Brown Knapweed

Centaurea jacea L.

Family: Asteraceae

Description and life cycle

Brown knapweed is a perennial that grows from a woody rootstock and reproduces strictly from seeds. Plants grow 60 – 120 cm tall, with branching occurring towards the upper 1/3 of the plant. Basal leaves are 15.2 cm long x 4 cm wide and are broadest in the middle and taper at both ends. Lance shaped, clasping stem leaves gradually become smaller towards the top and have shallow lobes and fine-toothed or smooth margins. Rose-purple coloured flowers appear from July to October, and are most abundant in August. On rare occasions the flowers can be white. Broad, oval, 2.5 cm flowers are produced singly at the top of branched stems. Overlapping bracts are dark brown with broad, thin, papery margins that appear ragged or torn. Light brown seeds that are 4 mm long have lengthwise ridges or lines. ‘Pappus’ appears small and scale-like.

Biological control agent

• Urophora jaceana: Fly

References

Pacific Northwest Extension Publication. No Date. Idenfication of knapweeds and starthistles in the Pacific Northwest. Washington State Univer., Oregon State Univ., Univer. Idaho and U. S. Dept. Agric. PNW432. 23 p.

Wilson, Linda M. and Carol Bell Randall. Biology and biological control of knapweed.