Japanese Knotweed advice

Japanese Knotweed: tips and advice for developers and land agents

1. Survey the site for Japanese Knotweed before purchase
   • With new stem injection technology, Knotweed can be controlled within one growing season. So if Knotweed is present, it should not stop you buying the site. However, you should employ a consultant to advise how profitable the development will be after taking control costs into account.
   • Has the site been skimmed or treated? Look for evidence of Knotweed material (may not be evident during the winter months.) Consider some form of legal protection from the potential subsequent cost of managing Japanese Knotweed within the purchase agreement
   • If there is Knotweed, we will advise whether you can treat on site or that you have sufficient land to contain on site; and factoring whether you have to treat as controlled waste; and advise on the most cost-effective controls.
2. Timetable for treatment and development
   • Plan to minimize the amount of Japanese Knotweed that you have to excavate
   • Allocate sufficient time within the project timescale to develop and apply a Japanese Knotweed management plan
   • Treat Japanese Knotweed as early and effectively as possible. This significantly cuts the chance of it growing again.
   • Consider a phased development to allow more time for treatment.
   • Use the best treatment methods, including the most effective herbicides. These will be determined by factors such as how close the site is to controlled waters, desirable trees and other vegetation.
3. Managing treated material
   • Just because plants have been treated, this does not mean that Japanese Knotweed cannot grow again. If plants are treated effectively, soil can be clean enough to be used for landscaping within the development, and a root viability test will confirm this.
   • Only use soil from a contaminated area in localised areas where control methods can be easily reintroduced if the knotweed starts to grow again. You should not use this material within 50m of a water course.
4. Long term management
   • Consider the chance that Japanese Knotweed could grow back when you are managing the site long-term
   • Current site owners must accurately record within the property deeds where any material is buried and make this available to all subsequent owners so that the material is not disturbed.
   • A summary of the treatment should be included within the vendor statement declaration.

Be wary of products and methods that claim to quickly eradicate Japanese Knotweed and guarantees that it will not appear again on the site. A BASIS-registered person should advise on all chemical treatment.

References: The Environment Agency