A hardy plant capable of living in a variety of environments, hemlock is widely naturalised in locations outside its native range, such as parts of Australia, West Asia, and North and South America, to which it has been introduced.

The hollow stems are usually spotted with a dark maroon colour and become dry and brown after completing its biennial lifecycle.

[2] Conium maculatum is a herbaceous flowering plant that grows to 1.5–2.5 metres (5–8 feet) tall, exceptionally 3.6 m (12 ft).

The leaves are two- to four-pinnate, finely divided and lacy, overall triangular in shape, up to 50 centimetres (20 inches) long and 40 cm (16 in) broad.

[6] Hemlock can be confused with the wild carrot plant (Daucus carota, sometimes called Queen Anne's lace).

[8][9] The species can also be confused with harmless cow parsley (Anthriscus sylvestris, also sometimes called Queen Anne's lace).

[10] The genus name "Conium" refers to koneios, the Greek word for 'spin' or 'whirl', alluding to the dizzying effects of the plant's poison after ingestion.

[16][14] Other local or infrequent names in the US include bunk, California-ferm, cashes, herb-bonnet, kill-cow, Nebraska-fern, poisonroot, poison-snakeweed, St. Bennet's-herb, snakeweed, stinkweed, and wode-whistle.

[20] Conium maculatum grows in quite damp soil,[13] but also on drier rough grassland, roadsides and disturbed ground.

[20][36] Coniine acts directly on the central nervous system through inhibitory action on nicotinic acetylcholine receptors.

[34] With its high potency, the ingestion of seemingly small doses can easily result in respiratory collapse and death.

[37] The alkaloid content in C. maculatum also affects the thermoregulatory centre by a phenomenon called peripheral vasoconstriction, resulting in hypothermia in calves.

[41] A short time after ingestion, the alkaloids induce potentially fatal neuromuscular dysfunction due to failure of the respiratory muscles.

[20] For an adult, the ingestion of more than 100 mg (0.1 gram) of coniine (about six to eight fresh leaves, or a smaller dose of the seeds or root) may be fatal.

Narcotic-like effects can be observed as soon as 30 minutes after ingestion of green leaf matter of the plant, with victims falling asleep and unconsciousness gradually deepening until death a few hours later.

Chronic toxicity affects only pregnant animals when they are poisoned at low levels by C. maculatum during the fetus' organ-formation period; in such cases the offspring is born with malformations, mainly palatoschisis and multiple congenital contractures (arthrogryposis).

It is a common myth that C. maculatum alkaloids can enter the human food chain via milk and fowl, and scientific studies have disproven these claims.

[45] Socrates, the most famous victim of hemlock poisoning, was accused of impiety and corrupting the minds of the young men of Athens in 399 BC, and was sentenced to death at his trial.