Conium is a genus of two species of highly poisonous perennial herbaceous flowering plants in the family Apiaceae, native to Europe and the Mediterranean region (C. maculatum), and to southern Africa (C. chaerophylloides).
By far the most familiar species is Conium maculatum (Hemlock or Poison Hemlock), the most common of several species of hemlock noted for their toxicity. It is a herbaceous biennial plant which grows between 1.5–2.5 m tall, with a smooth green stem, usually spotted or streaked with red or purple on the lower half of the stem. The leaves are finely divided and lacy, overall triangular in shape, up to 50 cm long and 40 cm broad. The flowers are small, white, clustered in umbels up to 10–15 cm across. The plant is often mistaken for fennel, parsley or wild carrot although the characteristic stem hairs of the wild carrots are missing. The Conium root is fleshy, white and often unbranched and can be mistaken for parsnip. When crushed, the leaves and root emit a rank, unpleasant odour often compared to that of parsnips.
The most important and toxic of these is coniine. Coniine is a neurotoxin, which disrupts the workings of the peripheral nervous system and is toxic to people and all classes of livestock. Coniine causes death by blocking the neuromuscular junction in a manner similar to curare; this results in an ascending muscular paralysis with eventually paralysis of the respiratory muscles which results in death due to lack of oxygen to the heart and brain. Death can easily be prevented by artificial ventilation until the effects have worn off 48-72 hours later. Ingestion of Poison Hemlock in any quantity can result in respiratory collapse and death. For an adult the ingestion of more than 100mg of coniine (approximately 6 to 8 fresh leaves, or a smaller dose of the seeds or root), may result in fatality.
Conium maculatum has been introduced and naturalised in many other areas, including much of Asia, North America and Australia. Poison hemlock is often found on poorly drained soils, particularly near streams, ditches, and other surface water. Often referred to by children in south east Ireland as 'Devil's Porridge.'
Poison hemlock flourishes in the spring, when most other forage is gone. All plant parts are poisonous but once the plant is dried, the poison is greatly reduced, however not gone completely. Hemlock is also known as "poison parsley" or "spotted parsley".
Osha does not do well in overly moist soils since it is a species dependent on mycorrhizal fungi to survive, but there are areas where Osha and Poison Hemlock can be found only a few feet from each other. Poison Hemlock lacks the "spicy celery" odor of Osha, and is easily distinguished from it due to the absence of hairlike dead leaf material present on the root crown of Osha roots. Poison Hemlock roots in many cases have no discernible odor, and are typically heavily branched rather than carrot-like, but this is not always the case. The plants themselves smell musty or "mousy", and in most instances will have purple blotches or shading on the lower stem of the plant if the plant is fairly mature, but again, this is not always the case.
In the Mountain West of North America, poison hemlock has become well established and invasive, and can be found in remote mountain areas anywhere water is present or soils are persistently moist. It is often found growing in the same habitat and side by side with Osha and Lomatium species, useful medicinal relatives in the Parsley family which Hemlock closely resembles, and can be very difficult to distinguish from Lomatium (an important historical food plant of Native Americans known as Biscuit Root).
A useful trick to determine whether a plant is poison hemlock rather than fennel, which it resembles, is to crush some leaves and smell the result. Fennel smells like anise or liquorice, whereas the smell of poison hemlock is often described as mouse-like or musty. Considering the high toxicity of poison hemlock, if the plant cannot be identified it must be discarded. Coniine can be absorbed through the skin, and it is well advised to wash your hands immediately after handling this plant and avoid touching your eyes or mouth if you have recently handled or come into contact with Poison Hemlock, or if you have crushed the leaves of this plant in your hand to perform a "smell test".
Poison hemlock is sometimes confused with water hemlocks in the related genus Cicuta, but are readily distinguished by the less finely divided leaves of the latter; the leaf veins of poison hemlock also run through the tips of the teeth, but those of the water hemlock run through the notches in between the teeth. The poison hemlock's root is long, white, and fleshy and is usually stringy and heavily branched, but can be carrot-like and unbranched in younger specimens of Conium. Water hemlock's roots are made up of several tubers, and are typically chambered, and excude a yellow, rank, highly toxic sap that contains cicutoxin.
These plants all have white flowers in large compound umbels. Therefore, these plants are confused with each other; the water parsnip, (swamp parsnip, sium suave) and the western water hemlock, (Cicuta douglasii, poison hemlock) or the spotted water hemlock (cicuta maculata, spotted water hemlock, spotted parsley, spotted cowbane). Water parsnip and water hemlock both have cluster of small white flowers shaped like umbrellas, and both have the same habitat near the shore line of lakes, and rivers. Water parsnip has leaves only once compound, and water hemlock has leaves which are three times compound. Water hemlock has a large swelling at the stem base. All water hemlock is highly poisonous. Water parsnip is not poisonous. The water hemlock has bracts at the base of each small flower cluster, not at the base of the main flower head. The Water parsnip has small bracts at the base of flowers and main flower head as well. The Yarrow, (Common Yarrow, Gordaldo, Nosebleed plant, Old Man's Pepper, Sanguinary, Milfoil, Soldier's Woundwort, Thousand-leaf (as its binomial name affirms), Thousand-seal or Achillea millefolium) also has many small white flowers in a cluster. However the yarrow has feathery looking leaves which are pinnately separated into small narrow segments. The cow parsnip (heracleum lanatum, Heracleum maxinium Indian Celery or Pushki, and Heracleum sphondylium, hogweed) is also confused in this group with similar flower groupings. However, the cow parsnip has large, broad leaves, and an unpleasant odour.
In ancient Greece, hemlock was used to poison condemned prisoners. The most famous victim of hemlock poisoning is the philosopher Socrates. After being condemned to death for impiety in 399 BC, Socrates was given a potent solution of the hemlock plant. Plato described Socrates' death in the Phaedo :
Although many have questioned whether this is a factual account, careful attention to Plato's words, modern and ancient medicine, and other ancient Greek sources point to the above account being consistent with Conium poisoning.
Sometimes the characteristic red spots found on the stem and branches are referred to as "the blood of Socrates" in reference to his death.