Habitat
St. John's wort is native to Europe and is now widespread in
the United States, being found in all but five states. It used to be
used for mainly ornamental
reasons. Hypericum
perforatum has the ability to grow in many environments. It
grows best in sunny, dry areas with little vegetation. It is
most commonly found in fields, pastures, waste areas, and on the
edge of forests.
Though most soils are adequate for this plant, it does struggle with warm, wet areas. This is the main reason why it struggles to grow during the summer in the US. The warm soil is too moist and it rots the plant roots. The plant grows best in heat zones 4-9. They are also unable to survive in shady areas, therefore, planting trees will suppress the plant's ability to grow.
Since they are able to survive in many different environments St. John's wort is classified as a noxious weed in the United States. It is nicknamed the Klamath weed since it grows abundantly near the Klamath River in California. It is able to grow rapidly in fields and pastures since it is mostly ignored by cattle.