English: Common Knapweed Centaurea nigra. A very common perennial of grassy places; about 60 cm in height with blue thistle-like flower heads and narrow-oval leaves. Knapweed is also known as Hardhead because of the hard flower head beneath the petals. The plants where once used as a poultice on wounds and have been used in traditional fabric dyeing. Despite its purple flower, yellow is the most typical colour of dye that it produces. These examples were growing on the roadside verge in well drained soil.