Eastern Forest Threat Center - St. Johnswort

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St. Johnswort flowers

St. Johnswort flowers

David Cappaert, Michigan State University, Bugwood.org

St. Johnswort
Hypericum perforatum

St. Johnswort is native to Europe, North Africa, and Asia, except for the Arctic regions. It was introduced into the U.S. during the 1700s as an ornamental.

Keywords: Clusiaceae, perennial, herb, spotted leaves, star-like flowers, creeping stems, rhizomes; Common names: Klamath weed, common goatweed, tipton weed

Distribution Map Distribution Source Image

Threat Description

St. Johnswort is an erect perennial herb typically growing 1-2½ ft. in height. Reddish stems are single or multiple, smooth, somewhat two-edged, woody at the base, and branching out toward the top of the plant. Narrow, lance-shaped leaves are 1-2 in. long, stalkless with pointed tips, and spotted with tiny translucent dots. Yellow star-like flowers have 5 petals with tiny black dots on the margins. Flowers occur in clusters at the ends of stems with 25-100 flowers per cluster. Fruit is a 3-sectioned pod with numerous dark brown seeds. One plant can produce up to 100,000 wind- and animal-dispersed seeds per year that are viable for ten years. St. Johnswort also spreads by its above-ground creeping stems and underground rhizomes. The root system consists of a long taproot with shallow rhizomes extending several inches from the crown. St. Johnswort prefers poor, sandy, dry soils and full sun, and can be found primarily in waste areas, railroad rights-of-way, sidewalk cracks, roadsides, meadows, dry pastures, rangelands, fields, open woods, dunes, and disturbed ground. This ecologically invasive plant crowds out native species and is toxic to livestock.