The Important Facts On Japanese Knotweed, The Gardener's Enemy

Japanese Knotweed is unusual in that it has three different scientific names - Fallopia japonica, Polygonum cuspidatum and Reynoutria japonica. The name Reynoutria japonica is its oldest name and was given to the plant in 1777 by the Dutch Botanist Houttuyn. This original name was then lost for over 100 years and the plant was named Polygonum cuspidatum in 1845 by Siebold and Zuchharini. The scientific name most used today is Fallopia japonica.

Other names for the Japanese Knotweed include Mexican bamboo, American bamboo, Japanese bamboo, fleeceflower, sally rhubarb, donkey rhubarb, pea shooters, elephant ears, monkeyweed, Hancock's curse, Huzhang, crimson beauty and wild rhubarb.

The Japanese Knotweed is a member of the Polygonaceae family of knotweeds whose members also include rhubarb (rheum), sorrel (rumex) and buckwheat (fagopyrum). It is a large, herbaceous plant and is a perennial. It can grow up to 2-3 metres high and it has hollow stems reminiscent of bamboo. Its leaves are oval in shape and its flowers, which are produced in late summer and early autumn, are a creamy, white colour.

It is native to Korea, Taiwan, China and Japan and its root extract is used in Chinese medicine. It was introduced to Europe and the USA in the 19th Century to feed grazing animals, as an ornamental plant and to prevent soil erosion. Unfortunately, it was found to be very invasive and is now considered a weed in Europe and the USA. In the UK, it is actually against the law to spread the plant (The Wildlife and Countryside Act 1981) and it has been labelled as one of the top 100 most invasive plants in the world by the World Conservation Union. It is also now causing problems in Australia and New Zealand and so is a widespread weed.

It can cause a great many problems including:-

1. Damage to pavements, roads, drains and even buildings - it manages to push its way through the smallest of cracks.
2. Damage to flood defences.
3. Damage to graveyards and historical buildings and archaeological sites.
4. Taking over river banks and preventing access to rivers.
5. Taking over and stopping native plants from growing.
6. Causing land values to plummet because land is covered by knotweed.
7. Erosion - removal of knotweed can cause soil erosion and the collapse of banks.
8. Expense - Japanese knotweed is tricky and expensive to get rid of.
9. It spreads easily and quickly.

How does Japanese Knotweed Spread? Underground, the Japanese Knotweed has extensive rhizomes, which are horizontal stems or roots with nodes that send out shoots. With the knotweed, these rhizomes can extend up to 7m away from the main plant and send out new shoots to make new plants. When old stems and leaves from the Japanese Knotweed die off, they slowly decompose and make a layer of thick mulch, which seeds from native plants find impossible to penetrate or germinate in. So, not only does a knotweed spread far and wide from its rhizomes, it also prevents native plants from growing in its area - a very clever and effective invasion. Rivers and flooding also help it to spread by washing parts of the plant to new areas where they can take hold.

It is against the law to aid the spread of Japanese Knotweed in the UK, but people do not even realise that they are helping the spread of this weed. People cut it down or dig up rhizomes and do not dispose of the rhizomes and fragments of the plant properly. It is thought that Japanese Knotweed has spread in urban areas because of fly-tipping and because people do not dispose of garden rubbish properly, even the smallest fragment can produce new shoots.

Although the Japanese Knotweed is a clever plant and can look pretty in pictures, it is a nuisance and needs to be dealt with by professionals for a long-term solution. Do not let this plant take hold in your garden or on your land.