Arctium lappa

Native to Asia, Europe, and the Indian subcontinent. Introduced and naturalized in the United States.

Disturbed sites, roadsides.

August to October

Purple

3′ to 9′, but usually 5′ or less

This is a 3′ to 9′ tall, but usually 5′ tall or less, 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 long leaf stalks. They are 10″ to 31″ long and 8″ to 27″ wide. The leaf stalk is 6″ to 14″ long, deeply grooved, and solid. The upper surface is green with sparse short hairs. The lower surface is light green or gray-green, with a thin covering of 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. They are nearly hairless on both the upper and lower surfaces.

The inflorescence is a cluster of long-stalked flower heads, 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. They are crowded and densely packed.

Flower heads have about 40 purple florets and are on 1″ to 2⅓″ long stalks. The whorl of overlapping bracts subtending the flower head is 1″ to 1½″ in diameter. It is hairless except for, usually, a few long, cobwebby hairs. 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) leaf stalks are hollow, at least at the base, and are not grooved. The flower heads are smaller, ½″ to 1″ in diameter, and are on short stalks or are stalkless.

Woolly burdock (Arctium tomentosum) flower heads are densely cobwebby.

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.