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. Agrilus hyperici were successfully introduced as biocontrol agents. Localized outbreaks of the plant sometimes occur after disturbances such as logging, fire, or during low population cycles of the flea and root-boring beetles.