White porterweed is an annual or perennial herb growing up to 2 m tall, becoming somewhat woody toward base. Stems are branched, younger stems moderately to densely short-pubescent. Leaves opposite, membranous or chartaceous, elliptic to ovate or narrowly ovate, 1.5-10 cm long, 1-5 cm wide, upper surface puberulent to strigillose or scaberulous, lower surface sparsely to moderately strigillose over entire surface, margins serrate to crenate-serrate, the teeth angled forward, apex sharply acute to acuminate, petioles 0.3-1 cm long. Spikes slender, 15-45 cm long, up to 3 mm in diameter, somewhat flaccid, usually short-pubescent throughout, the furrows nearly as wide as the rachis, bracts narrowly lancelike, 5 mm long; sepal-cup 6 mm long, the teeth subequal. Flowers pale blue to pale violet, or lavender, occasionally white.