Hypericum // is a genus of flowering plants in the family Hypericaceae (formerly considered a subfamily of Clusiaceae). Hypericum is unusual for a genus of its size because a worldwide taxonomic monograph was produced for it by Norman Robson (working at the Natural History Museum, London). Robson recognizes 36 sections within Hypericum.
The genus has a nearly worldwide distribution, missing only from tropical lowlands, deserts and polar regions. All members of the genus may be referred to as St. John's wort, and some are known as tutsan. The white or pink flowered marsh St. John's-worts of North American and eastern Asia are now separated into the genus Triadenum.
There are over 490 species in the genus. The name hypericum may derive from the Greek for "above pictures," for its use over shrines to repel evil spirits, though some have translated it as "above the heath". Hypericums range from herbaceous annuals or perennials 5–10 cm tall to shrubs and small trees up to 2 m (6.6 ft) tall. The leaves are opposite, simple oval, 1–8 cm long, either deciduous or evergreen. The flowers vary from pale to dark yellow, and from 0.5–6 cm in diameter, with five (rarely four) petals, most having prominent stamens. The fruit is usually a dry capsule which splits to release the numerous small seeds.
Hypericum is a large genus, containing 400–500 species.
Hypericum is broken up into 36 sections, each with its own subsections and species. They include:
Hypericum species are the only known food plants of the caterpillar of the treble-bar, a species of moth. Other Lepidoptera species whose larvae sometimes feed on Hypericum include the common emerald, the engrailed (recorded on imperforate St. John's-wort, H. maculatum), the grey pug and the setaceous Hebrew character. A leaf beetle, Paria sellata, feeds on the foliage of Hypericum adpressum, while ant species Formica montana and F. subsericea decorate their nests with its bright yellow petals. A small, reddish-brown weevil, Anthonomous rutilus breeds in the inflorescences of Hypericum kalmianum and H. swinkianum, the larvae developing within the fruit capsules.
St. John's-worts can occur as nuisance weeds in farmland and gardens. On pastures, some can be more than a nuisance, causing debilitating photosensitivity, and sometimes abortion in livestock. The beetles Chrysolina quadrigemina, Chrysolina hyperici and the St. John's-wort Root Borer (Agrilus hyperici) like to feed on common St. John's-wort (H. perforatum) plants and have been used for biocontrol where the plant has become an invasive weed.
Common St. John's-wort (H. perforatum) has long been used in herbalism. It was known to have medical properties in Classical Antiquity and was a standard component of theriacs, from the Mithridate of Aulus Cornelius Celsus' De Medicina (ca. 30 CE) to the Venice treacle of d'Amsterdammer Apotheek in 1686. Folk usages included oily extract ("St. John's oil") and Hypericum snaps, a small alcoholic drink. Hypericum perforatum is the most potent species and it is today grown commercially for use in herbalism and medicine.
Two main compounds found in Hypericum species have been studied in more detail: hyperforin and hypericin. As psychiatric medication, it is usually taken in pill form, but may also be consumed as a tea. Standardised preparations are commercially available and research has mainly studied alcoholic extracts and isolated compounds. Research has found a noticeable effect in many cases of light and medium clinical depression, but no significant improvement of severe depression and OCD.
The red, oily extract of H. perforatum may help heal wounds. Both hypericin and hyperforin are reported to have antibiotic properties. Justifying this view with the then-current doctrine of signatures, herbalist William Coles (1626–1662) wrote in the 17th century that:
"The little holes where of the leaves of Saint Johns wort are full, doe resemble all the pores of the skin and therefore it is profitable for all hurts and wounds that can happen thereunto."
Hypericum perforatum may also be capable of reducing the physical signs of opiate withdrawal.
Hypericum extract, by inducing both the CYP3A4 and the P-glycoprotein (P-gp), can reduce the plasma concentrations of different antineoplastic agents such as imatinib, irinotecan and docetaxel, thus reducing the clinical efficacy of these drugs.
Numerous hybrids and cultivars have been developed for use in horticulture, such as H. × moserianum (H. calycinum × H. patulum), H. 'Hidcote' and H. 'Rowallane'. All of the above cultivars have gained the Royal Horticultural Society's Award of Garden Merit.
Fossil seeds from the early Miocene of †Hypericum septestum have been found in the Czech part of the Zittau Basin.  Many fossil seeds of †Hypericum holyi have been described from middle Miocene strata of the Fasterholt area near Silkeborg in Central Jutland, Denmark.
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|Wikispecies has information related to Hypericum|
- "Hypericum Online".
- University of Illinois Extension. "Selecting Shrubs for Your Home - Kalm St. Johnswort (Hypericum kalmianum)".