Japanese Knotweed – Much More Than a Summer Nuisance

admin / October 23, 2018

Fallopia Japonica, more commonly known as Japanese Knotweed was introduced to the UK in 1850. It was chosen by gardeners as the ideal landscaping plant due to its dense bamboo-like appearance and capability of growing rapidly almost anywhere. Since Japanese Knotweed infested soil has been classified as contaminated waste under the Environmental Protection Act 1990. Japanese Knotweed is now known as a plant that can quickly colonise an area, crowding out domestic herbaceous species. The problem of Knotweed infestations have become more prevalent in the UK recently with increased fly tipping. Properties situated adjacent to Rail networks are particularly at risk to Japanese Knotweed infestation.


The species has a green, medium size, spade shaped leaf which has a waxy texture. The stem of the plant can be likened to bamboo and has purple specks. The top of the plant is likely to blossom with tiny white flowers in the summer and root of the plant has a fleshy orange centre when inspected closely.


The species does not disperse seeds in order to reproduce but instead regenerates from cuttings from the stem, root or shoots as small 0.2g in weight. Japanese Knotweed, like many other plants, lies dormant until around March where the early growth stage occurs until about April. During this period visual characteristics may not have developed fully with only small shoots being visible. Between May and October the plant grows rapidly, up to 3cm per day. During this period identifying features become visible and stems may grow to approximately 10 feet in height. Roots can extend as deep as 6m with a 3m radius around the central crown.

In September the plant begins to die down leaving brown stems which can remain up until November. Until the next early growth stage the plant is very difficult to identify.

The Problem

Japanese knotweed is notoriously difficult to treat due to its ability to spread and grow rapidly when cut back. It can penetrate masonry and tarmac, leaving boundaries, driveways, outbuildings, retaining walls and services vulnerable.

Insurers often don’t enquire specifically about Japanese Knotweed when trying to obtain buildings insurance, but they are reluctant to honour a claim which relates to damage associated with this. This leaves the onus on the lender and homeowner to rectify Knotweed infestations without the aid of the insurer.

Many of major lenders in the UK will refuse to lend on properties known to be affected by Japanese Knotweed. This depends on the severity and proximity of the infestation to the subject property (See Fig1 below). In the case of Category 3 or 4 infestations, the property is unlikely to be deemed suitable as mortgage security to most lenders. A property being affected by a category 1 or 2 infestation is likely to be acceptable providing an appropriate treatment/management plan is in place accompanied by a long term guarantee.


Excavation and Incineration

Infested soil can be excavated and incinerated on site and disposed of in landfill. There is little data surrounding the efficiency of this method and it is likely that additional treatment methods will be require in conjunction with incineration.

Excavation and Burial on site

This method involves excavation of infested soil and encasing this within an anti-penetration sack. The sack of contaminated waste can then be buried on site at a minimum depth of 5m. It is important that soil surrounding infestations is excavated to avoid any possibility of leaving small sections of rhizome behind which may regenerate.

Chemical Treatment

Chemical treatment involves repeat treatment with a herbicide over a period of time. This can be an effective way of successfully treating Japanese Knotweed however some of the chemicals used in the treatment have been banned by Local Authorities and there is some speculation as to whether the remaining useful herbicidal treatments will be banned also.

Predator introduction

In East Asia, Japanese Knotweed is not considered a pest and lacks the ability to spread rapidly like it does in the UK. The reason for this is there is an abundance of natural predators such as Aphalara Itadori, a native sap-sucking louse which control the growth of such plants to an acceptable level. The introduction of natural predators has been trailed in the UK several times however in each trail, the predators introduced were found to die out before the Knotweed could be sufficiently controlled.

If you’re concerned about the risks of Japanese Knotweed affecting the sale of your house or a house you’re looking to purchase then we recommend you consult a RICS accredited surveyor who will be able to quickly identify the problem.

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