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 a perennial herb growing up to about a metre in height.
The leaves are up to 25 centimetres long, usually deeply lobed, and hairy. The lower leaves are stalked, whilst the upper ones are stalkless.
The inflorescence contains a few flower heads, each a hemisphere of black or brown bristly phyllaries. The head bears many small bright purple flowers. The fruit is a tan, hairy achene 2 or 3 millimetres long, sometimes with a tiny, dark pappus. Flowers July until September. flowers sometimes are yellow, or white
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