Hypericum perforatum, commonly known as St John's wort, is a flowering plant in the family Hypericaceae and the type species of the genus Hypericum. It is a perennial that grows up to one metre tall, with many yellow flowers that have conspicuous black glands around their edges, long stamens, and three pistils. Probably a hybrid between close relatives H. attenuatum and H. maculatum that originated in Siberia, the species now has a cosmopolitan distribution. It is native to temperate regions across Eurasia and North Africa, and has been introduced to East Asia, Australia, New Zealand, and parts of North and South America. In many areas where it is not native, H. perforatum is considered a noxious weed. It densely covers open areas to the exclusion of native plants, and is poor grazing material. As such, methods for biocontrol have been introduced in an attempt to slow or reverse the spread of the species.
The species produces numerous phytochemicals in large amounts that are highly active. These chemicals are harmful to large animals, especially sheep, and help to deter herbivores from consuming the plant. Other chemicals in the plant, such as hypericin and hyperforin, have various uses in medicine. St John's wort has been used in traditional medicine since at least the first century AD, often as a cure-all or panacea. The oil from its glands can be extracted, or its above-ground parts can be ground into a powder called herba hyperici. In modern times, its use as an antidepressant has been the focus of numerous studies and clinical trials; however, the active ingredients can be very harmful or even lethal when taken alongside other medicines.
Hypericum perforatum is an herbaceous perennial plant with hairless (glabrous) stems and leaves. The root of each plant is slender and woody with many small, fibrous small side roots and also extensive, creeping rhizomes. The taproot will grow to a depth of 0.6–1.5 meters into the soil depending on conditions. The crown of the root is woody.