The spread and growth of Japanese knotweed is why it is classed as an invasive weed

Japanese knotweed (Fallopia japonica) is a highly invasive plant species that is native to Asia but has spread to many parts of the world, including North America and Europe.

It is known for its rapid growth and ability to outcompete native plant species, making it a major problem for landowners and conservationists. But where did this invasive species come from, and how did it spread to other parts of the world?

In this blog post, we will explore the history and origins of Japanese knotweed.

Introduction to Japanese Knotweed

Japanese knotweed is a perennial plant that can grow up to 10 feet tall and has bamboo-like stems with large, heart-shaped leaves. It is often found growing along riverbanks, in wetlands, and in other disturbed areas.

The plant produces large clusters of small white or cream-coloured flowers in the summer, and it spreads through underground rhizomes (horizontal roots) that can extend up to 20 feet away from the main plant.

Japanese knotweed is incredibly difficult to control and eradicate once it becomes established in an area. The rhizomes can survive in the soil for up to 20 years, and even small fragments of the plant can grow into new plants. This makes it a major problem for landowners, as it can damage foundations, walls, and other structures, and it can also make it difficult to develop land or sell a property.

Leaves of the Japanese knotweed plant reach across and shading any other vegetation
Leaves of the Japanese knotweed plant reach across and shading any other vegetation

The Origins of Japanese Knotweed

Japanese knotweed is native to Japan, China, and Korea, where it has been used for centuries as a traditional medicine and food source. The plant was first described by Japanese botanist Philipp Franz von Siebold in 1835, and it was later introduced to Europe and North America in the late 19th century as an ornamental plant.

It was first introduced to the United Kingdom in 1825 by the Victorians as an ornamental plant, at the Royal Botanic Gardens, Kew. Knotweed was also sold in nurseries as a garden plant and was used as an ornamental plant in many gardens and parks.

The Spread of Japanese Knotweed

Japanese knotweed quickly escaped from cultivation and began to spread throughout Europe and North America. The plant’s ability to survive in a wide range of conditions and its ability to spread rapidly through underground rhizomes made it well-suited to colonize new areas.

In the UK, it spread rapidly due to its ability to colonize river banks and canal sides, as well as its ability to grow in a wide range of soil conditions. The plant was also spread by the movement of contaminated soil and by the disposal of garden waste.

In the United States, it was first reported in the wild in the late 1800s and has since spread to most of the eastern and western states. It is particularly prevalent along the East Coast and in the Pacific Northwest.

Japanese Knotweed as an Invasive Species

Japanese knotweed is now considered one of the most invasive plant species in the world. It can outcompete native plant species and change the structure and function of ecosystems. It can also cause significant damage to infrastructure, including buildings, bridges, and roads, as well as making it difficult to develop land or sell a property.

In the UK, Japanese knotweed is listed as an invasive species under the Wildlife and Countryside Act 1981, making it an offence to plant or cause the plant to grow in the wild. Similarly, in the United States, it is listed as a noxious weed in several states, and it is illegal to transport, sell, or buy the plant.

Japanese knotweed spreads as such a rate that it dominates everything around it
Japanese knotweed spreads as such a rate that it dominates everything around it

Control and Eradication of Japanese Knotweed

Due to the invasive nature of Japanese knotweed, control and eradication efforts have been ongoing for many years. However, due to the plant’s deep rhizomes and ability to regrow from even small fragments, completely eradicating Japanese knotweed can be difficult and costly.

There are several methods for controlling and managing Japanese knotweed, including:

  • Chemical control: Using herbicides such as glyphosate or triclopyr can be effective in controlling the growth of Japanese knotweed. However, multiple applications may be required, and care must be taken to avoid harming non-target species.
  • Physical control: Cutting or mowing the plant can help to control its growth, but it will not kill the roots. Digging or excavating the plant can be effective if all the rhizomes are removed, but this can be costly and difficult in areas where the plant has spread widely.
  • Biological control: Researchers are currently exploring the use of insects or pathogens as a method of controlling Japanese knotweed. A psyllid insect (Aphalara itadori) that feeds on the plant’s leaves has been shown to be effective in controlling its growth, but it has not yet been approved for release in the United States.
  • Combination of methods: The most effective control strategy for Japanese knotweed is often a combination of chemical, physical, and biological control methods. This can include a combination of cutting, treating with herbicides and using insects.
Japanese knotweed knows no boundary as it passes from one property to the next
Japanese knotweed knows no boundary as it passes from one property to the next

In conclusion

Japanese knotweed is a highly invasive plant species that has spread from its native range in Asia to many parts of the world, including Europe and North America. The plant’s ability to outcompete native plant species and cause damage to infrastructure makes it a major problem for landowners and conservationists.

While control and eradication efforts have been ongoing for many years, the plant’s deep rhizomes and ability to regrow from small fragments make it difficult to completely eliminate. The most effective control strategy for Japanese knotweed is often a combination of chemical, physical, and biological control methods.