Thistles have spines on stalks leaves or flowerheads. Burdocks have hooked spines on the involucral bracts. Flowerheads have disk flowers only, usually purple in color. Thistle leaves are alternate, often finely divided Burdock leaves are alternate and heart-shaped.
Guide to Identify Presented Species of the Thistle group
FLOWERHEADS WITH HOOKED SPINES
Arctium minus – Common Burdock Stout, coarse, branching biennial, up to 150 cm tall. Disturbed areas.
Flowerheads purple, round, bur-like, with hooks, 15-25 mm wide, short-stalked.
Leaves large, ovate to heart-shaped with thin, woolly hairs underneath.
FLOWERHEADS AND LEAVES WITH STRAIGHT SPINES
Cirsium – Thistle Biennials or perennials, medium to tall plants. Stems simple or branched.
Flowerheads spiny, mostly pink to purple, in terminal or side clusters.
Leaves alternate, spiny-toothed and often woolly, often pinnately lobed.
