This annual plant can achieve an impressive size. It closely resembles several Aureolaria spp. (False Foxgloves), which are also parasitic on the roots of oaks. The flowers of Mullein Foxglove are a little shorter than the flowers of the latter species (which can exceed ¾' in length), and the throat of its flowers are partially obstructed by an abundance of fine hairs. The flowers of the False Foxgloves are quite open at the throat because they lack these hairs. Another scientific name for Mullein Foxglove is Seymeria macrophylla.