Erect noxious perennial, to 1.2 m tall, with rhizomes and showy, bright yellow flowers. Foliage is dotted with tiny translucent and black oil glands that contain hypericin, a fluorescent red pigment that is toxic to livestock when consumed in quantity, especially to animals with light-colored skin. Toxicity symptoms include skin photosensitivity of light-colored areas and loss of condition. Most animals graze plants only when more desirable forage is unavailable. In herbal medicine, hypericin is the antidepressant ingredient in St. Johnswort remedies. There are several regional varieties of common St. Johnswort. The variety in the Pacific Northwest is aggressively competitive and can spread rapidly by seed and rhizomes. By 1940, more than 2 million hectares (~ 1 million ha in California) of rangelands were infested. Several years later, the leaf-feeding flea beetles Chrysolina quadrigemina and C. hyperici and the root-boring beetle Agrilus hyperici were successfully introduced as biocontrol agents.

SEEDLINGS: Cotyledons lanceolate to ovate, 1.5-3 mm long, 1-2 mm wide. Subsequent leaves opposite, oval to elliptic, increasingly larger. Underside leaf margins dotted with a few elevated black glands

MATURE PLANT: Stems with numerous sterile shoots 2-10 cm long from the lower leaf axils, highly branched near the top, glabrous, often reddish, with black glands along 2 longitudinal ridges. Leaves opposite, elliptic-oblong to linear, 1-3 cm long, sessile, flat, glabrous, green, 3- to 5-veined from the base, dotted with numerous, tiny translucent glands that are visible when a leaf is held up to the light. Edges of leaf lower surfaces lined with elevated black glands. Margins rolled under (revolute).

ROOTS and UNDERGROUND STRUCTURES: Taproots stout, with many branched lateral roots, to ~ 1.5 m deep. Rhizomes develop just below the soil surface from the crown and can extend outwards to ~ 0.5 m. New shoots grow from the crown and rhizomes in early spring. Fragmented rhizomes can develop new plants. Under favorable conditions, roots grow deeper and fewer rhizomes develop. Specialized corky tissue (polyderm), found only in a few plant families, protects the roots and crowns

FLOWERS: June-September. Flowers bright yellow, ~ 2 cm in diameter, clustered at the stem tips. Petals 5, separate, 8-12 mm long, typically dotted with black glands along the margins. Sepals 5, linear-lanceolate, 4-5 mm long, much shorter than petals. Stamens yellow, numerous. Styles 3, 3-10 mm long. Plants typically do not flower the first year. Insect pollinated and apomictic (seed develops without pollination).

HABITAT: Rangeland areas and pastures (especially when poorly managed), fields, roadsides, forest clearings in temperate regions with cool, moist winters and dry summers. Grows best in open, disturbed sites and on slightly acidic to neutral soils. Does not tolerate saturated soils.