Poison Hemlock

Conium maculatum

Apiaceae, Apiaceae (Parsley family)


Poison hemlock is a biennial of the parsley family. It has stout, erect, hollow stems that may be purple streaked or splotched and may grow to up to 10 feet tall. Leaves can be 6 inches wide and 12 inches long, with many oval to broadly oval leaflets opposite each other. The leaf stems clasp the main stem at their junction. White flowers are arranged in umbrella-shaped clusters.


These plants are found in dense stands in roadside ditches and stream banks in the southern half of Texas.

Toxic Agent

Poison hemlock contains pyridine alkaloids. The stems and leaves are the most toxic parts of the plant. Cattle and swine are the species affected most often. This plant is hazardous to humans and was used in political executions in ancient Greece (Socrates). Cattle seldom graze the plant, but may be poisoned by it in hay or green chop. The roots or young leaves may poison swine. Hay containing poison hemlock is considered hazardous.

Signs of Livestock Ingestion

Signs of acute poisoning occur within a few hours of consumption; these include initial stimulation followed by progressive central nervous system depression.

Stimulation: Nervousness; Muscle tremors; Incoordination; Salivation; Colic.

Depression: Partial paralysis; Slow heart rate; Low body temperature; Slow respiration rate; Coma; Death.

Low-level intake during a specific period of gestation (50 to 75 days in cattle) may cause birth defects. "Crooked calf disease" may include cleft palate and skeletal deformities. The amount of plant needed to produce birth defects may not be enough to cause illness in the dam.

Management Strategies

Do not cut hay or green chop from areas containing this plant. This is especially of concern when roadsides are used to harvest lowquality hay during drought. Severely poisoned animals may be given stimulants and supportive care.