Symphytum x uplandicum (Knitbone or Blackworth) Unlike the common comfrey, Bocking 14 is sterile and will not set seed and invade your garden. It also has the highest potash content of all comfreys, making it the best for liquid manure. Mauve flowers spring and early summer. Hardy perennial. Height: 1 metre. Thrives in most soils or situation, does best under shade of trees but will tolerate full sun. Priced for: 9, cm pot