Conium maculatum
Conium maculatum, the plant better known as hemlock, was once swallowed on purpose by an English physician named John Harley. In 1869 he published what he learned in a book called The Old Vegetable Neurotics. He had tested a hemlock preparation, succus conii, on himself, and he recorded the result: "The mind remained perfectly clear and calm, and the brain active throughout; but the body seemed heavy, and well-nigh asleep."
That steady tone hides a plant with a deadly reputation. This flowering weed belongs to the carrot family, and it now grows on several continents far from where it began. Its seeds, roots, leaves and stems have all been blamed for poisonings.
How does a weed from the carrot family become a poison? Why do people still mistake it for food? And how did a plant that pharmacists once listed as a medicine end up treated as a hazard to cattle, gardeners and anyone who touches it?
In its first year, Conium maculatum has no stems at all, only a large rosette of leaves. The second year changes everything. Coarse, heavily branched stems rise to as much as 8 ft, hollow except at the joints where the leaves attach. They are generally spotted or streaked with purple, and they turn dry and brown once the plant finishes its two-year cycle.
Under the right conditions the growth is quite rapid, and a long, white, fleshy taproot anchors the plant. That root is usually unbranched. Hemlock is a biennial that can occasionally live as a perennial, and it is a nitrophile, a plant that favours nitrogen-rich ground. Most people notice the odour before anything else. It is usually judged unpleasant, and it carries on the wind.
The leaves are lacy and finely divided, ranging from one- to three-pinnate, with leaflets attached in opposite pairs along the central veins. The lower leaves are the largest, broad and triangular, and more divided than the upper ones, which are often only partly divided. Every part of the plant is glabrous, meaning free of hairs, although the lower parts sometimes carry a thin blue-grey wax.
Each flower is small and white, with five petals and no sepals. The flowers sit in umbrella-shaped clusters called umbels, found at the tips of branches and in the axils, where a leaf stem meets the main stem. Every umbel has ten to twenty rays, short stems between 1 and 3.5 cm long. The fruit is gray-brown, ridged and egg-shaped, measuring 2.5 to 3.5 mm. It splits easily into two parts, a type known as a schizocarp.
Those small fruits, and the seeds inside them, turn out to be among the most dangerous parts of the plant.
Cultivated carrots, wild carrot, parsnip, fennel and parsley can all be confused with hemlock, with potentially fatal results. The scientific names are Daucus carota for wild carrot, Pastinaca sativa for parsnip, Foeniculum vulgare for fennel and Petroselinum crispum for parsley. All of them share a family with the poison.
Wild carrot is the most instructive comparison. Its stem is hairy and lacks purple markings, and it grows less than 1 m tall. Hemlock has a smooth texture and a vivid mid-green colour, with purple spotting on its stems and leaf stalks. Its flowering stems typically stand at least 1.5 m high.
Harmless plants cause confusion too. Cow parsley, Anthriscus sylvestris, is one. Another is the wild herb Chaerophyllum macropodum, which Turkish cheesemakers use. Neither carries the danger of the plant it resembles.
The worst mix-up involves water hemlock, Cicuta, which is deadly in its own right. It lacks the purple spots and the disagreeable, mouse-like smell of hemlock. Its tuber branches and grows sideways in the soil, while hemlock sends down a vertical taproot.
The genus name Cicuta even appears in the tangled paper trail of hemlock's own names, where Cicuta major, from 1779, is one of its old synonyms.
William Shakespeare is credited with the first spelling of hemlock, in his play Henry V in 1623. The word itself is older. Old English hymlice, hymlic and hemlic probably referred to Conium. By the 1500s the word clearly meant Conium maculatum, and it appeared in herbalist texts. In Middle English it took forms such as hemeluc, hemlok, hemlake and hemblock.
The plant has no relation to the conifers of the genus Tsuga, which are also called hemlocks. People once thought those trees had a similar smell.
In British, Australian and New Zealand English, hemlock is the prominent name. American and Canadian speakers usually say poison hemlock, a usage that dates to 1757. Local names pile up beyond that. In the United States the plant has been called kill-cow, wode-whistle, St. Bennet's-herb, Nebraska-fern, California-fern, poisonroot and stinkweed. Australians sometimes call it wild carrot, wild parsnip or snakeweed. Canadians add fool's-parsley, deadly hemlock and spotted parsley.
The scientific name Conium comes from the Ancient Greek κώνειον, kṓneion, meaning hemlock. It may be related to konas, meaning to whirl, a nod to the vertigo that the plant causes. Carl Linnaeus described the species in his 1753 publication Species Plantarum, and it was the first species of its genus to be described. Maculatum comes from the Latin macula and means spotted, a reference to the blotches on the stalks.
Plants of the World Online counts 16 synonyms at species rank. They include Coriandrum maculatum from 1788 and Sium conium from 1805. The most recent on the list, subspecies viride, dates only to 2002.
In the 1800s, poison hemlock arrived in the United States from Europe as an ornamental garden plant called winter fern. Its native range is Europe, North Africa, Central Asia and the Mediterranean region, and it occurs in many British Isles counties, including in Northern Ireland. It has since become naturalised in West Asia, North and South America, Australia and New Zealand. Along rivers in southeast Australia and Tasmania it is considered invasive. In the United States, infestations and human contact with the plant sometimes make the news.
Poorly drained soil near streams and ditches suits it well. It also grows on roadsides, at the edges of cultivated fields and in waste areas. Yet it does not need to be wet. Drier rough grassland and disturbed ground serve it too.
Seed is its main way of travelling. The plant spreads rapidly by seed, and the seeds can lie dormant for up to six years. That patience makes eradication hard.
Insects have their own stake in the plant. The poison hemlock moth, Agonopterix alstroemeriana, arrived in the United States along with the plant and has spread across most of the country. People have widely used it as a biological control agent, but its success at destroying the plant has been limited. Larvae of silver-ground carpet moths also feed on hemlock.
Butterflies make use of it as well. Caterpillars of the North American black swallowtail eat hemlock, though they do better on wild carrots and parsnips. In urban and suburban California, the anise swallowtail, Papilio zelicaon, depends largely on non-native plants such as hemlock and fennel.
The plant may even be adapting to the moth by becoming more toxic, which raises the question of how much poison hemlock really carries.
The roots of young, first-year plants hold only traces of poison, or none at all. Seedling leaves carry less than those of slightly older plants. In the spring of the second year the leaves are highly toxic, though not as poisonous as the flowers and seeds that follow later in the season. Growing conditions matter as well. Plants raised in sun can be twice as poisonous as those from wet, cloudy places.
The poison outlasts the plant. Hollow stems remain deadly for up to three years after the plant has died. Drying, however, strips away a large part of the toxic compounds.
The chemistry centres on piperidine alkaloids: coniine, N-methylconiine, conhydrine and γ-coniceine. The last is the precursor of the others. Flower buds hold mostly γ-coniceine, and it changes into coniine in the later stages of fruit development. Researchers assume that these volatile alkaloids help attract pollinators such as butterflies and bees. In mice, injected γ-coniceine was lethal at 0.14 of the equivalent dose of coniine, while N-methylconiine took 1.5 times the equivalent dose. Given by mouth, coniine took about five times as much to be lethal.
Animals suffer widely. Poisoning has been reported in pigs, bovids, turkeys, equines, rabbits, cats and dogs. Songbirds are less sensitive, and they have reportedly become toxic themselves by absorbing coniine from hemlock. Grazing animals are most at risk in spring, when other forage is scarce, or when hemlock gets mixed into grain, hay or silage. In cattle it typically causes intense and fatal seizures. Even very small quantities can cause birth defects when eaten 40-100 days into gestation.
Medieval writers listed hemlock among the suspected causes of coturnism, a disease linked to eating common quail in certain seasons. Modern research points to annual woundwort, Stachys annua, as the likelier source of the toxin. The true cause is still unknown.
Narcosis-like effects can appear as soon as 30 minutes after someone eats green hemlock leaves. Victims fall asleep and gradually lose consciousness. Death follows a few hours later. The onset resembles poisoning by curare, with muscular paralysis that ascends through the body until the respiratory muscles stop. The ultimate cause of death is oxygen deprivation.
Coniine has a chemical structure and pharmacological properties similar to nicotine. It acts on the central nervous system by inhibiting nicotinic acetylcholine receptors. The source puts the potentially fatal dose for an adult at more than 100 mg, or 0.1 gram, of coniine. That is about six to eight fresh leaves, or a smaller amount of seed or root.
Survival is possible. If the poisoning is not lethal, spontaneous recovery can follow, provided the victim avoids further exposure. Artificial ventilation can prevent death until the effects wear off after 48-72 hours. There is no specific antidote.
The kidneys take damage too. Rhabdomyolysis and acute tubular necrosis have been found in patients who died of hemlock poisoning, and some of them had acute kidney injury. Touching the leaves with bare skin can bring a rash and persistent blisters, because the plant makes skin sensitive to sunlight.
Other animals show different effects. In calves the alkaloids cause hypothermia through peripheral vasoconstriction. In rats and rabbits they raise the heart rate by stimulating the sympathetic ganglia and weakening the influence of the parasympathetic ones.
Because no antidote exists, prevention is the only answer to agricultural losses. Herbicides and grazing with less susceptible animals, such as sheep, have been suggested. A small infestation can be dug out, roots included, by someone wearing clothing that covers the skin, eye protection and gloves. A popular belief holds that the alkaloids enter the human food chain through milk and fowl, but scientific studies have disproven it.
Theramenes, Socrates, Polemarchus and Phocion all died of the same plant. In ancient Greece, hemlock was used to poison condemned prisoners. Socrates, the most famous victim, was sentenced to death at his trial and took an infusion of it. Greek myth gave poisonous plants like hemlock to the goddess Hecate and to her daughters Circe and Medea.
In the high mountain areas of Georgia, people once ate hemlock leaves as a spring food after long winters. The leaves needed careful cooking, often in several changes of water. Cow parsnips, potatoes and lilies were prepared in a similar way. Locals told ethnobotanical researchers that they knew the plants were poisonous, and that the practice had largely died out as roads improved and markets stocked cultivated foods.
As a medicine, hemlock demanded great care because of its toxicity. In medieval Europe it was given only against rabies, then called "the bite of mad dogge", and it was taken in wine with betony and fennel seed. Later it served as a last, desperate attempt to cure virulent poisons such as strychnine. In the 1400s and 1500s, European monks roasted the root and applied it to the feet, hands and wrists for gout pain.
The herbalist Nicholas Culpeper wrote in the 1650s that hemlock was under the control of Saturn, which gave it a cold and dangerous character. He recommended it externally for inflammation and swelling. From the 1760s, it was applied to cancerous ulcers.
Victorians gave hemlock flowers a meaning in the language of flowers: "You will be the death of me." Officially, the London and Edinburgh pharmacopoeias listed it as a medicine from 1864 to 1898. The last listing of all came in the British Pharmaceutical Codex in 1934.
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Common questions
What is Conium maculatum?
Conium maculatum is a highly poisonous flowering plant in the carrot family, commonly called hemlock in British English and poison hemlock in North America. It is a biennial, nitrophile weed that can grow to 8 ft.
How can you tell Conium maculatum from wild carrot?
Conium maculatum has a smooth stem with purple spotting, while wild carrot has a hairy stem without purple markings. Hemlock flowering stems typically reach at least 1.5 m, and wild carrot grows less than 1 m tall.
How poisonous is Conium maculatum to humans?
Conium maculatum can kill an adult who ingests more than 100 mg of coniine, roughly six to eight fresh leaves. The alkaloids cause ascending muscular paralysis, and death comes from failure of the respiratory muscles.
Where does Conium maculatum grow?
Conium maculatum is native to Europe, North Africa, Central Asia and the Mediterranean region. It has become naturalised in West Asia, North and South America, Australia and New Zealand, where it can be an invasive weed.
Did Conium maculatum kill Socrates?
Yes. Socrates was sentenced to death at his trial in ancient Greece and took an infusion of Conium maculatum. The plant also killed Theramenes, Polemarchus and Phocion.
Is there an antidote for Conium maculatum poisoning?
No specific antidote exists for Conium maculatum poisoning. Artificial ventilation can prevent death until the effects wear off after 48-72 hours, and spontaneous recovery is possible if exposure stops.
Why is Conium maculatum called hemlock?
The name hemlock derives from Old English words such as hymlice, hymlic and hemlic. By the 1500s it clearly referred to Conium maculatum in herbalist texts, and the plant is unrelated to the Tsuga conifers also called hemlocks.
All sources
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