There are several accepted methods to remove Japanese knotweed and many people have spent a long time debating which solution is best. The answer we suggest is specific to the property or site in question. Generally other demands put upon the land in question will define how Japanese knotweed should be controlled. Unfortunately money as with everything comes into the Japanese knotweed control equation. The cost of Excavation far exceeds that of the chemical control the normal alternative way to remove Japanese knotweed. So other than money what are the deciding factors that need to be considered? Here we have created a simple chart to help those considering who they should remove Japanese knotweed.
Chemical Control considerations:
· Cost effective
· Beneficial to the environment
· Time; takes 3-4 years to control and even more years for the plant fully decompose so land can be considered re-mediated and/or knotweed eradicated
· Changes in future land use are restricted in areas formally impacted with knotweed. Impacted land will always be considered as knotweed contaminated from a waste perspective
· Requires specialist training and certification for professional treatment
· Restrictions on chemical type under or near trees and EA approval needed near water. Natural England need to be informed in certain situations as well
Considerations for excavation and removal off site:
· Expensive
· Land fill is a scarce resource so excavation that requires waste taken off site will need to go to a specialist landfill site
· Excavation and removal off site is instant and the only true method of eradication
· Once knotweed impacted material is excavated and removed the location is fully re-mediated and can has not restriction on future land use
· Requires experience and expertise so that efficient excavation can be implemented. A knotweed specialist must manage and act as a watching brief at the very least.
· Excavation is restricted near foundation to buildings or other built structures. Excavation will be restricted under or close to trees and may well be considered inappropriate as such an operation my destabilise or make a trees for retention un-safe
There are alternative approaches which is where specialist advice should be employed. Other methods which fall in the middle of the two options detailed above include; on site relocation, on site burial and partial excavation and capping with root barrier; these options are briefly discussed below.
On site relocation: Excavate and move impacted material from one location where redevelopment is perhaps planned to another location within the same legal piece of land where it can be treated chemically over a longer period. The advantage here is that knotweed can be treated economically over time diverting waste from landfill which is generally more cost effective and better on the environment.
On site burial: being able to bury Japanese knotweed impacted material on site removes the need for it to go to landfill and is accepted by the Environment Agency as a viable method of Japanese knotweed control. The impacted material must be free from other forms of contamination and the EA do need to be informed. Normally a root barrier is used o encapsulate the knotweed which is buried to a depth of more than 2 metres
Partial excavation and capping with root barrier: In some circumstances it is possible to excavate to formation level and then use a root barrier to cap the knotweed in situ. This is more economical then complete removal and so long a suitable specialist root barrier is used and correctly installed. This method can be an effective and economical solution but may need to be combined with chemical control.
In conclusion consideration must be given to the site and the demands that will be put on it through future changes in re-development and use. The recommendation is that a proper survey is documented and a strategy developed to ensure that proper consideration has been given to the methodology of Japanese knotweed control. This is often delivered in the form of a management plan. Again it is very important that any such plan contains survey information, risk assessment and proper evaluation of possible control methods.
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