[Wikipedia] Eurasia native: Possibly a hybrid between H. maculatum and H. attenuatum, the species can be found across temperate areas of Eurasia and has been introduced as an invasive weed to much of North and South America, as well as South Africa and Australia. While the species is harmful to livestock and can interfere with prescription drugs, it has been used in folk medicine over centuries, and remains commercially cultivated in the 21st century. (link added by Mary Ann Machi)

[Cal-IPC] Invasive: Hypericum perforatum (common St. Johnswort, klamathweed) is a perennial plant (family Clusiaceae) grown for medicinal use, but it can be toxic to light colored livestock when consumed in large quantities. By 1940, more than 1 million hectares of California were infested by St. Johnswort, but biological control agents have eliminated most populations below 1500 m elevation. (link added by Mary Ann Machi)