Japanese Knotweed

Japanese knotweed is a very invasive ‘super weed’, originally introduced as an ornamental plant in the 19th Century; this soon escaped, and spread rapidly throughout the UK. This is particularly problematic due to its rapid growth and hardy ability to penetrate tarmac, walls and foundations causing costly damage to roads and buildings.

It has also impacted developers, where it has been known to add around 10% to a development cost, for its control/ removal and has caused significant project delays.

Japanese knotweed (Fallopia japonica) is an invasive herbaceous perennial (leafy plant that lives for more than two years), which is native to eastern Asia, China and Korea.

It grows at an alarming rate of more than a metre a month and spreads easily by natural or human activity. It is particularly difficult to eradicate completely due to its ability to spread from small fragments of rhizome (underground stems).

 

There are 7 types of knotweed, including amongst others, Giant knotweed (Fallopia sachalinensis, Knotweed Hybrid (Fallopia x bohemica) and Himalayan Knotweed (persicaria walli chii)

Japanese knotweed can have disastrous consequences, particularly if you are looking at buying a property or selling your home, as banks/ building societies may be unwilling to lend a mortgage on an infected property. Consequently, this can significantly reduce the value of your home.

It should also be borne in mind that allowing the weed to spread onto a neighbouring owners land is a criminal offence and regarded as a nuisance under common law.

 

Identifying Japanese Knotweed

Generally, Japanese knotweed generally colonizes in temperature climates, frequently alongside roads, wasteland and riparian ecosystems (alongside river banks). It is tolerant of a wide variety of soil types, ph and salinity and the rhizomes can survive in temperatures as low as –35 degrees.

Japanese Knotweed has spread to countries including the UK, vegetatively, through growth of established clumps and or part of the crown, stem or rhizome; as such new occurrences of Japanese Knotweed growth can only start in areas which have Knotweed remnants in the soil, e.g. through use of imported topsoil, or disturbance of existing contaminated soil.

Identifying Japanese Knotweed is no easy task, however there are some physical characteristics that can be taken into account below:

  1. Green, shield-shaped, varying size leaf
  2. Distinctive zig zag leaf stem, often pink/purple in colour
  3. Stem is bamboo like, green, hollow and jointed
  4. Crown is a knotty mass, partially visible above ground
  5. Rhizome underground stem system can spread several metres
  6. Flowers are small, white, five petalled, appearing in Aug till Oct/Nov

This is no guarantee to ensuring an accurate identification, as there are other plants which look similar to Japanese Knotweed, such as the Russian vine and Hedge Bindweed which are often confused with Japanese Knotweed

Another key aspect is to look out for the renowned characteristics that make Japanese Knotweed so intrusive, that is its ability to grow rapidly, in poor soil conditions and resilience to herbicides.

It should also be noted, that many surveyors’ terms and conditions exclude its identification; therefore, this should be clarified prior to engagement with a professional.

 

Japanese Knotweed treatment

Japanese Knotweed is notoriously difficult to eradicate, and has a high potential for re-growth (as rhizome material can lay dormant for up to 20 years) however there are treatment options available for dealing with the problematic plant.

The weed is resistant to a number of weed killers, however it can be killed by the herbicide Glysophate (which is the active ingredient in many commercial products) However, despite this, repeated applications area still required over a period of 3 years or more to completely eradicate the weed.  It is also common for the weed to be buried or burned in situ, (in an underground cell, using a root barrier membrane)

  1. Chemical treatment (24-36 month)
  2. Relocation and herbicidal programme (24-36 months)
  3. Reduced dig and herbicidal treatment (12-24 months)
  4. In situ capping (instant eradication)
  5. On site burial (instant eradication)
  6. Excavation and removal (instant eradication)
  7. Stem Cell Injection

 

Further Advice

If you suspect Japanese Knotweed may be present, you should obtain professional advice immediately.