Common St. Johnsworth

Common St. Johnswort is an herbaceous perennial in the Hypericaceae/Clusiaceae (St. Johnswort) family. It produces one or multiple erect stems from the crown that may reach 2 to 5 feet tall. These stems are woody at the base and branched toward the top. Leaves occur opposite each other and are oblong, reaching about 1 inch long, and they have many transparent glands that are visible when held up to the light. Common St. Johnswort flowers from early to late summer. Flowers occur in flat-topped cymes (clusters) at the end of stems. Each flower is yellow and star-shaped, containing 5 petals and many protruding stamen. Common St. Johnswort has both a deep taproot and lateral rhizomes. Seeds are produced en masse in 3-celled capsules and are small, black, round, and shiny. Seeds may be viable in the soil for more than 50 years.

Common St. Johnswort is native to Europe, western Asia, and Northern Africa and was introduced to North America in 1793 likely for medicinal use. It creates dense infestations by spreading rhizomes and seeds and spreads to new locations primarily by seeds. It is primarily found in disturbed sites, roadsides, woodland, rangeland, and meadows.

Prevention strategies and cultural control strategies should be utilized as much as possible.

There are two biological control agents available for common St. Johnswort; however, infestations of common St. Johnswort in Teton County are not always dense or extensive enough to support viable populations of these agents. Agents in Teton County include leaf feeding moths or “inchworms” (Aplocera plagiata) and leaf feeding beetles (Chrysolina quadrigemina).

Hand pulling and digging out of the taproot and lateral rhizomes can effectively small patches of common St. Johnswort, but these must be removed below the crown while also removing as much of the rhizome as possible. Wear gloves and long sleeves when handling common St. Johnswort as it is poisonous outside of a medicinal setting. Mowing at regular, two week intervals can effectively deplete food stores in roots. Grazing is not recommended as common St. Johnswort is toxic to livestock and wildlife.

Larger infestations can be controlled with herbicides. Spring and early summer treatments prior to bloom are effective. Read the label before using any herbicide. Contact TCWP if you have any questions about application rates or how to use an herbicide.

• Common St. Johnswort (2013) DiTomaso, J.M., G.B. Kyser et al. Weed Control in Natural Areas in the Western United States. Weed Research and Information Center, University of California.
• Common St. Johnswort (2023) Washington State Noxious Weed Control Board.
• Best Practice Management Guide: St. Johnswort (2020) Best Practice Management Guide for Environmental Weeds. Cooperative Research Centre for Weed Management Systems. University of Adelaide. ISSN 1442-7192
• Common St. John’s-wort. Montana Field Guide. Montana Natural Heritage Program