But other south west areas are at risk from this weed. St. John's wort can overrun grazing land, particularly when pastures get denuded. The leaves of St. John's wort contain a toxin which causes photosensitization amongst white faced stock. The plant also affects animals nervous systems, causing unthriftiness and sometimes death. St Johns wort is a perennial plant which reproduces from seed and from creeping underground rhizomes.
Seeds germinate in either spring or autumn.
The plants do not flower in their first year, but flower and seed prolifically in later years. Seeds may survive in the soil up to six years before germination. New infestations are formed from seed which has been spread in water or mud, on vehicles, stock and machinery and in hay, silage and chaff. Existing infestations slowly spread as the rhizomes creep outwards. New plants also form from pieces of root spread by cultivating machinery.