Botany

Taxonomy and Description
St. John's wort is a member of the genus Hypericum, of which there are 400 species worldwide. There is some disagreement as to the plant's family, some placing Hypericum in the segregate family Hypericaceae, while others place it in the family Guttiferae. However, most researchers now think that the morphological and chemical differences of the two families are insufficient to justify separating them (1,2).

The plants are described as glabrous perennials, erect and usually woody at the base. The ovate to linear leaves are sessile, opposite, and well-supplied with translucent glandular dots. The regular flowers have five short, subequal, entire, imbricate, basally connate sepals, and five persistent-withering yellow petals. The ovary is superior, capsicular, and three-styled. Stamens are many, arranged in bundles of threes, and the flowers are profuse, arranged in branched cymes which bloom from June until September. In the absence of insect pollination, apomixis commonly occurs.

St. John's wort should not be confused with rose of sharon (H. calycinum), a common ornamental ground-cover in the United States. Rose of sharon flowers and leaves are much larger than those of St. John's wort (though interestingly, anti-biotic substances have been extracted from H. calycinum that are similar in activity to substances in H. perforatum (3).

Range and Habitat
St. John's wort is native to Europe, West Asia, North Africa, Madeira and the Azores, and is naturalized in many parts of the world, notably North America and Australia (4,5). The plant spreads rapidly by means of runners or from the prodigous seed production and can invade pastures, disturbed sites, dirt roads, the sides of roads and highways, and sparse woods.

In the western United States, St. John's wort is especially prevalent in northern California and southern Oregon, hence one of its common names, "Klamath Weed". Because of the known photosensitizing properties of the plant, which can be toxic to cows and sheep, it has been considered a pest in some places. Prior to 1949, it was estimated to inhabit 2.34 million acres of rangeland in northern California. For years an attempt was made to control the plant with herbicides6, but with little success.

The solution to the problem with St. John's wort in northern California finally proved to be with biological methods of control, not pesticides. In 1946, the leaf-beetles Chrysolina quadrigemina Rossi, and to a lesser extent C. hyperici Forst, were introduced from Australia, where it had been observed that they had a voracious appetite for Hypericum. Their appetite proved to be so voracious, in fact, that by 1957 northern California's stands of St. John's wort were reduced to only 1% of their original number (5).