Centaurea nigra

Centaurea nigra is a species of flowering plant in the family Asteraceae known by the common names lesser knapweed, common knapweed and black knapweed. A local vernacular name is hardheads.

It is native to Europe but it is known on other continents as an introduced species and often a noxious weed.

Although the plant is often unwanted by landowners because it is considered a weed by many, it provides a great deal of nectar for pollinators. It was rated in the top five for most nectar production (nectar per unit cover per year) in a UK plants survey. It also placed second as a producer of nectar sugar per floral unit, among the meadow perennials, in another study in Britain.