Japanese Knotweed

20 October 2014

Vipul Raichura comments on the problem of Japanese Knotweed:


What is Japanese Knotweed?

Japanese Knotweed is native to Eastern Asia in Japan, China and Korea and was brought to Britain from Japan as an ornamental garden plant in the mid nineteenth century.  As  with other such introductions of alien plant species during this period, the implications of introducing this species was not fully understood. The plant spread much further than expected and thrived in an environment that lacked natural predators.

Japanese Knotweed grows up to 3 metres high, has bamboo like stems, heart shaped leaves measuring between 5 to 12cm wide and flowers around September (depending on geographical location).  It is currently an offence to plant or allow the spread of Japanese Knotweed.




Why is it a problem?

Japanese Knotweed is a Rhizamotaus plant, much like the dandelion, and produces underground stems, which can regrow from just a small amount of the stem.  It is understood that a section as small as 1cm can produce a new plant within about 10 days and that the stem could lay dormant for up to 20 years before starting this process.  Japanese Knotweed can grow up to 7 metres away from the crown and to a depth of 2 to 3 metres, depending on soil conditions.  Japanese Knotweed is a highly invasive plant that can damage paths, drives, walls and even foundations, which is why it is so important to ensure it is identified early and remedied or treated accordingly.

During the lending crisis in 2007, building societies and banks had no choice with their restricting  finances but to seek only absolute clean loans. Japanese Knotweed infestations were within the category of perceived risks and subsequently mortgage lenders expected the presence of Japanese Knotweed to be included on a residential valuation report.  RICS red book guidance requires the valuer to indicate the presence of “invasive vegetation”.

The presence of Japanese Knot Weed on a property could affect a property purchaser / developer receiving funding from mortgage providers and could have an effect on the insurance of an owner. Eradication of Japanese Knotweed could be expensive and affect programme, depending on the where it is located and the space available on the site.

If you are a developer or are purchasing a property you will want to avoid properties that have Japanese Knotweed on the site.  If you already own a property that has Knotweed, you will probably want to have it removed in order to avoid costly repairs or in some cases even the complete replacement of the affected structure.

It is not an illegal offence to have Japanese Knotweed growing on your land, however, allowing it to spread to neighbouring properties could be considered a nuisance under common law.

Whilst it is an offence to allow the spread of Japanese Knotweed, if your neighbour has it they are not legally required to eradicate it.  This can impact on adjoining properties given the risk associated.  New legislation is being proposed that could help those who may have a property they cannot sell due to Japanese Knotweed in neighbouring properties.

The Infrastructure Bill amends the Wildlife and Countryside Act so that “species control orders” can be introduced.  As a result this may compel landowners to control invasive species on their land or to allow Local Authorities to survey and carry out work.

Although it is not compulsory and there is no blanket policy for lenders, it would be advisable to obtain an insured guarantee as provided by Guarantee Protection Insurance Ltd (GPI) or by a similar FSA approved and regulated insurance company. This would remove one of the obstacles to obtain funding, assist in future sales and insures the contractors’ guarantee for 10 years.


 Eradicating Knotweed

Wherever possible, Japanese Knotweed should be burned in situ, or tightly wrapped in root barrier membrane and buried on site. There are various methods available for eradicating Japanese Knotweed and the method of eradication will mainly depend on how quickly the Japanese Knotweed needs to be removed, the budget, location of the Japanese Knotweed and the space available on the site. Knotweed is resistant to most types of weedkiller, although glyphosphate will work if applied repeatedly for three years. However, there still remains the problem of the underground rhizomes, which could emerge again.

Japanese Knotweed eradication should be undertaken by specialist companies and the main methods of eradicating it are listed below:-

  1. Chemical treatment – usually lasting between 2-3 years – either by foliar method involving application across the leaf or directly into the stem of the plant.
  2. Relocation and application of herbicide – Over a period of 1-2 years.
  3. Small scale excavation of the topsoil and the application of herbicide – Over a period of 1-2 years.
  4. In situ capping of the affected soil – Instant.
  5. Elevation and cell burial on site – Instant.
  6. Full scale excavation, often to a depth of 3 metres (10ft) and removal from site. – Instant but very expensive. The waste from the excavation works would be treated as contaminated waste for specialist disposal.

The spread of Japanese Knotweed in its native Japan is controlled by a jumping louse/Psyllid called Aphalarn Itadors, which likes nothing more than sucking the sap from Japanese Knotweed. However, it only serves to control the spread of the plant and does not eradicate the problem. Defra are undertaking trials to a couple of sites.

The cost of treatment of Japanese Knotweed can also be offset by recovery of cost under the Land Remediation Act.  Rules and restrictions apply but this should be considered if it is located on any of your sites.


Price Comparisons for Removal

Whilst any treatment will be dependent upon specific site variables, herbicide treatments are usually 75-50% cheaper than programmes that involve moving the Japanese Knotweed and soil about the site or away from it.

With regards to the treatment of Japanese Knotweed, Giant Knotweed and other hybrid variations is a specialist area, it is recommended that you try to  identify the species and typical characteristics so that you are alert to potential risks.  Specialist advice should then be obtained to confirm the species.  The various options and best treatment method can then be discussed and tailored to suit the proposed work and minimise the potential risk and cost associated.