Toxic Parts:
piperidine alkaloids
Flower Color:
  • flower color
meadows, haybales

Time of Greatest Risk


Geographical Distribution

Poison hemlock distribution - United States

Related Species

Poison Hemlock

Conium maculatum

Poison Parsley, Spotted Hemlock, Winter Fern, California Fern, Nebraska Fern,Wild Carrot, Fool's Parsley, Snake Weed, Poison Root, Cige, Bunk, Cashes, Cicuta, Winter Fern
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Poison hemlock (Conium maculatum) is an annual or biennial herb of the Apiaceae family. It is native to Europe and western Asia and was introduced into North America in the 1800s as an ornamental but has since then, escaped cultivation and now widely distributed across much of the United States and adjacent areas in Canada. As a biennial, poison hemlock produces leaves as a basal rosette it's first year of growth, which develops into an upright flower stalk during the second year.

Poison Hemlock Toxic Components

Poison hemlock contains eight piperidine alkaloids, however y-coniceine and coniine are generally the most abundant and the predominant cause of acute and chronic toxicity in horses. Coniine is a pyridine derivative similar in structure and function to nicotine. These toxins have a direct effect on the horse's nervous system, neuromuscular junction, and brain. Poison hemlock toxicity varies depending on the stage of growth, environmental conditions (rain, temperature, cloud cover), and soil. Toxicity increases throughout the growing season, especially in the roots, which resemble parsnips. Although all parts of the plant are toxic, the most toxic parts are the fruits when they are still green, and stems.

Signs of Poison Hemlock Poisoning in Horses

Signs of poison hemlock poisoning in horses develops 1 to 2 hours after ingestion. Horses don't typically graze hemlock unless food is scarce, or it is contaminated in hay. The lethal dose of poison hemlock ranges from 0.2 to 0.8% body weight or 2 to 8 lbs per 1,000-pound horse.

What Poison Hemlock Looks Like

  • Height: 2 to 3 m (6.6 to 10 ft) tall
  • Stem: Hollow, erect, branched, upright flower stalk the second year of growth. Light green, hairless, with distinctive purplish spots or blotches.
  • Leaves: Delicate, parsley or fern-like, large and lacy, alternate and basal, with the upper leaves progressively smaller. Leaflets are glossy green (that are darker on the top side), minute, lance-shaped, and have serrated edges. Leaves are attached to stems by leaf stalks (petioles).
  • Flowers: Small, white, 5-petaled, in terminal, umbrella-shaped clusters that are between 1 to 3 inches in diameter.
  • Root System: Long white taproot with fibrous secondary roots.


  • Nervousness
  • Muscle Tremors
  • Excessive Salivation
  • Incoordination
  • Increased Respiratory Rate
  • Listlessness
  • Rapid Heart Rate
  • Paralysis
  • Seizures
  • Dilated Pupils
  • Colic Signs
  • Frequent Defecation
  • Excessive Urination
  • Odor On Breath
  • Weakness
  • Muscular Paralysis
  • Death


MECHANICAL: Mow prior to seed production. Hand pull only while wearing gloves as the plant is highly poisonous.

BIOLOGICAL: Hemlock moth, a defoliating moth, gives inconsistent but sometimes good control.

CHEMICAL: Use a broadleaf weed killer such as 2, 4-D to protect bank-stabilizing grasses. Make sure the chemical is labeled for use around water when poison hemlock is growing in a ditch, stream, or wetland.