Japanese Knotweed sounds like the sort of Zen plant you might choose to plant in the garden. In fact this plant can damage footpaths, and patios; in the worse cases, it can get into the property. It is very difficult to get rid of this plant as evidenced on this Homebuyer Report.
The surveyor found the it growing within close proximity to the property and although evidence of treatment was noted.
The vendor, who was present during the survey, confirmed that they had paid for a significant excavation of the plant which had necessitated moving tonnes of earth and the laying of a root impenetrable membrane.
It clearly had not worked. Further professional investigation was recommended and the surveyor notified the client.
The surveyor also advised that the buyer inform their solicitor to enquire about any warranties to the treatment and whether the warranties were transferable.
Knotweed is so serious that it can take a number of years to defeat and may affect future saleability, mortgage-ability and value of the home. RICS have issued a 16 page notification to surveyors about the plant.