Native Range

Europe, Asia, and northern Africa

Appearance

Poison hemlock is an herbaceous biennial (sometimes a winter annual or short-lived perennial) typically growing 3–6 ft (0.9– 1.8 m) tall from a long, stout taproot. Stems are hollow with faint longitudinal ridges, light green, and covered in purple spots or splotches. Leaves are 2–3 times compound, and fern-like leaflets are glossy dark green, finely divided, and have prominent veins on their undersides. Flowers are produced in spring through summer in umbels 3 in (7½ cm) across. Each umbel is further divided into umbellets, each with approximately 15 flowers that are tiny and have five white petals. Each fruit splits into two seeds that are oval, flattened on one side, and have conspicuous wavy ribs.

Impact

Poison hemlock is highly competitive, displacing native species in natural areas and reducing pasture and crop production. It is also an alternate host for many crop pests. All parts of the plant contain alkaloids that are highly toxic to livestock and humans and are responsible for numerous deaths and birth defects. Although livestock tend to avoid grazing the fresh plant, they may ingest dried poison hemlock present in hay or silage. Symptoms of poisoning in either livestock or humans may present as a lack of muscle control, nervousness, trembling, joint knuckling, dilated pupils, and a slow or weak heart beat. Death can occur in 30 minutes to two hours, depending on the species and size of the animal. In sensitive individuals, mere physical contact with foliage or roots may result in respiratory and skin reactions.

Reference

Randall, C.B., J.E. Andreas, and J. Milan. 2024. Poison Hemlock (Conium maculatum): 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-2024-22- POISON HEMLOCK-P. https://bugwoodcloud.org/resource/files/31428.pdf

Resources

• Element Stewardship Abstract - The Nature Conservancy
• Weed of the Week - USDA Forest Service
• Weed Field Guide - USDA Forest Service

Selected Images

Maps

Invasive Listing Sources

• California Invasive Plant Council
• Colorado Noxious Weeds
• Delaware Invasive Species Council
• Idaho Noxious Weeds
• Illinois Exotic Weed Act
• Indiana Invasive Species Council - Invasive Plant List
• Indiana Noxious Weeds
• Indiana Terrestrial Plant Rule
• Invasive Plant Species of West Virginia
• Iowa Noxious Weeds
• 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 - Severe Threat
• Maryland Invasive Species Council - Invasive Species of Concern in Maryland
• Mid-Atlantic Field Guide to Aquatic Invasive Species
• Minnesota Noxious Weeds
• National Park Service, Mid-Atlantic Exotic Plant Management Team Invasive Plant List
• Nevada Noxious Weeds
• New Jersey Invasive Species Strike Team 2017 Invasive Species List
• New Mexico Noxious Weeds
• Non-Native Invasive Plants of the City of Alexandria, Virginia
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• Ohio Invasive Species Council
• Ohio Noxious Weeds
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Pennsylvania Department of Conservation and Natural Resources Invasive Plants
• Pennsylvania Noxious Weeds
• Pennsylvania Regulations for Controlled Plants and Noxious Weeds
• Utah Noxious Weeds - Contain
• Washington Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States
• West Virginia Department of Agriculture Regulations Governing the Distribution of Plant Materials
• 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
• West Virginia Noxious Weeds
• Wisconsin's Invasive species rule – NR 40

References