as a consequence it can be very difficult to eradicate. The stems are reddish in colour, ridged, jointed and hollow. The leaves are alternate on the stem, broadly truncate at the base and up to 7cm wide. The leaf veins are often reddish and the petioles are long and ridged. The flowers bloom in late summer and are small and greenish white. It isn’t a particularly good-looking plant so how it became so popular is anyone’s guess!

Japanese knotweed is very difficult to kill when growing strongly due to the persistence of its extensive underground rhizomes and their ability to grow even with limited nutrients. It was dug up from volcanic ash near Nagasaki, where it thrived in the lava and poisonous gases. Rhizomes can also remain dormant in the ground for many years. Some companies offer the services of “Knotweed detection dogs” who have much better olfactory senses than humans and can sniff out even dormant rhizomes hidden underground.

What is surprising after all we know about its thuggish behaviour, is that it’s not illegal to grow it but there is an obligation in law to let your estate agent know you have it when selling your property. You are prohibited to allow it to spread from your land onto other land and the Council is subject to the same Japanese knotweed laws as the public so they are prohibited to allow it to spready from public land onto privately owned land. In that way it has to be controlled.

The law comes down heavily on you if you fly tip it or any material that contains the plant; it is a criminal offence that can be punishable with up to 2 years in prison and an unlimited fine. However fly tipping is notoriously difficult to pin onto an individual or organisation so prosecution is only likely if they are caught red handed. This can be done by calling the Environment Agency on their 24-hour free number 0800 80 70 60.

There is some very good advice and in-depth information on all aspects of its culture and the law from Knotweed Help in Liverpool. Liverpool must be a hot spot for Japanese knotweed judging by the lengths they go to explain the ins and outs of the law surrounding it.
Thankfully in Bury we have very little of it and it’s mostly under control. Garden centres and nurseries do not sell it anymore and it will soon be confined to the history of the very singular habits of Victorian England.

Jillian Macready, November 2021