This species is characterized by erect stems with verticillate leaves, spiciform to cymose inflorescences, yellow corolla 3.5-8.5 mm long, and capsules ca. 1 mm long. It is variable, and Steyermark (1963) recognized a number of varieties and forms based on density and distribution of pubescence, flower size, and leaf size and form. These were all later synonymized by Taylor et al. (2004: 696-697), as discussed there.