Flannel Weed (Sida cordifolia) is an upright herb or subshrub about 0.3 to 3 m tall, and is as wide as tall when a shrub. The stem is woody, branching several times and with a well developed tap root. The leaves of Flannel Weed are heart shaped with serrated margins and have a dense covering of hairs which give a light green, felt-like appearance, and are up to 65 mm long.

Flowers are about 15 to 18 mm across, yellow or orange, and are borne in dense clusters at the end of branches. The outer whorl of the flowers (the calyx) enlarges and encloses the mature fruit, leaving only the awns on the mericarps showing. Petals are yellow or orange throughout or sometimes have a deeper orange or reddish centre and are longer than the calyx, being 7-8 mm long. The fruit, or seed capsule, divides into 10 portions (the mericarps). Each mericarp has two fine bristles at one end (Barker, unpublished manuscript; Pitt 2002). These bristles have a number of stiff downward-pointing hairs along their length, which allow the mericarps to become attached to objects and to thus act as a seed dispersal unit.