Diffuse knapweed, Centaurea Maculosa

This weed is a diffusely branched annual or short-lived perennial, 1 to 2 feet tall, stems are rough to the touch. Leaves are pinnately divided; the reduced leaves of the inflorescence are mostly entire. Flowering heads are numerous and narrow. Flowers are white to rose or sometimes purplish; margins of involucral bracts are divided like the teeth of a comb, and bracts are tipped with a definite slender spine. Achenes are brown or grayish; pappus is lacking.

Centaurea is a large genus of over 400 species, most originating in the Mediterranean region. All of the species treated here have been introduced from Eurasia and now represent a threat to pastures and rangelands. Diffuse knapweed infests roadsides, waste areas and dry rangelands, and as a highly competitive plant, threatens to exclude many desirable species. Flowering occurs from July to September.

If you would like more information, email MWCA at acamm@montana.edu with your questions. A weed control expert will be happy to answer your questions.