By CELESTINE DUNCAN, TechLine Editor
Twenty two different knapweed (Centaurea sp) species are well established in the United States. Four of these knapweed overlap in distribution and share similar morphological characteristics. This includes spotted (C. stoebe), brown (C. jacea), black (C. nigra) and meadow knapweed (C. xmoncktonii). The key to separating these and other knapweed species are the involucre bracts.1 These four knapweed spread by seed, but brown, black, and meadow knapweed can also spread from woody root crowns.
1. Involucre bracts are small leaf- or scale-like structures surrounding an inflorescence, located just below flower petals. Bract images from Pacific Northwest Extension Publication PNW432.
2. "Entire leaf margins" refers to leaves that are neither toothed, lobed, nor compound, i.e. simple with smooth margins.
Published November 2018; reviewed May, 2019.