When is a weed not a weed? Well not very often if it is growing in the wrong place. If you have planted Persicaria then that is Knot a weed it is a notweed or not depending on your spelling.
Persicaria affinis aka Polygonum affine Common Name: ‘Knotweed’ flower from late summer until the frosts with spires of red, pink or white flowers held aloft as with these massed plants by the lake at Grewlthorpe.
There are over 150 species of Knotweed which get their common name from the swollen nodes on the stems.