poison-hemlock USDA PLANTS Symbol: COMA2
U.S. Nativity: Exotic
Habit: Forbs/Herbs
Conium maculatum L.

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Taxonomic Rank: Magnoliopsida: Apiales: Apiaceae
Synonym(s): poison hemlock, deadly hemlock, poison parsley
Native Range: Europe (BAIL);

Poison hemlock is a biennial (usually) herb that can grow from 3-10 ft. (1-3 m) in height. Stems are hollow, ribbed and purple-spotted. Plants begin as a rosette of leaves and flower in the second year of growth. Leaves are opposite, finely dissected, 8-16 in. long, triangular and emit a foul odor when crushed. The petioles often sheath the stem. Flowering occurs from May to August, when many umbrella-shaped heads (umbels) of small, white flowers develop at the apex of the stems. Umbels are 2-2.5 in. (5-6.2 cm) in diameter and contain many 5-petaled flowers. One plant can produce over 30,000 seeds. Plants, when eaten, are poisonous to most animals. Poison hemlock is native to Europe and was introduced into North America in the 1800s as an ornamental. Poison hemlock can sometimes be confused with water hemlock (Cicuta maculata) and giant hogweed (Heracleum mantegazzianum). The leaf veins in water hemlock end in the notches between the teeth of the leaflets and in poison hemlock the veins end at the tips of the teeth. Giant hogweed has larger, less divided leaves and a hairy stem.

Identification, Biology, Control and Management Resources

Selected Images from Invasive.orgView All Images at Invasive.org


Plant(s); in flower
Joseph M. DiTomaso, University of California - Davis, Bugwood.org
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Flower(s);
Steve Dewey, Utah State University, Bugwood.org
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Flower(s);
John D. Byrd, Mississippi State University, Bugwood.org
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Foliage;
John D. Byrd, Mississippi State University, Bugwood.org
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Foliage; the leaf on the right is from poison hemlock (Conium maculatum), and the leaf on the left is from western water hemlock (Cicuta douglasii).
Steve Dewey, Utah State University, Bugwood.org
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Feature(s); Purple speckles
Steve Dewey, Utah State University, Bugwood.org
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Plant(s); in flower
Jan Samanek, State Phytosanitary Administration, Bugwood.org
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Fruit(s);
Jan Samanek, State Phytosanitary Administration, Bugwood.org
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Fruit(s);
Jan Samanek, State Phytosanitary Administration, Bugwood.org
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Stem(s); close-up of main stem
Jan Samanek, State Phytosanitary Administration, Bugwood.org
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Infestation;
Eric Coombs, Oregon Department of Agriculture, Bugwood.org
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Fruit(s); Adaxial side of three mericarps. Apex at top.
D. Walters and C. Southwick, USDA, Bugwood.org
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Fruit(s); Abaxial (left) and adaxial (right) sides of two mericarps. Apex at top.
D. Walters and C. Southwick, USDA, Bugwood.org
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Plant(s); Britton, N.L., and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 2: 653.
USDA PLANTS Database, USDA NRCS PLANTS Database, Bugwood.org
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Plant(s); USDA NRCS. Wetland flora: Field office illustrated guide to plant species. USDA Natural Resources Conservation Service.
USDA PLANTS Database, USDA NRCS PLANTS Database, Bugwood.org
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EDDMapS Distribution:
This map is incomplete and is based only on current site and county level reports made by experts and records obtained from USDA Plants Database. For more information, visit www.eddmaps.org
 


State(s) Where Reported invasive.
Based on state level agency and organization lists of invasive plants from WeedUS database.

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U.S. National Parks where reported invasive:
Antietam National Battlefield (Maryland)
Colonial National Historical Park (Virginia)
Grand Canyon National Park (Arizona)
Harpers Ferry National Historical Park (West Virginia)
Monocacy National Battlefield Park (Maryland)
Stones River National Battlefield (Tennessee)



Invasive Listing Sources:
California Invasive Plant Council
Faith Campbell, 1998
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.
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
Mid-Atlantic Exotic Pest Plant Council, 2005
Pacific Northwest Exotic Pest Plant Council, 1998
Pennypack Ecological Restoration Trust, Pennsylvania.
Tennessee Exotic Pest Plant Council
Virginia Department of Conservation and Recreation, 2009