Common St. Johnswort (Hypericum perforatum)
Photo by Robert Videki, bugwood.org.
Common St. Johnswort, also called goatweed or Klamath weed, is a perennial forb in the Clusiaceae family that was intentionally brought to North America as a medicinal and ornamental plant. Promoted mostly as a natural antidepressant, its other intriguing uses have included treatment of skin disorders, wound healing, vodka distillation, and protection from evil spirits. Originally introduced to the eastern U.S. in the late 1600s, St. Johnswort spread to the West by the early 1900s.
Several features of St. Johnswort are key for identification. First, St. Johnswort has bright yellow flowers with five petals and numerous stamens; petals have glands along the edge which appear as black dots. Second, leaves are opposite and about an inch long; when held up to the light, glands appear as perforations or “pin pricks.” Third, stems are up to 2 ½’ tall, somewhat woody at the base and rust colored; this rust color is most apparent in autumn.
Photo by Ohio State Weed Lab, bugwood.org.
Native Look Alikes
There are three native Hypericum species in Montana that could be mistaken for common St. Johnswort. The most likely to cause confusion is western St. Johnswort (Hypericum formosum var. scouleri). Its sepals (leaf-like structures at base of petals) are rounded with amber veins, while those of common St. Johnswort are narrower and more pointed. Western St. Johnswort tends to be shorter (<1’ tall) and grows in wetter habitats and at higher elevation than common St. Johnswort. Another native, Tinker’s penny (H. anagalloides), grows low to the ground (1-5” tall) in montane and subalpine wet areas. The final native, larger Canadian St. Johnswort (H. majus), does not have black glands on its flower petals and prefers wetter habitats than common St. Johnswort. Read more about each species in the Montana Field Guide.
Photo by Norman E. Rees, bugwood.org.
St. Johnswort is toxic. All parts of the plant contain the pigment hypericin which causes photosensitivity, especially in areas around the mouth, nose, and ears. Light-haired cattle, horses, and sheep are particularly sensitive. Photosensitivity can lead to excessive scratching, skin blistering, elevated temperature, rapid respiration and pulse, and convulsions. Photosensitivity may extend to humans who are taking St. Johnswort as a dietary supplement. Like many invasive plants, common St. Johnswort can also form dense monocultures which limit biodiversity and reduce habitat for wildlife and forage for livestock.Impacts
A variety of options are available for managing common St. Johnswort including mechanical, chemical, and biological control. Details can be found in the MSU Extension publication St. Johnswort: Identification, biology, and integrated management. Fun trivia to share: Importation of the Chrysolina beetle in 1944 was the first attempt in North America to control weeds with insects. The beetle is native to Europe but obtaining insects from Europe was not possible during World War 2, so insects were imported from Australia where they had previously established from European introductions. Beetles had to be conditioned to sync their life cycle to the Northern Hemisphere.
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