Knapweed [Centaurea nigra]

Pest profile

About the weed

It is a downy, perennial dicotyledon, with upright, tough, usually branched stems, 30 - 60cm tall. It has a rosette of leaves at the base and purple thistle-like flowerheads.

Key features

Young plant: The first true leaves have a dark colour.

Biology

Knapweed is more common in older pastures and is usually found on the margins of arable land. Although the plant dies back overwinter it is a perennial. Plants mainly reproduce by seed, which may survive for several years, if they survive predation by insects or small mammals. Plants may reproduce vegetatively if side shoots become detached from the parent plant.

Symptoms & Diagnosis

Life cycle

Seed longevity: 1 - 5 years

Seed weight: 2.5 mg

Seeds/flower: 1

Seeds/plant: Up to 1000

Treatment

Management

It does not persist in cultivated soils and is readily controlled with glyphosate in uncropped breaks.