A native of Europe, Klamath weed (Hypericum perforatum) most likely entered the United States through the Pacific Northwest seaports and has now become a noxious weed or pretty wildflower, depending on your view, throughout Oregon, Washington and Northern California. This bright yellow flower is also know, particularly to herbalists and gardeners, as common St. Johnswort (it is also written as Saint John’s Wort in the literature).
A perennial that reproduces by seeds or short runners, Klamath weed prefers open hillsides, rangelands, and forest openings where it can get full sunlight. Growing one to three feet in height, the stems of Klamath weed have numerous branches. The opposite leaves are sessile, meaning they do not have stalks, and often the leaf base wraps around the stem slightly. In addition to prominent veins, the leaves are covered with transparent dots that can be seen when the leaf is held to the light. The numerous flowers, with five separate petals and numerous stamens arranged in three groups, form flat-topped flower clusters (cymes). Purplish glands containing hypericin form on the petals and leaves and cause a red stain when the foliage is rubbed between the fingers.
Klamath weed is not edible. It has been used in medicine since ancient times to treat a variety of ailments including depression, wounds, diarrhea, intestinal worms and dysentery. Chemically, common St. Johnswort is one of the most extensively studied plants in modern medicine. Herbalists use it to treat depression and anxiety, for circulatory and heart problems, as a sleep aid and to treat PMS and gastrointestinal complaints. It also has shown some promise as a retrovirus inhibitor, prompting study of hypericin for use in HIV treatment.
Hypericin, thus Klamath weed, causes skin irritation and photosensitive reactions in livestock that eat it. The affected animals rarely die but they do lose weight and develop skin irritations when exposed to sunlight. This is especially true for light-skinned animals. Not only are animals subject to this photosensitive reaction, humans ingesting hypericin also experience the reaction.
Personally I will not use or experiment with Klamath weed – St. Johnswort. Anything that can kill intestinal worms and causes severe photosensitive reactions should not be casually consumed.
These Klamath weed plants were growing near a road south of Fall River Mills CA (Shasta County). The glands containing hypericin are visible on the leaf and flower closeups.