H. perforatum, commonly called the common St. John's wort, klamathweed, tiptonweed, goatweed, and enolaweed (1), is a native of Europe, but has spread to temperate locations in Asia, Africa, North and South America, and Australia (2,3). It thrives in poor soils, and is commonly found in meadows, fields, waste areas, roadsides, and abandoned mines and quarries (1,2,4).
Individuals of St. John's wort are freely branching perennials that typically range from 40 to 80 cm tall (Fig. 1) (1,2). The stems are herbaceous, though the bases are somewhat woody. The stems and branches are densely covered by oblong, smooth-margined leaves that range from 1 to 3 cm long and 0.3 to 1.0 cm wide. The leaves are interrupted by minute translucent spots that are evident when held up to the light. The upper portions of mature plants can produce several dozen five-petaled yellow flowers that are typically 1.0-
Figure 1 Diagram of St. John's wort, Hypericum perforatum. (Image from http://www.thorne.com/altmedrev/hypericum.jpg.)
2.0 cm wide. The edges of the petals are usually black-dotted. Crushed flowers produce a blood-red pigment. By late summer, the flowers produce capsules that contain dozens of tiny dark-brown seeds. St. John's wort reproduces both by seed and by ground-level rhizomes.
Because of concerns over phototoxicity to livestock, H. perforatum is listed as a noxious weed in seven western states in the United States. Programs promoting its eradication are underway in Canada, California, and Australia. Some of those measures include use of the Chysolina beetle, which is a natural predator (3).
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