What is Japanese Knotweed? 

Japanese Knotweed (Fallopia japonica) is a herbaceous perennial plant. This species is extremely common and widespread across Ireland. It is most prominent in areas of wasteland, roadsides, landfills, wetlands and riparian (river) areas across the entire country.

Where did it come from?  

Japanese Knotweed is native to East Asia in countries such as Japan, China and some regions of Korean and Taiwan. It is thought to have been introduced to Europe as an ornamental plant in the 1800s. Due to its value as an ‘ornamental’ plant, it was purposefully planted in gardens, public parks and forest areas which led to this species becoming widespread very quickly. It is now abundant throughout Europe – except for the South of Europe where it does not seem to have taken off.

Dispersal Mechanism

This plant disperses naturally by means of Rhizomes. These Rhizomes are modified stems that run underground at a horizontal angle. Rhizomes create new roots of their own nodes down into the Earth’s soil. This type of activity assists with reproduction of the plant. These Rhizomes can break off of the Japanese Knotweed plants and travel across the environment by natural methods such as wind carrying the Rhizomes or river flow. Additionally, birds can assist with travel of Rhizomes as they carry them and can accidentally drop them into a new environment, helping the spread of this plant. Rhizomes can also stick to the clothing of humans that hike through the areas where Japanese Knotweed is growing and can fall off at any stage if loosened, which means the spread is also assisted by humans. This is just one example of how humans can assist the spread of Rhizomes, but there are endless ways in which we do so without even realising!

The above image shows the regeneration of Japanese Knotweed Rhizomes.

Influence on the environment

Japanese Knotweed has a huge impact on its surrounding environment. This species of flora negatively impacts the supporting services of the environment that it has invaded. Supporting services refer to the services provided by the environment to maintain a habitat that can be used by native species. Japanese Knotweed interferes with the supporting services by transforming species diversity and by altering the physio-chemical properties and structure of the environment.

This plant out-competes native plant species in Ireland by restricting light availability. This is done by the Japanese Knotweed growing higher and larger than native plant species and consuming all the available light first, while lower down shrubs and plants do not reach the sunlight. This results in native species dying back and allowing the Japanese Knotweed to thrive even more.

Management & Control

The management of this invasive species is extremely expensive and very labour-intensive. There are many different types of controls put in place in European countries (including Ireland) that face this invasive species. Chemical controls such as the use of herbicides and mechanical controls such as digging up the plants and burning them. However, the use of herbicides can be damaging to the environment, as can removal of the roots from the soil physically, as a lot of disturbance can be caused to the environment.

Biological control has been talked about for this species. This involves introducing a non-native insect into Europe to eat the leaves of the Japanese Knotweed to suppress its growth. The reason it thrives so well in new environments is because there is no insect that feeds on it in these new environments. Therefore, by introducing a natural insect that will suppress its growth by feeding on the plant, we are managing it with little labour or cost put in place rather than impossibly trying to eradicate it.

Regulations 49 & 50 of the European Communities (Birds & Habitats) Regulations 2011 make it an offence to plant, disperse or allow dispersal of Japanese Knotweed in Europe. It is also an offence to own this plant in Europe under these regulations.