Survey
If you think there is Japanese knotweed on the plot or near it, or indeed the valuer thinks that may be the case, then you will need to get a survey completed by a professional & qualified firm. They will need to be members of the PCA (Property Care Association).
A survey will provide a formal Japanese knotweed identification report showing where the knotweed is located, whether it is on or near the boundary, the extent of the risk areas and suggest any remedial work or treatment that would need to be done accordingly.
Depending who you use to do the survey some firms offer this for free but may charge a fee if there is no evidence of Japanese knotweed.
Management Plan & Guarantee
A Knotweed Management Plan is required for house buying and selling where the subject property has evidence of Japanese knotweed. Most mortgage lenders will insist on proof that there is a management plan in place certainly if there is evidence of it within the boundary, and possibly if there is evidence the other side of the boundary.
The treatment plan can be over a period of typically 5, 7, 10 or 15 years. They tend to come with an insurance backed guarantee. Most mortgage lenders require a minimum 10-year guarantee for the treatment plan to be acceptable. Depending on the amount of work & treatment involved the cost can run into many thousands of pounds.
Herbicide
A common and cost-effective treatment for Japanese knotweed is the use of a chemical herbicide over a long period of time to prevent further growth and regrowth. The extent and severity of the knotweed might indicate how long the treatment might take to eradicate it. In most cases it takes many years of treatment to achieve this.
The type of herbicide used to treat the knotweed will depend on a number of factors such as the concentration and extent of the knotweed, what trees or plants are nearby, its proximity to streams, river or the sea, or indeed what wildlife might be close to the location.
Herbicides should only be applied by individuals with a recognised National Proficiency Test Council (NPTC) Certificate of Competence. This ensures that the appropriate products are used at the appropriate levels with consideration of course to the above factors. You should NOT try doing this yourself and must use a suitably qualified company.
If there are plans to unsettle the ground for example to develop the ground for plants, building a cabin or installing a hot tub, then a herbicide treatment might not be the best solution because disturbing the ground can promote new growth. The solution might be to excavate the ground.
Excavation
If herbicidal treatment is likely to be inadequate or not appropriate, then excavating the ground might be the best solution in dealing with the knotweed.
Digging and removing the area of ground to a certain depth will completely remove the knotweed. This is not very sustainable and is expensive. It would be classified as ‘controlled waste’ so can only be removed by firms that are licenced to remove this type of waste. It would also need to be taken to landfill with the additional cost of the Landfill Tax. Not all landfill sites can accept this type of waste so depending on how far the nearest site is could affect the cost of disposal.
A more sustainable approach would be to remove the area of affected land and bury it elsewhere on the plot. This method is only usually a consideration for large land projects where for example a house builder is building many homes with spare land available. For house builders too if they need to dig to a lower foundation level then this reduced area can be covered with a non-permeable membrane.
A survey would suggest the recommended the most appropriate course of treatment.