Common burdock (Arctium minus (Hill) Bernh.)
Common burdock is a member of the Aster or Sunflower family, Thistle tribe. It is an introduced biennial which reproduces by seeds. In the first year of growth the plant forms a rosette. The second year the plant is erect. The stem is stout, grooved, rough, has multiple branches, and grows to 2-6 feet tall.
The leaves are alternate, dark green, smooth above, whitish green, and woolly hairy beneath. The flowers are purple or white in numerous heads. The head is enclosed in a prickly bur, an involucre, composed of numerous smooth or wooly bracts tipped with hooked spines.
Burdock grows along roadsides, ditch banks, and neglected areas. This plant is a very serious threat to sheep as the burs can significantly damage the quality of the wool. Burdock is very common in central and north central Colorado.