Main stem erect, single or several from the base of the plant, usually accompanied by many slender weak sterile basal stems. 1-3 feet tall. Numerous, borne in pairs, if held to light appearing as if pinpricked and with tiny black dots on the margins. Flowering period is during June and July. Splitting at maturity into 3 segments.
Gelatinous coating helps seeds stick to fur and clothing when wet.
St. Johnswort is a perennial that propagates vegetatively by underground stems or runners. It is cumulatively poisonous to livestock. Light-pigmented animals develop blisters and scabs around the mouth, eyes, ears, nose, and feet, especially when exposed to sunlight. Dark-colored animals are less affected. It invades farm and pasture lands.