In order to determine the knotweed infected area in sq m on a subject property measure/approximate the length and breadth of the visible Japanese knotweed at ground level. Multiply the length x breadth to give you the total visible in sq m. As knotweed rhizome spreads laterally underground the actual area of infestation is greater than is visible above ground. A buffer zone is therefore added in each direction to the visible length and breadth of 1m in normal circumstances, 2m or more in exceptional cases.
For example, a stand of visible knotweed measuring 2m x 1m with a buffer of 1m in each direction would be 4m x 3m, to give a total 12 sq m. It is not necessary to add the buffer if the buffer falls outside of the property boundary. So for example if the above was in the corner of the garden, the area would be 3m x 2m , to give a total on the subject property of 6 sq m.
If there are 2 strands of Japanese knotweed within 2m of each other, it is likely that these are interconnected underground and should be classed as 1 stand when measuring the length and breadth.