Russian knapweed, Centaurea repens L..[CENRE. centaurée de Russie, centaurée toujours fleurie] Perennial, but differs from other perennial knapweeds by reproducing both by seed and from deeply penetrating, widely spreading, dark brown or black roots which can form extensive patches and be extremely difficult to eradicate once established. It is a finer, more slender plant than the other knapweeds, with narrow, undivided but shallowly and irregularly toothed leaves; stems and leaves covered with a white woolliness which peels off in strips as the stems get older; flower heads small (1 - 1.5 cm, 2/5 - 3/5 in. across), silvery green before opening; ray florets absent; disk florets pinkish to light purplish; involucral bracts with a papery tip which is neither fringed nor irregularly divided. Flowers from July to September.

Russian knapweed occurs in cultivated fields and pastures in the central part of southern Ontario.

It is distinguished by its ability to reproduce from deeply penetrating and widely spreading roots, these with dark brown to black bark, its grey-green stems and undivided leaves. its small flower heads with silvery-green involucral bracts which are not fringed at the tip, and by the extremely persistent bitter taste of its stem and the inner parts of its roots.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)