Horehound (Marrubium vulgare) is an erect, multi-stemmed, semi-woody, perennial herb to about 60 cm high. The stems are densely covered with white hairs. The leaves are arranged in opposite pairs, finely wrinkled, more or less round or oval in shape, 10-35 mm long and 10-30 mm wide and densely covered on the lower surface with white hairs. The flowers are densely clustered at the bases of leaf pairs. Each flower has a tube of white petals, 6-12 mm long surrounded by a densely hairy calyx with 10 hooked lobes. After flowering, the calyx becomes a dry, brown, spiny, burr-like structure that surrounds the fruit and aids in seed dispersal.