Japanese Knotweed has devalued UK real estate by £20 billion according to the Independent (2018). But what is this ornamental plant and how is it affecting the industry in such a big way?
Japanese Knotweed is a hardy bamboo-like perennial plant that grows quickly and can spread underground through its roots, appearing in thick clumps or stands that can rapidly grow to heights of over two meters. Prior to the non-native plant being included in the Wildlife and Countryside Act 1981 making it an offence to plant or otherwise cause Japanese Knotweed to grow the plant was heavily used by landscapers and marketed as a useful plant to grow due to its ability to spread quickly and form a dense screen. In 1905 the Royal Horticultural Society warned against the use of Japanese Knotweed as its ability to out-compete indigenous flora started to become a problem. In recent years this uneasy relationship with the plant has become a problem with mortgage lenders and has caused depreciation in worth to affected properties due to concerns regarding the damaging effects of this invasive plant. Such concerns according to the Royal Institute of Chartered Surveyors are often based on misunderstanding and overreactions, however, some of the more serious effects include;
- Drains and other buried services
Japanese knotweed can exploit existing cracks and gaps in underground pipework, using its roots to search for water. This causes further damage and, in some cases, can result in blocked drains as the plant clumps.
- Patios, Pathways and Driveways
This non-native plant can grow between patio slabs and joints in paving causing movement which will require the removal of the existing slabs and bedding material to be able to treat the knotweed effectively.
- Boundary and Retaining Walls
Closely packed stands of knotweed can undermine garden walls with shallow foundations. The mass can ‘push over’ retaining walls, which can lead to sudden collapses.
Lightweight outbuildings with poor foundations can be affected by serious stands of Japanese Knotweed, which can result in structural damage occurring.
Conservatories have similar problems to outbuildings, however, due to them being attached to the main building have greater importance.
The invasive nature of Japanese Knotweed can spoil any garden. It is not uncommon for thousands of pounds to be spent renovating garden spaces with beautiful landscaping and water features, however, if untreated Japanese Knotweed can ruin much of this.
EFFECTIVE TREATMENT OF JAPANESE KNOTWEED
Once this invasive plant has become well established it can be challenging to remove. The Royal Horticultural Society states: ‘eradication requires steely determination’. There are, however, treatment options available.
Excavation of the plant and its roots
According to the RICS the volume of excavated soils can extend up to 3 meters vertically and 7 meters horizontally from the above ground growth. Thus, likely resulting in large quantities of soil removal if this method was to be used which is priced per ton to dispose of the plant material.
On-site Burial and/Encapsulation with Membranes
If the site allows, the plant can be covered with a minimum of 5 meters of overburden, however, if this is not a viable option a barrier membrane should be installed to encapsulate the Japanese Knotweed affected soil. If there is no option to bury the plant at all it is possible to use a root barrier membrane. These burial techniques are used as barriers to prevent cross boundary spreading, however, these are not always effective and advice from an appropriately qualified professional should be sought after.
This method of treatment can be the most financially viable, however, can take years for the treatment process to be complete. Chemical control uses specialist herbicides applied to the Japanese Knotweed to eradicate the plant.
THE LAW ON INVASIVE NON-NATIVE PLANTS
It is an offence to grow or dump invasive plants in local streams or woodlands under Schedule 9 of the Wildlife and Countryside Act 1981. The government prohibits the sale of such species and anyone caught doing so will face a government implemented penalty. Note that growing controlled plants is considered to be a criminal offence which carries a fine of up to £5,000 and/or 2 years of imprisonment.
Because of the law, people are encouraged to follow strict guidelines to help eradicate these plant species. The formation of the Global Invasive Species Programme in 1997, conceptualised strategies on how to eradicate them which was published in 2001.
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