H. perforatum is a medicinal plant which is poisonous to livestock, and which is considered a weed even in some of the countries where it is native. Such countries include Turkey, Italy, France, Hungary and Sweden, where it is a weed mostly in poorer pastures, neglected areas and occasionally in crops (Parsons and Cuthbertson, 1992). It has been widely introduced, and has become a serious invasive problem in parts of Canada, USA, Mexico, Argentina, Chile, South Africa, Reunion, Japan, Australia, New Zealand and Hawaii, often because of the damaging photosensitivity it can cause in livestock. Once established in an area it spreads vegetatively and is hard to control without damaging other pasture species. It is invasive in natural ecosystems as well as in agricultural land. This species is considered to have a number of herbal uses and has therefore been spread to many places as a useful medicinal or ornamental plant as well as an accidental hitchhiker on agricultural produce.