Common St. John’s-wort is a shrublike, much branched, perennial herb, with sometimes woody stem bases, and leafy shoots. Flowers are many, yellow, in flat or domed inflorescences. Petals 5, broad at base, with relatively few black dots, usually at or near the margins; sepals with or without a few yellowish-brown to black dots. Blooms May–September. Leaves many, crowded, opposite; each pair is at right angles with those above and below (decussate), with many translucent spots; sessile, linear to oblong, to 1½ inches long.
Similar species: There are 14 species of Hypericum recorded for Missouri. Some of the most common are these natives: shrubby St. John’s-wort (H. prolificum), St. Andrew’s cross (H. hypericoides), nits-and-lice (H. drummondii), pineweed (H. gentianoides), dwarf St. John’s-wort (H. mutilum), spotted St. John’s-wort (H. punctatum), and round-fruited St. John’s-wort (H. sphaerocarpum).
Height: to 3 feet.
Habitat and Conservation
Occurs in glades, prairies, and forest edges; also fields, pastures, levees, ditches, roadsides, railroads, and open, disturbed areas. A native of Europe. In Missouri, this species mainly colonizes roadsides and other disturbed areas. In western states, however, it is a major weed of rangelands. Pigment chemicals, including hypericin, which account for the dark dots on the petals and leaves, cause livestock to develop swollen mouths and ears and body sores upon sunlight exposure.
Over the years, St. John’s-worts have had different family names. Today, most scientists agree they should be in the Clusiaceae. In the past, they’ve been in the Hypericaceae, a more restricted family excluding many tropical members. An alternative name for the Clusiaceae, which you sometimes see, is Guttiferae. That name, which came from these plants’ guttiferous (resinous-sap-yielding) character, was used before botanists standardized all family names to have the ending “-aceae.”
In Europe, this species had a long history of various medicinal uses and to protect against witchcraft. Modern studies have shown that the active ingredient, hypericin, is effective in treating mild to moderate depression. But check with your doctor, as it can interact with other drugs.
Like many weedy Eurasian plants, this species traveled with Europeans as they spread across the globe. In many countries, and some US states, it is a genuine invasive weed, outcompeting native plants and degrading native ecosystems. Its poisoning of livestock makes it even less welcome.