Japanese Knotweed

ByKyle Marsh

Japanese Knotweed

Asian knotweed or Japanese knotweed is a large, grassy herb perennial plant of the knotweed family. It’s native to East Asia in Japan, Korea and China.

Although very lovely to look at, Japanese knotweed is a real vandal as it spreads wildly. In winter, the plant dies back below ground but by early summer the bamboo-like stems shoot to over 2.1m (7ft), overpowering all other growth.

Fallopia japonica more familiar Japanese Knotweed was transported from Japan to Europe during the mid 19th century by a chap called Philipp von Siebold. A German botanist, who found Japanese knotweed growing around the outer rim of the volcanos.

By 1854, the plant had been sent to the Royal Botanic Gardens in Edinburgh and was then sold commercially by nurseries.

During 1850, the Botanical Gardens Kew in Richmond was presented with various plants from Siebold from his travels. The head gardener read a note that was attached to the box containing the plant it read, “Plant this Japanese knotweed and see how quickly it grows, this plant may be used for planting along railways to help prevent landslides.”

The gardener at Kev subsequently sent samples of Japanese knotweed out to fellow botanist across the country. They too were very impressed with the plant’s ability to grow at a fast rate.

Sometime around 1854, Japanese knotweed was sent to Edinburgh’s Botanic Gardens and sold to the general public through nurseries.

Japanese knotweed went undiscovered and evaded us for many years, through steadfast planting.

Scientists, at the University of Leicestershire, said, “People using cuttings and disposing of Japanese knotweed was the main reason for the spread”. Japanese Knotweed was escalating through the water courses of Britain, and also from contaminated soil used in construction and road building.

Fly tipping and animals were also responsible for spreading Japanese knotweed.

Japanese Knotweed professional Ann Connolly, who passed away in 2010, discovered examples of Japanese knotweed being planted deliberately near gardens. In and around the valleys in the Walsh coal-mining towns in the 1960s/1970s because it was excellent at supporting loose soil.

Japanese Knotweed, starts to die off around the end of September, the plant goes into hibernation over the cold winter months. The only evidence that the plant was there, is the bamboo brown canes and dark brown crowns. New asparagus type shoots begin to re-emerge in late February, by the end of June the Japanese knotweed can have reached 2 meters’ high. By the end of September, the plant can have grown as high as 3 to 4 meters.

According to DEFRA, this is what we should be looking for when trying to identify Japanese Knotweed……

  • February: Chubby light red shoots as it first appears from the ground (asparagus in appearance).
  • April to June: Big spade or heart-shaped leaves that will be light green in colour.
  • The Japanese Knotweed leaves will be carried on a short zig-zag stem attached to the main canes.
  • Bamboo like canes, light green in colour with pink flecks running vertically. The thickness of the canes can vary depending on the age of the plant and conditions. Some canes have been noted to be more than 2 inches in diameter and more than 50 years old.
  • Crowns are thick clumps, which are usually sat on the surface of the soil. Usually, the canes have been snapped out of the crowns, leaving lots of holes in the clump.
  • Japanese Knotweed produces pale, creamy coloured Nectar-rich flowers usually around the end of July, beginning of August which entices bees.
  • The Japanese Knotweed dwindles back during the autumn months leaving stems that are brown in colour.

Why is Japanese knotweed so troublesome?

In its homeland of Japan, where it grows in the volcanic landscape. The weather conditions and regular deposits of volcanic ashes tend to keep the Japanese knotweed plants relatively small. In addition, there are many types of insects that also eat Japanese knotweed. The plant can only survive thanks to its deep-rooted energy stores in the rhizomes. It should also be noted, that Japanese knotweed plants can be cross-pollinated in Japan, as there are male and female plants. All of the plants in the UK and Europe are female, the seeds produced by the plant are none viable and will not grow.

In Britain, Europe, and other parts of the world, Japanese knotweed has grown in intensity and strength. In the middle of the growing season, depending on the conditions, Japanese knotweed can potentially grow up to 15-20cm every day. Currently, there are no natural predators that can eat the plant and keep it under control in the UK.

Japanese knotweed rhizomes can potentially grow up to 7 meters in all directions from the main plant. As well as up to 3 meters down, the rhizomes can grow through damaged concrete/ foundations and badly laid tarmac paths.

With no other natural devourer, this allows the weed to grow undiminished with overwhelming force. Thus, hindering all other plants getting any light to them, upsetting the ecology of the area. Japanese knotweed doesn’t produce any viable seeds, it will grow from a very small fragment of the rhizomes. Beneath the surface of the ground, there is a network of stems and roots, meaning it can easily spread.

You might be thinking, it’s a good idea to dig the Japanese knotweed out of the ground. Bear in mind, if you leave just a tiny trace of the rhizome system, it will grow again. It takes just a small 0.8g fragment of rhizome and you will have a whole new plant.

Be careful, once Japanese knotweed has been dug up you have to dispose of it in the correct manner. Under the Environmental Protection Act 1990, Japanese knotweed is classified as controlled waste and it can only be discarded at authorised landfill sites. You may be able to dry it all out, then burn it. If you decide to bury it, the hole must be at least 5m deep, to prevent regrowth. For most gardeners, this depth would not be practical.

In 2010, experts introduced a Japanese bug, Aphalara itadori, to the UK, that feasts almost exclusively on knotweed.

It’s hoped this will become available to gardeners if it works. One of the main drawbacks, in this country, is the weather, the Aphalara Itadori prefer warmer weather. It may survive better down south, where the climate is warmer.

You can turn to chemicals, especially treatments containing glyphosate. But beware, it can take up to five years treatment to finally kill it off. If you don’t use a professional treatment, it can set you back thousands of pounds.

Want to find out about Japanese Knotweed Removal Costs? Read our article!

About the author

Kyle Marsh administrator