Arctium tomentosum is a biennial herbaceous plant. The stem is erect, with ascending branches. It can reach an height of about 2 metres (6 ft 7 in). Leaves are grayish white and quite felted, green and glabrous toward the stem. Basal leaves are petiolate. Leaf blade is heart-shaped, with rather denticulater margins. They can reach a length of 20–50 centimetres (7.9–19.7 in) and a width of 10–30 centimetres (3.9–11.8 in). Flowers are purplish red, with a diameter of 0.9–1.2 centimetres (0.35–0.47 in). This species can be distinguished from related ones because the underside of the leaves is covered with white woolly hairs. Flowers and fruits appear from July to September.
Kell DJ. 2006. Arctium Linnaeus, Sp. Pl. 2: 816. 1753; Gen. Pl. ed. 5, 357. 1754. In Flora of North America Editorial Committee (Eds.) Flora of North America North of Mexico, Vol. 21: Asteraceae. New York and Oxford.
Lopez-Vinyallonga S, Romaschenko K, Susanna A and Garcia-Jacas N. 2011. Systematics of the Arctioid group: Disentangling Arctium and Cousinia (Cardueae, Carduinae). Taxon 60(2): 539-554.