Plant description. Common burdock is a biennial that grows as a rosette of leaves the first year and then produces a 5-foot-tall, erect, bushy flowering stem. Rosette leaves are distinctive due to their large size, heart-shaped base, wooly undersurface, and hollow leaf stalks (petioles). Stem leaves are similar to but smaller than rosette leaves. Located at the ends of branches or at leaf axils on the flower stem are flower heads comprised of a bur with hooked bristles beneath a closely packed cluster of tubular, purplish flowers. The weed is best known for the hooked bristles on its burs that stick to fur and clothing. The only means by which common burdock reproduces are its seeds.
Seedlings and shoots. First to emerge are two leaves (cotyledons) that are large, spoon-shaped, and have a waxy surface. Subsequent leaves are alternate, egg-shaped, flocked with short hairs, puckered between the veins, and bitter tasting. Attached to each leaf is a flared stalk (petiole) that clasps the basal stem of the rosette.
Stems. During the rosette stage of growth, the stem remains compressed and close to the soil surface. As flowering is initiated, the stem elongates producing an erect flower stem that is 2- to 6-feet tall, much-branched, rough-hairy, hollow, and grooved lengthwise or angular.
Leaves. The large rosette leaves are 20 inches long, 12 inches wide, and attached to the stem by way of hollow petioles that may be purple-tinged. The upper leaf surface is dark green and coarse while the underside is pale gray-green and wooly. Rosette leaves have a heart-shaped base and wavy edges. Stem leaves are alternate (1 leaf per node). Lower stem leaves resemble rosette leaves except they are smaller. Leaves gradually become smaller, less heart-shaped, and tapered at both ends as their location progresses up toward the tip of the stem. Also, their petioles become shorter and solid rather than hollow.
Flowers. Each flower head consists of bristles with hooked tips that form a round, 3/4-inch-wide bur beneath a closely packed cluster of many individual, tube-shaped, reddish-purple flowers. Flowers appear alone or grouped on short stalks attached to the end of main branches or at leaf axils on the stem.
Fruits and seeds. In each bur are many single-seeded, 1/4-inch-long, brown, oblong, angular fruits having a short, stiff bristle at one end.