Arctium tomentosum

Native to Asia and Europe. Introduced and naturalized in the United States. Potentially invasive.

Disturbed sites, roadsides.

July to October

Rose purple, occasionally white

2′ to 4′, rarely taller

This is a 2′ to 4′ tall, rarely taller, erect, biennial forb that rises on a single stem from a fleshy taproot. In its first year of growth this produces a rosette of basal leaves. In the second year it produces a hollow stalk that is branched, hairy, and ridged.

Basal leaves are large, heart shaped and indented at the base where they attach to the leaf stalks. They are 12″ to 15″ long and 6″ to 11″ wide. The leaf stalk is 4″ to 6″ long, mostly hollow, and is covered with glandular hairs. The upper surface of the blade is green with sparse short hairs. The lower surface is light green or gray-green, with a thin covering of white, matted, short, soft, woolly hairs. The leaf margins are wavy. By the time the flowers are fully open the basal leaves are usually withered.

Stem leaves are much smaller, alternate, and egg-shaped, getting progressively smaller toward the top of the stem.

The inflorescence is a cluster of stalked flower heads with the outer ones on longer stalks, forming a flat-topped or convex cluster. The clusters appear at the end of the stem and in the upper leaf axils.

Flower heads have about 30 rose purple, occasionally white, florets, are ½″ to 1″ wide, and are on ½″ to 4¾″ long stalks. The whorl of overlapping bracts subtending the flower head is 1″ to 1½″ in diameter and is densely woolly or cobwebby. The bracts are hooked at the tip. When dry the flower head becomes a bur resembling a thistle. Thistles, however, do not have hooked bracts.

Common burdock (Arctium minus) flower heads are not densely cobwebby and are on short stalks or no stalks at all.

Great burdock (Arctium lappa) has flower heads on long stalks. The inflorescence is a group of stalked flower heads with the outer ones on longer stalks, forming a flat-topped or convex cluster.

Sources: 2, 3, 5, 7.

The only recorded occurrences in the state are in a meadow adjacent to the Red River and along the bank of the Red River, both in St. Vincent. However, it can also be seen at the north side of the parking area near the beach house in Flandrau State Park.

Taxonomy
Cardueae is a synonym of the tribe name. Cynareae was published first and has precedence. Nevertheless, most sources use the name Cardueae for the tribe.