Native Range

Europe, northern Africa, Asia

Appearance

Common St. Johnswort is an herbaceous perennial forb typically growing 1–3 ft (0.3–1 m) tall with numerous, rust-colored stems that are somewhat woody at their base. The plant has both a taproot and lateral roots. Leaves are opposite, without stems or lobes, and are up to 1 in (2½ cm) long. Leaves have numerous transparent glands as well as tiny black glands along their margins. Flowers are numerous, bright yellow, ¾ in (1.9 cm) in diameter, have many stamens, and have petals with additional black glands along margins. Seed pods are sticky, 3-celled, less than ½ in (1¼ cm) long, and filled with several small brown seeds.

Impact

Common St. Johnswort is a vigorous competitor in pastures, rangelands, and natural areas, displacing native and/or more desirable forage species throughout the West. At its peak densities in the mid-1900s, it was considered the leading cause of economic loss to California agriculture, attributed both to stock fatalities and the loss of pasture and rangeland. Glands on the weed’s foliage produce hypericin, an oil and phototoxin. Upon ingestion, animals become sensitive to sunlight, often leading to dermatitis, inflammation of the mucus membranes, itching, swelling, blisters, and open sores. Animals with lighter pigmentation, a thinner fur or wool covering, and softer skin (young) are affected most, as are nursing animals whose mothers are exposed to hypericin. If consumed in large quantities, starvation, dehydration, and death may occur. Humans may also experience the same reactions when using herbal remedies containing hypericin.

Reference

Randall, C.B., J.E. Andreas, J. Milan, and C.E. Moffat. 2024. Common St. Johnswort (Hypericum perforatum): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2024-18-COMMON ST JOHNSWORT-P. https://bugwoodcloud.org/resource/files/33585.pdf

Resources

• Fire Effects Information System - USDA Forest Service

Selected Images

Maps

Invasive Listing Sources

• Alberta Weed Control Act
• California Invasive Plant Council
• City of Ann Arbor Michigan Parks and Recreation
• Colorado Noxious Weeds
• Hoffman, R. & K. Kearns, Eds. 1997. Wisconsin manual of control recommendations for ecologically invasive plants. Wisconsin Dept. Natural Resources, Bureau of Endangered Resources. Madison, Wisconsin. 102pp.
• Indiana Invasive Species Council - Invasive Plant List
• Invasive Plant Association of Wisconsin
• Invasive Plant Species of West Virginia
• Jil M. Swearingen, Survey of invasive plants occurring on National Park Service lands, 2000-2007
• John Randall, The Nature Conservancy, Survey of TNC Preserves, 1995.
• Kentucky Exotic Pest Plant Council - Moderate Threat
• Montana Noxious Weeds
• National Park Service, Mid-Atlantic Exotic Plant Management Team Invasive Plant List
• National Park Service, National Capital Region Exotic Plant Management Team Invasive Plant List
• Native Plant Society of Oregon, 2008
• Nevada EDRR
• Nevada Noxious Weeds
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• Ontario’s Invading Species Awareness Program Tracked Species List
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Utah Noxious Weeds - Early Detection
• Washington Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States
• West Virginia Invasive Species Strategic Plan and Volunteer Guidelines 2014
• West Virginia Native Plant Society, Flora West Virginia Project, and West Virginia Curatorial Database System, September 3, 1999
• Wyoming Noxious Weeds

References