Poison Hemlock

April 11, 2011 Print Friendly and PDF
Poison Hemlock (Conium maculatum), courtesy of USDA NRCS PLANTS Database

Scientific Name: Conium maculatum
Distribution: Throughout the United States, Canada, and Mexico
Type of Poison: Coniine, a volatile alkaloid, as well as other related alkaloids
Signs of Poisoning:

  • Abortions may occur in pregnant animals that recover
  • Nervous trembling
  • Salivation
  • Lack of coordination
  • Bloating
  • Dilation of pupils
  • Rapid, weak pulse
  • Blue coloration of lining of the mouth
  • Respiratory paralysis
  • Coma


About Poison Hemlock

Growth Characteristics: A forb, native to Europe, growing 5 to 10 feet tall. Stems are erect, stout, and purple-spotted with distinct ridges and extensively branched. Foliage has a strong musky odor.

Flowers/Inflorescence: Flowers are borne in many umbrella-shaped clusters, each supported by a stalk (pedicel). Flowers are white and lacking sepals.

Fruits/Seeds: Seeds are paired, one-eighth inches long, light brown, ribbed, and concave.

Leaves: Shiny, green, with the appearance of parsley leaves, growing 6 to 12 inches long. Lower leaves occur on long stalks that clasp the stem. Upper leaves on short stalks.

Where and When It Grows

Poison hemlock is a range plant that may kill sheep, cattle, horses, and other domestic animals. All are affected by eating a small amount. It is extremely poisonous to humans. The roots of poison hemlock may easily be mistaken for wild parsnips.

Because of its attractive flowers, poison hemlock was brought to the United States from Europe as a garden plant. It has become widespread on many ranges. Poison hemlock is found on roadsides, on edges of cultivated fields, in creek beds, in irrigation ditches, and in waste areas.

Poison hemlock starts growing in the early spring. It usually grows for two years, but in favorable locations it may be perennial.

Poison hemlock is sometimes confused with western waterhemlock, a more deadly species, because the names are similar.

How It Affects Livestock

All parts of poison hemlock — leaves, stem, fruit, root — are dangerous. Leaves are especially poisonous in the spring, up to the time the plant flowers. Fresh leaves have a nauseating taste, so livestock seldom eat hemlock when other feed is available. The root becomes more toxic during the plant's first year of growth.

Poison hemlock ingestion frequently is fatal. Sheep may be poisoned by eating as little as 4 to 8 ounces of green leaves. Cattle that eat 10 to 16 ounces may be affected.

Signs usually appear within an hour after an animal eats the plant. Animals die from respiratory paralysis in two to three hours. Convulsions, which are common in western waterhemlock poisoning, seldom occur with poison hemlock.

When cows eat poison hemlock between the 40th and 70th day of gestation, calves may be congenitally deformed, with symptoms indistinguishable from the lupine-induced crooked calf disease.

How to Reduce Losses

Give stimulants and a large dose of mineral oil to affected animals as soon as possible. This treatment may save animals that have not eaten an excessive amount of poison hemlock.

Animals that recover seldom show aftereffects, although pregnant animals may abort or give birth to deformed offspring.

Research results show that poison hemlock may be controlled by treating plants with herbicide. Contact your local Cooperative Extension agent for more information on specific herbicides. Follow all precautions when handling herbicides.

Hemlock Poisoning in Humans

People may be poisoned by eating any part of a hemlock plant. Often, poisoning occurs after the victim confuses hemlock root with wild parsnips, or hemlock seed with anise. Whistles made from hollow stems of poison hemlock have caused death in children.

If a person is poisoned by hemlock, take the affected person to the emergency room of the nearest hospital. Induce vomiting, if possible. Call a physician.


L.F. James, R.F. Keeler, A.E. Johnson, M.C. Williams, E.H. Cronin, and J.D. Olsen. Plants Poisonous to Livestock in the Western States. U.S. Department of Agriculture, Agriculture Information Bulletin 415, 90 pp. 1980.

Pratt, Mindy S. Poison hemlock. Range Plants of Utah. 2008. Utah State University Extension Service. 14 September 2009. http://extension.usu.edu/rangeplants/htm/poison-hemlock

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.