n.a weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized
同义词：lesser knapweedblack knapweedCentaurea nigra
英汉双解大词典
hardheads/ˈhɑːdˌhɛdz/
1.
N（名词）
a thistle-like plant, Centaurea nigra, native to Europe and introduced into North America and New Zealand, that has reddish-purple flower heads: family Asteraceae (composites) 矢车菊 (Also called knapweed)