Japanese Knotweed

Japanese Knotweed (Fallopia japonica) also known as Mexican Bamboo or False Bamboo, is an extremely invasive species that if left uncontrolled may adversely affect the value of your home. Although its appearance is somewhat aesthetically pleasing to the eye; brilliant heart-shaped leaves, bamboo stems and small, delicate white-flower tassels, the Japanese knotweed has a relentless killer instinct. This large, herbaceous perennial plant is of the Knotweed and buckwheat family Polygonaceae and grows at an alarming rate of up to 10cm per day in any type of soil. It forms dense clumps of up to 3m (10ft) in height. It is native to East Asia in Japan, China and Korea.

Japanese Knotweed is listed by the World Conservation Union as one of the world’s worst invasive species and has a plethora of imaginative english names, including fleeceflower, monkeyweed, Hancock’s curse, and elephant ears. It can grow up to an impressive 6.5 feet tall and 65 feet wide. It’s untiring and ferocious army of unseen roots cause the Japanese knotweed to spread out underground, forcing its way up through every concrete crack imaginable, even walls and floors cannot stop this unruly force of nature. The exceptionally invasive root system and unstoppable growth has been known to damage concrete foundations, entire buildings, flood defences, roads, paving, retaining walls and architectural sites. Another risk factor is that it reduces the capacity of channels in flood defences to carry water. Management and control of Japanese Knotweed truly is an ongoing process. In fact, it can take anywhere from three to five years to bring an infestation under control. What’s more, re-contamination can occur from even the tiniest fragment of leftover root. When it comes to digging up knotweed it is more of a large – scale evacuation than a mere matter of making a hole.

Non-native species of invasive plants have many transmission vectors, including biogenic vectors, however most invasions are correlated with human activity. Whilst there are no horticultural reports of animals dispersing propagules in the introduced range, it is possible that animals play a role in redistributing small pieces of rhizome. Vectors include plants or seeds imported for horticulture, Debris and waste associated with human activities, floating vegetation/debris, land vehicles, machinery/equipment, mail/post via internet plant sales, Soil, sand, and gravel.

In Japan, Japanese knotweed is managed naturally through a holistic combination of fungus and insects. Unfortunately, however, no natural enemies for Japanese Knotweed exist in the UK and Ireland, and it outcompetes all of the native species for light, water and nutrients.

When it comes to invasive species, it “Pays to be Proactive; Not Reactive”. For instance, vehicles should be inspected when moving from infected sites to new ones. Eradication of Japanese Knotweed comes with many roadblocks and risk factors. Unlike other common plants and grasses that we are all familiar with, Japanese Knotweed has a very zealous growing underground rhizome system. Whilst it is possible to eradicate Knotweed if a new infestation of rhizome is spotted quickly and the resultant plants pulled or treated before roots have become well established, however this is rarely the case. Usually upon discovery of this aggressive invasive plant in their property, people tend to under estimate the problem, not fully realising what they were dealing with. This causes them to panic and try to tackle the problem on their own by either cutting or strimming the aggressive invasive plant. Horticulturist warn that strimming and cutting is one of the major causes of spread in Ireland.

Eradication and where not possible, effective control of Japanese Knotweed can not only be difficult and costly, but it also requires careful planning and subsequent follow up treatments are necessary. One needs to also consider management and disposal of the dead plant material, and to the treatment of contaminated soils left behind. All cut or pulled stems of Japanese Knotweed should be handled with extreme caution as they can potentially re-sprout and cause further damage. They should be kept on site, or disposed of in a licensed landfill site that can implement deep burial.

Horticultural experts advise to prepare a management plan, and to get expert assistance before taking on any significant infestation of this invasive species. In other words, never ever attempt to control Japanese Knotweed yourself, seek professional advice first.

Japanese Knotweed has the unique ability to survive most herbicides. Even more worryingly, when these plants treated non-professionally with commercially available herbicides and shop bought weed killers they tend to die back for a short period of time before fighting the chemical and thus growing back stronger than nearby untreated infestations.

It is crucial when dealing with Japanese Knotweed to develop and produce a site-specific control/management plan with the help of a horticulturist.

‘Biosecurity’ refers to the protection afforded from the risks that invasive species afford to not only the environment, but also the economy and public health; through management, eradication, and control. Invasive Non-Native Species, sometimes called “invasive alien species” has a wide variety of negative effects including structural damage, aesthetic degradation, biodiversity loss, loss of land function, access restrictions and increased risk to human and animal health and safety. Biosecurity protocols such as detailed site investigation/mapping, non-native species monitoring And the placement of restricted access signs should be seriously considered. it is also worthwhile to consider potential pathways of introductions onto your site from elsewhere so that mitigation procedures can be put into place for biosecurity purposes.

There is a list of ‘usual suspects’ that are commonly mistaken for the infamous Japanese Knotweed.


Also known as Cornus, is a flowering tree that grows 20 to 40 feet tall and wide at maturity. The leaf shape of this woody shrubs and small/young trees can easily be confused as Japanese Knotweed to the untrained eye. One of the key differences lies in the stems. Knotweed stems are not at all woody, meaning anything with bark that can be stripped or twigs that snap to reveal a solid, woody core are not Knotweed .


Buddleia (Buddleja davidii) is a common flowering plant that can often be seen growing along roadsides, railway lines and areas of urban wasteland. The underside of the leaves and the stem of Buddleia has a felt like feel and appearance, due to the fact that they are covered in fine white hairs.

Ground Elder

Ground Elder (Aegopodium Podagraria) is a rapidly growing and invasive, perennial weed. It can spread very quickly resulting in a carpet of foliage that will soon crowd out less-vigorous plants in beds and borders. Early in the year, shoots with dark green leaves erupt through the soil. These are followed in late spring and early summer by numerous flat white flowers supported by tall stalks.


Cowparsley, sometimes referred to as “the countryside killer”, is a member of the Umbelliferae or carrot family. Right through the month of May, most roadsides are fringed with the white flowers, seemingly immune to traffic pollution, salt-spray and regular mowing by the highway authorities.