[3] The white or pink flowered marsh St. John's worts of North America and eastern Asia are generally accepted as belonging to the separate genus Triadenum Raf.

[6][7] Hypericum is unusual for a genus of its size because a worldwide taxonomic monograph[8] was produced for it by Norman Robson (working at the Natural History Museum, London).

The pale glands, forming the pellucid dots, are each a schizogenous intracellular space lined with flattened cells that secrete oils and phloroglucinol derivates, including hyperforin.

When a species has a tree or shrub habit, the internodes become mostly terete with age, though some trace of lines can still be detected in mature plants.

The leaves lack stipules and can be sessile or shortly petiolar, though long petioles exist in sections Adenosepalum and Hypericum.

[9] Almost all Hypericum petals are yellow, though a range of color exists from a pale lemony hue to a deep orangish-yellow.

In those two sections, sterile bodies have developed between the stamen fascicles, working as lodicules to spread the petals of the pseudotubular flower, a specialized pollination mechanism.

For example, H. lloydii is susceptible to a fungal infection as a seedling if conditions are too moist, whereas other species including H. chapmanii can grow underwater.

They include: H. perforatum is an invasive species and noxious weed in farmland and gardens in the humid and sub-humid temperate zones of several continents.

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.

[14] 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.

[12] A small, reddish-brown weevil, Anthonomous rutilus breeds in the inflorescences of Hypericum kalmianum and H. swinkianum, the larvae developing within the fruit capsules.

[15] H. perforatum is the most commonly used species – especially in Europe – as an herbal substitute for prescription drugs to treat depression, and is also sold as a dietary supplement.

[15] One meta-analysis found that St John's wort had similar efficacy and safety as prescriptions drugs to treat mild-to-moderate depression, such as selective serotonin reuptake inhibitors.

[16] There is evidence that combining St. John’s wort with prescription antidepressants may cause adverse effects, such as a life-threatening increase of serotonin, the brain chemical targeted by some drugs used for depression.

[15] St. John's wort interacts with hormonal contraceptives, reducing their effectiveness and increasing the risk of unplanned pregnancy.

The following have gained the Royal Horticultural Society's Award of Garden Merit: Most species of Hypericum are prone to thrips, scale, anthracnose, rust, and leaf spots.

[1] Fossil seeds from the early Miocene of †Hypericum septestum have been found in the Czech part of the Zittau Basin.

[24] Many fossil seeds of †Hypericum holyi have been described from middle Miocene strata of the Fasterholt area near Silkeborg in Central Jutland, Denmark.