poison hemlock

Apiales > Apiaceae > Conium maculatum L.
Synonym(s): deadly hemlock, poison parsley

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

  • Weeds of the Week - USDA Forest Service, Northeastern Area, Forest Health Protection

Selected Images from Invasive.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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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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Feature(s); Purple speckles
Steve Dewey, Utah State University, Bugwood.org
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Feature(s); close-up of main stem
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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Plant(s); in flower
Jan Samanek, State Phytosanitary Administration, Bugwood.org
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Infestation;
Eric Coombs, Oregon Department of Agriculture, Bugwood.org
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Seed(s);
Steve Hurst, USDA NRCS PLANTS Database, Bugwood.org
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Diagram or Graphic; 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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Diagram or Graphic; 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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Invasive Reference(s):

Check Invasive.org for most current lists.
  • California - Invasive Plant Inventory
  • Colorado - Noxious Weed Law
  • Iowa - Noxious Weed Law
  • Kentucky - EPPC List
  • New Mexico - Noxious Weed Law
  • Nevada - Noxious Weed Law
  • Ohio - Noxious Weed Law
  • Oregon - Noxious Weed Law
  • Tennessee - EPPC List
  • Texas - Invasive Plant List
  • Virginia - Invasive Alien Plant Species
  • Washington - Noxious Weed Law
  • Mid-Atlantic - EPPC List
  • Invasive Plants: Western North America
  • Invasive Plants: Guide to Identification and the Impacts and Control of Common North American Species
  • Invasive Plants of the Upper Midwest


External Links


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