burdock

:  a tall weed related to the thistles that has prickly purplish heads of flowers

1:  any of a genus (Arctium) of coarse composite herbs bearing globular flower heads with prickly bracts and including one (A. lappa) that has an edible root used as a vegetable and in herbal medicine

2 or burdock root :  the edible root of the burdock (Arctium lappa) Herbalists use dried burdock as a diuretic and a blood-purifying agent.—Kathryn Shattuck, The New York Times, 20 July 2008—called also lappa