Japanese Knotweed

Japanese Knotweed, as the name suggests, came from Japan. It was introduced to Europe by a German born botanist called Phillip Von Siebold who discovered it growing on the sides of volcanoes in Japan and thought it would make a good plant for Victorian gardeners.

In Holland in 1847 it won an award as the most interesting new ornamental plant. In 1850 Phillip Von Siebold sent some to Kew Gardens. By 1854 it had reached Edinburgh and was being sold from plant nurseries. In the 1960s and 1970s Japanese Knotweed was planted in South Wales to stabilise steep ground in mining areas.

It is an attractive herb with big green leaves, hollow red stems (which children can use as pea shooters) and dangling white flowers that produce plenty of nectar for bees. The young shoots are edible. It dies back every autumn and all that remains is the extensive network of rhizomes under the ground. These can be the size of an underground tree which makes it easy to see why the plant is so difficult to eradicate. All the Japanese Knotweed in Britain is a clone of infertile male plants and it cannot produce seeds. This means that the only way that it can spread is through human intervention. The foliage can be killed by glyphosate but this systemic herbicide cannot reach all of the extensive root network. Even small pieces of rhizomes can grow into a new plant and so careless attempts to remove it will lead to it spreading. By law it can only be disposed of at licensed landfill sites where it must be buried below 5 metres. It is a notifiable plant and if you do anything that spreads it you are breaking the law.

In Britain it does not have natural enemies - this is a common tale for invasive plants. If a plant or indeed animal is transported from its ecosystem to another part of the world there may not be the usual constraints. Scientists have studied what controls Japanese a Knotweed in Japan and there are about 200 candidates including fungi. There are plans afoot to use biological control via the good offices of a Psyllid bug whose Latin name is Aphalaria itadori. Rigorous tests have been undertaken to try to ensure that the insect will only attack Japanese Knotweed and not other plants. There are many instances of ill thought out biological control going wrong and the introduced predator reading havoc on unintended species.

The reason why Japanese Knotweed is thought of a such a threat is because of its potential damage to buildings. This is probably exaggerated - researchers at Leistershire University say that it cannot grow through concrete although it can bulge tarmac and exploit cracks between bricks.

There are a couple of places where Japanese Knotweed occurs at Rannoch - near the dam at Kinloch Rannoch and at Dall. It also occurs along the river Tay near Aberfeldy and at a site near Strathtay.

Mark Purrmann-Charles is the Scottish Invasive Species Initiative project officer for the Rivers Tay and Esk. His email is markeskrivers@gmail.com

Should you come across Japanese Knotweed, Giant Hogweed or Himalayan Balsam you could contact him to let him know the location.

Photo - Creative Commons - credit MdE