Common St. Johnswort is a perennial plant about 1–2½’ tall with frequent branches. Stems are round, hairless, and light green; the larger stems have a pair of small longitudinal ridges. The opposite leaves are oblong, hairless, and sessile. Flowers are about ¾” across, consisting of 5 yellow petals, 5 green sepals, 3 styles, and numerous stamens (more than 20). The root system is rhizomatous and produces numerous basal offshoots around the base of the plant.