Flowers are bright yellow and about 3/4 inch. They have five petals and numerous visible stamens. Common St. johnswort, also known as Klamath weed, is a member of the St. johnswort family. It is an introduced, creeping perennial which reproduces by seeds and short horizontal roots. The multi-branched stems are 1 to 2 ½ feet tall and emerge from a woody base.

Leaves are opposite, elliptic to oblong, with translucent dots along the leaf margins. The bright yellow flowers have five petals which have black dots along their margins, an occur on showy terminal clusters.

Common St. johnswort is an aggressive invader of overgrazed rangeland. It is found on gravelly or dry sandy soils. This plant may be toxic to white-haired animals (causes photosensitization), such as white-faced sheep.

Common St. johnswort is a "List C" species on the Colorado Noxious Weed List.