The Collaborative International Dictionary
Centaurea nigra
Ironheads \I"ron*heads`\, n. (Bot.)
A European composite herb ( Centaurea nigra); -- so called
from the resemblance of its knobbed head to an iron ball
fixed on a long handle.
--Dr. Prior.
Centaurea nigra
Knapweed \Knap"weed`\, n. (Bot.) The black centaury ( Centaurea nigra); -- so called from the knoblike heads of flowers. Called also bullweed.
Centaurea nigra
Matfelon \Mat"fel*on\, n. [W. madfelen.] (Bot.) The knapweed ( Centaurea nigra).
Wikipedia
Centaurea nigra
Centaurea nigra is a species of flowering plant in the daisy family 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.