It finally happened. We found a house that suited all our requirements and while we were in the garden, the estate agent casually mentioned Japanese Knotwood. No worries I thought but my other half (who should have a medal for research) started coughing and we quickly left. Surely it can’t be that bad I thought but he shared the research!
Japanese Knotwood is native to Japan, China and Taiwan and was introduced to Britain by the Victorians in the 1800s as an ornamental garden plant. It is described as ‘the UK’s largest female’ as the male plant was never brought over, and all the Japanese Knotwood in the UK has reproduced a-sexually and spread from that import.
In its native countries, the growth is controlled by predators such as fungus and insects, but in the UK, Japanese Knotwood has no natural enemies which means it can spread. And it has done, becoming one of the most invasive weeds in the UK and one of the top 100 in the world. It has what is called a Rhizome root system where the roots are horizontal, below ground stems, which then branch out shoots and more roots. It can grow up through tarmac and other substances and it is capable of causing damage to foundations and brickwork.
And it also grows fast. During its growth period between May and October, it can grow 10cm a day up to 2-3 metres. Its roots can also spread up to 7 metres horizontally and 3 metres deep, making it difficult to get rid of.
In fact, Japanese Knotwood is so invasive that all parts of it are controlled waste and must be disposed of correctly. It is against the law to contaminate other land and other waste with it and if you knowingly have Japanese Knotwood on your land and allow it to spread onto neighbouring land, you can be legally prosecuted.
Jason Harker from Japanese Knotweed Experts says: “There are several methods of removing the plant from gardens including using strong chemicals, excavating and removing it, and permanently burying it. We can stop the spread onto neighbouring land by digging vertical trenches approximately 3 metres deep and then putting in a root membrane. This can be filled with sand or wood for protection and support and prevent the rhizomes from spreading.”
Top tips to recognise Japanese Knotwood in your garden are the new red/purple shoots that appear in Spring. As the leaves unfurl, they have a reddish tinge or are green with red veins. As we head into Summer, the leaves are heart or shield shaped with stems that are green and later develop purple speckles and in the late Summer, white or creamy flowers appear.
Make sure you do your research and don’t get caught out – either when buying or if you own a property. If you’re not sure, bring in an expert. Best to check and be informed than have something take over your garden and potentially your home – even if it is an attractive looking plant!
If you think you may have Japanese Knotweed, you may find some of these links useful:
If you are concerned that you may have Japanese Knotweed, or other nuisance plants.
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