Hypericum perforatum, also known as St John's wort, is a flowering plant species of the genus Hypericum and a medicinal herb that is sold over-the-counter as a treatment for depression. Other names for it include Tipton's weed, rosin rose, goatweed, chase-devil, or Klamath weed. With qualifiers, St John's wort is used to refer to any species of the genus Hypericum. Therefore, H. perforatum is sometimes called common St John's wort or perforate St John's wort to differentiate it.
Hypericum is classified in the family Hypericaceae, having previously been classified as Guttiferae or Clusiaceae. Approximately 370 species of the genus Hypericum exist worldwide with a native geographical distribution including temperate and subtropical regions of Europe, Turkey, Ukraine, Russia, Middle East, India, and China.
St John's wort is a perennial plant with extensive, creeping rhizomes. Its stems are erect, branched in the upper section, and can grow to 1 m high. It has opposing, stalkless, narrow, oblong leaves that are 12 mm long or slightly larger. The leaves are yellow-green in colour, with transparent dots throughout the tissue and occasionally with a few black dots on the lower surface. Leaves exhibit obvious translucent dots when held up to the light, giving them a ‘perforated’ appearance, hence the plant's Latin name.
Its flowers measure up to 2.5 cm across, have five petals, and are coloured bright yellow with conspicuous black dots. The flowers appear in broad cymes at the ends of the upper branches, between late spring and early to mid summer. The sepals are pointed, with glandular dots in the tissue. There are many stamens, which are united at the base into three bundles. The pollen grains are ellipsoidal. When flower buds (not the flowers themselves) or seed pods are crushed, a reddish/purple liquid is produced.