Water Hemlock ☠️

What Is Water Hemlock?

Water hemlock is a poisonous perennial of the carrot family that grows with its feet in water, in marshes, ditches and wet meadows across western North America ^{13Reference 13Knoke DCicuta douglasiiView study →}. The USDA calls it the most violently toxic plant that grows in North America ^{12Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}.

It has the finely divided leaves and flat, umbrella-like clusters of small white flowers typical of its family ^{12Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}. Below ground it has a swollen rootstock divided into small chambers, which hold a highly poisonous straw-coloured to brown liquid ^{12Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}. The chambered rootstock identifies the plant, and the tubers are its most toxic part ^{3,12Reference 3Orlando-Goulart CFP et al. · 2018AnimalNeurobehavioral evaluation of mice dosed with water hemlock green seeds and tubers — animalView study →Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}.

Its toxin, cicutoxin, blocks the brain’s main calming signal and causes violent seizures ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. Water hemlock kills cattle, sheep and other grazing animals, and people have died after mistaking the root for an edible one ^{7,10,11,12Reference 7Centers for Disease Control et al. · 1994Case reportWater hemlock poisoning — Maine, 1992 — case reportView study →Reference 10Heath KB · 2001Case reportA fatal case of apparent water hemlock poisoning — case reportView study →Reference 11Landers D et al. · 1985Case reportSeizures and death on a white river float trip: report of water hemlock poisoning — case reportView study →Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}.

Human poisonings are uncommon, and the outlook is good with prompt supportive care ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. There is no antidote ^{7Reference 7Centers for Disease Control et al. · 1994Case reportWater hemlock poisoning — Maine, 1992 — case reportView study →}.

Identification & Lookalikes

The decisive character is the chambered rootstock. Cut lengthwise, the swollen stem base and rootstock of Cicuta show a series of small hollow chambers separated by cross-partitions ^{12,13Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →Reference 13Knoke DCicuta douglasiiView study →}. The chambers hold a highly poisonous straw-coloured to brown liquid ^{12Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}.

The root is the most poisonous part, and the liquid in the chambers carries the toxin ^{3,12Reference 3Orlando-Goulart CFP et al. · 2018AnimalNeurobehavioral evaluation of mice dosed with water hemlock green seeds and tubers — animalView study →Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}. The liquid is toxic on contact, so cut roots should not be handled bare-handed. It can be sparse in older, dried or early-season plants, so its presence confirms Cicuta while its apparent absence rules nothing out.

Plant confused with it	How it actually differs
Water parsnip (Sium suave)	Leaves once-pinnate, with a single row of 7–13 toothed leaflets 14Reference 14Knoke D et al.Sium suave, Heracleum maximum, Perideridia gairdneri and Conium maculatumView study →, against Cicuta’s 1–3 times divided leaves 13Reference 13Knoke DCicuta douglasiiView study →. Roots fibrous, not chambered 14Reference 14Knoke D et al.Sium suave, Heracleum maximum, Perideridia gairdneri and Conium maculatumView study →. Some Sium leaves are deeply cut and can mimic Cicuta 14Reference 14Knoke D et al.Sium suave, Heracleum maximum, Perideridia gairdneri and Conium maculatumView study →, so judge from well-developed foliage and the root
Cow parsnip (Heracleum maximum)	Leaves once ternate: three very large, coarsely toothed, lobed leaflets 10–30 cm long and wide, on a plant 1–3 m tall 14Reference 14Knoke D et al.Sium suave, Heracleum maximum, Perideridia gairdneri and Conium maculatumView study →. Barely confusable except as a young rosette
Sweet cicely (Osmorhiza spp.)	Crushed root and foliage smell strongly of anise. A positive test for Osmorhiza, not an exclusion test for Cicuta
Yampah (Perideridia gairdneri)	Narrow, elongate leaf segments; a single edible tuberous root, not chambered 14Reference 14Knoke D et al.Sium suave, Heracleum maximum, Perideridia gairdneri and Conium maculatumView study →. An important traditional food plant
Poison hemlock (Conium maculatum)	A stout taproot, and a purple-spotted hollow stem 14Reference 14Knoke D et al.Sium suave, Heracleum maximum, Perideridia gairdneri and Conium maculatumView study →. It grows in drier, disturbed ground. Water hemlock can also show purple on the stem (see below)

Two widely repeated tells are less reliable than they seem.

Purple stem blotching does not separate water hemlock from poison hemlock, because spotted water hemlock (Cicuta maculata) has purple-mottled stems too.

Veins running to the notches between the teeth, not to the tooth tips, is a real feature of Cicuta ^{12,13Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →Reference 13Knoke DCicuta douglasiiView study →}. It is subtle, though, and needs well-developed leaflets and good light, so it is best used alongside the root. The chambered rootstock plus wetland habitat is the reliable test.

Botany

Western water hemlock (Cicuta douglasii (DC.) J.M.Coult. & Rose) is a perennial in the Apiaceae ^{13Reference 13Knoke DCicuta douglasiiView study →}. Mulligan’s revision of the genus recognised four North American species: C. maculata, C. douglasii, C. virosa and C. bulbifera ^{1Reference 1Mulligan GA · 1980The genus Cicuta in North AmericaView study →}.

C. douglasii has 44 chromosomes. Its set combines 22 large chromosomes, like those of C. maculata, with 22 small ones, like those of C. virosa ^{1Reference 1Mulligan GA · 1980The genus Cicuta in North AmericaView study →}. It has also been treated as a variety of C. maculata (var. californica) ^{13Reference 13Knoke DCicuta douglasiiView study →}.

It is a stout perennial, often with a waxy bloom, 5–20 dm tall ^{13Reference 13Knoke DCicuta douglasiiView study →}. The stems rise singly or a few together from a tuberous, thickened, chambered base, and several of the roots are thickened as well ^{13Reference 13Knoke DCicuta douglasiiView study →}.

The leaves are 1–3 times divided. The leaflets are lance-shaped to ovate, toothed, and 1.6–12.6 cm long ^{13Reference 13Knoke DCicuta douglasiiView study →}. Their main side veins run to the notches between the teeth ^{13Reference 13Knoke DCicuta douglasiiView study →}.

The small white flowers grow in several compound umbels ^{13Reference 13Knoke DCicuta douglasiiView study →}. The fruit is round and ribbed, 2–4 mm long ^{13Reference 13Knoke DCicuta douglasiiView study →}.

Distribution

Western water hemlock grows from Alaska south to Oregon and east to Alberta, Montana and Wyoming ^{13Reference 13Knoke DCicuta douglasiiView study →}. It is found in all the western United States ^{11Reference 11Landers D et al. · 1985Case reportSeizures and death on a white river float trip: report of water hemlock poisoning — case reportView study →}.

It grows in marshes, ditches and wet low places, from low to middle elevations in the mountains ^{13Reference 13Knoke DCicuta douglasiiView study →}. It also grows in wet meadows and pastures and along stream banks ^{12Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}.

It often grows at the edge of pasture. Its leaves and stems are most toxic early in the growing season and lose toxicity as they mature, while the tubers stay the most toxic part ^{12Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}.

It is not of conservation concern ^{13Reference 13Knoke DCicuta douglasiiView study →}.

Phytochemistry

Cicutoxin is a C17 polyacetylene alcohol — a long-chain, highly unsaturated compound, not an alkaloid ^{4,6Reference 4Uwai K et al. · 2000In vitroExploring the structural basis of neurotoxicity in C17-polyacetylenes isolated from water hemlock — animal and in vitroView study →Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. Cicutol, isocicutoxin and other C17 polyacetylenes accompany it ^{2Reference 2Stonecipher CA et al. · 2020Geographical and seasonal variation in water hemlock (Cicuta maculata) toxinsView study →}.

In spotted water hemlock (C. maculata), the tubers held the highest concentrations of cicutoxin and total C17 polyacetylenes, and green seeds the second highest ^{2Reference 2Stonecipher CA et al. · 2020Geographical and seasonal variation in water hemlock (Cicuta maculata) toxinsView study →}. Tuber concentrations rose until the green-seed stage, then fell as the seeds matured and the plant began to die back ^{2Reference 2Stonecipher CA et al. · 2020Geographical and seasonal variation in water hemlock (Cicuta maculata) toxinsView study →}. Concentrations also varied between the six collection sites ^{2Reference 2Stonecipher CA et al. · 2020Geographical and seasonal variation in water hemlock (Cicuta maculata) toxinsView study →}.

The best measure of potency comes from mice given water extracts of C. douglasii by mouth. The median lethal dose was 17 mg/kg for tuber extract and 1,320 mg/kg for green seed, making the tuber roughly 78 times more lethal ^{3Reference 3Orlando-Goulart CFP et al. · 2018AnimalNeurobehavioral evaluation of mice dosed with water hemlock green seeds and tubers — animalView study →}. Extracts of dry seed, flower and stem killed no mice at any dose tested ^{3Reference 3Orlando-Goulart CFP et al. · 2018AnimalNeurobehavioral evaluation of mice dosed with water hemlock green seeds and tubers — animalView study →}.

Constituent Summary

Toxicology

Class of poison. Polyacetylene convulsant; a GABA-A receptor antagonist.

Mechanism. Cicutoxin blocks the GABA-A receptor in a non-competitive way, removing the brain’s main inhibitory signal ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. The result is unchecked excitation of nerve cells, which leads to seizures ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}.

In mice, the ability of cicutoxin and related compounds to bind the GABA-gated chloride channel tracked their lethality ^{4Reference 4Uwai K et al. · 2000In vitroExploring the structural basis of neurotoxicity in C17-polyacetylenes isolated from water hemlock — animal and in vitroView study →}. A water extract of C. douglasii tuber blocked GABA-A receptors non-competitively in cultured cells, and the benzodiazepine midazolam modified that effect ^{5Reference 5Green BT et al. · 2015In vitroThe non-competitive blockade of GABAA receptors by an aqueous extract of water hemlock (Cicuta douglasii) tubers — in vitroView study →}.

Course. First symptoms typically appear 15–60 minutes after eating the plant: nausea, vomiting, abdominal pain and heavy salivation ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. Grand mal seizures can follow quickly, often within 30–90 minutes, with dilated pupils and a racing heart ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}.

Status epilepticus, rhabdomyolysis, kidney failure, coma, breathing failure and heart rhythm disturbances can follow ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. When death occurs, it usually results from status epilepticus ^{7Reference 7Centers for Disease Control et al. · 1994Case reportWater hemlock poisoning — Maine, 1992 — case reportView study →}, typically within 1–8 hours ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. Survivors may have retrograde amnesia ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}.

Potency in human terms. No human lethal dose has been established; the medical literature consists almost entirely of case reports ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. The best-documented case is from Maine in October 1992. Two brothers foraging for wild ginseng ate from the same water hemlock root. The 23-year-old took three bites, vomited and convulsed within 30 minutes, and died about 3 hours after eating it. His 39-year-old brother took one bite, developed seizures and delirium, and survived ^{7Reference 7Centers for Disease Control et al. · 1994Case reportWater hemlock poisoning — Maine, 1992 — case reportView study →}.

In goats, 1–2 fresh tubers were estimated to be a lethal dose ^{15Reference 15Welch KD et al. · 2020The acute toxicity of water hemlock (Cicuta douglasii) in a goat model — animalView study →}.

Antidote — none. Treatment is supportive, centred on seizure control and protecting the airway ^{6,7Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →Reference 7Centers for Disease Control et al. · 1994Case reportWater hemlock poisoning — Maine, 1992 — case reportView study →}. Benzodiazepines are first-line. In goats dosed with C. maculata tubers, intravenous diazepam controlled convulsions almost instantly ^{8Reference 8Green BT et al. · 2022Evaluation of diazepam as a drug treatment for water hemlock (Cicuta species) poisoning in Spanish goats — animalView study →}.

A 9-year-old boy in Berlin whose seizures began about an hour after he bit into a water hemlock root needed midazolam, lorazepam and finally propofol to stop them ^{9Reference 9Arsenova R et al. · 2022Case reportBeware of neurotoxins in common plants: water hemlock poisoning presenting as convulsive status epilepticus — case reportView study →}. Activated charcoal is used only if it can be given early, after the patient is stabilised ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}. Haemodialysis has been used successfully when the kidneys fail ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}.

Human deaths. The CDC gave a case-fatality rate of 30% for poisonings reported between 1900 and 1975 ^{7Reference 7Centers for Disease Control et al. · 1994Case reportWater hemlock poisoning — Maine, 1992 — case reportView study →}. With prompt supportive care, the outlook is now good ^{6Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →}.

Documented deaths include the Maine case of 1992 ^{7Reference 7Centers for Disease Control et al. · 1994Case reportWater hemlock poisoning — Maine, 1992 — case reportView study →}. A 14-year-old Indigenous boy in Canada died about 20 hours after eating “wild carrot” while gathering plants with his family; the plant sample contained cicutoxin, though it could not be found in his tissues ^{10Reference 10Heath KB · 2001Case reportA fatal case of apparent water hemlock poisoning — case reportView study →}. A death attributed to C. douglasii on a white-water river trip was reported in 1985 ^{11Reference 11Landers D et al. · 1985Case reportSeizures and death on a white river float trip: report of water hemlock poisoning — case reportView study →}.

Traditional & Historical Use

Native American Medicine

Many peoples of western North America knew the plant as a poison. The Okanagan-Colville called it “a very bad poison”, and the Thompson knew its roots as poisonous to both people and animals ^{16Reference 16Moerman DE · 1990Native American Ethnobotany Database: Cicuta douglasii (entries from Turner et alView study →}.

It was also used as a medicine, mostly on the outside of the body. The Thompson applied a poultice of the root for severe pain in the legs and back, and boiled, mashed roots to the back of bedridden patients, with the warning not to leave the poultice on for more than half an hour ^{16Reference 16Moerman DE · 1990Native American Ethnobotany Database: Cicuta douglasii (entries from Turner et alView study →}.

The Kwakiutl used a root poultice to draw out thorns and splinters, and the Paiute used a decoction of mashed roots as a wash for rheumatism and tired, aching feet ^{16Reference 16Moerman DE · 1990Native American Ethnobotany Database: Cicuta douglasii (entries from Turner et alView study →}. In Montana, a poultice of split roots was used for rattlesnake bites ^{16Reference 16Moerman DE · 1990Native American Ethnobotany Database: Cicuta douglasii (entries from Turner et alView study →}.

A few peoples took it internally as an emetic. The Kutenai drank an infusion of the roots with large amounts of warm water to cause vomiting, using it cautiously because of its poisonous effects, and the Coast Salish also used it with caution to induce vomiting ^{16Reference 16Moerman DE · 1990Native American Ethnobotany Database: Cicuta douglasii (entries from Turner et alView study →}.

Cicutoxin has no pharmaceutical use. It is used in research to study the convulsant site of the GABA-A receptor ^{4Reference 4Uwai K et al. · 2000In vitroExploring the structural basis of neurotoxicity in C17-polyacetylenes isolated from water hemlock — animal and in vitroView study →}.

Safety & Pregnancy

Full Safety Detail

The typical picture. Someone who has eaten a wild root, vomits within the hour and then has a seizure fits the pattern of water hemlock poisoning. Poison hemlock, by contrast, causes paralysis.

The two hemlocks are treated differently. Water hemlock needs aggressive seizure control with benzodiazepines ^{6,8Reference 6Schep LJ et al. · 2009ReviewPoisoning due to water hemlock — reviewView study →Reference 8Green BT et al. · 2022Evaluation of diazepam as a drug treatment for water hemlock (Cicuta species) poisoning in Spanish goats — animalView study →}. Poison hemlock needs support for breathing through a paralysis that wears off. Saying “suspected water hemlock, seizing” rather than “hemlock” gives emergency services the right picture.

The root identifies the plant. A photograph of leaves may not be enough, while the chambered rootstock is diagnostic, so a sample of the root is the most useful thing to bring.

Livestock. Grazing animals show signs of poisoning 15 minutes to 6 hours after eating the plant, and can die within 15 minutes to 2 hours of the first signs ^{12Reference 12USDA Agricultural Research ServiceWater hemlock (Cicuta douglasii)View study →}.

References

1. Mulligan GA. (1980). The genus Cicuta in North America. Canadian Journal of Botany 58:1755–1767. https://doi.org/10.1139/b80-204
2. Stonecipher CA, Welch KD, Lee ST, Cook D, Pfister JA. (2020). Geographical and seasonal variation in water hemlock (Cicuta maculata) toxins. Biochemical Systematics and Ecology 89:104012. https://doi.org/10.1016/j.bse.2020.104012
3. Orlando-Goulart CFP, Welch KD, Pfister JA, Goulart DS, Damasceno AD, Lee ST. (2018). Neurobehavioral evaluation of mice dosed with water hemlock green seeds and tubers — animal. Poisonous Plant Research 1:1–13. https://doi.org/10.26077/sk8v-xz71
4. Uwai K, Ohashi K, Takaya Y, et al. (2000). Exploring the structural basis of neurotoxicity in C17-polyacetylenes isolated from water hemlock — animal and in vitro. Journal of Medicinal Chemistry. https://pubmed.ncbi.nlm.nih.gov/11087575/
5. Green BT, Goulart C, Welch KD, Pfister JA, McCollum I, Gardner DR. (2015). The non-competitive blockade of GABAA receptors by an aqueous extract of water hemlock (Cicuta douglasii) tubers — in vitro. Toxicon. https://pubmed.ncbi.nlm.nih.gov/26415905/
6. Schep LJ, Slaughter RJ, Becket G, Beasley DM. (2009). Poisoning due to water hemlock — review. Clinical Toxicology. https://pubmed.ncbi.nlm.nih.gov/19514873/
7. Centers for Disease Control and Prevention. (1994). Water hemlock poisoning — Maine, 1992 — case report. MMWR 43(13):229–231. https://www.cdc.gov/mmwr/preview/mmwrhtml/00026056.htm
8. Green BT, Stonecipher CA, Welch KD, Lee ST, Cook D. (2022). Evaluation of diazepam as a drug treatment for water hemlock (Cicuta species) poisoning in Spanish goats — animal. Toxicon. https://pubmed.ncbi.nlm.nih.gov/34871669/
9. Arsenova R, Prager C, Becker LL, Scholtis S, Methling M, Kaindl AM. (2022). Beware of neurotoxins in common plants: water hemlock poisoning presenting as convulsive status epilepticus — case report. Pediatric Neurology. https://pubmed.ncbi.nlm.nih.gov/34954473/
10. Heath KB. (2001). A fatal case of apparent water hemlock poisoning — case report. Veterinary and Human Toxicology. https://pubmed.ncbi.nlm.nih.gov/11205076/
11. Landers D, Seppi K, Blauer W. (1985). Seizures and death on a white river float trip: report of water hemlock poisoning — case report. Western Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/4013278/
12. USDA Agricultural Research Service, Poisonous Plant Research Laboratory. Water hemlock (Cicuta douglasii). https://www.ars.usda.gov/pacific-west-area/logan-ut/poisonous-plant-research/docs/water-hemlock-cicuta-douglasii/
13. Knoke D, Giblin D. Cicuta douglasii. Burke Herbarium Image Collection, University of Washington (with the Flora of North America description). https://burkeherbarium.org/imagecollection/taxon.php?Taxon=Cicuta%20douglasii
14. Knoke D, Giblin D, et al. Sium suave, Heracleum maximum, Perideridia gairdneri and Conium maculatum. Burke Herbarium Image Collection, University of Washington. https://burkeherbarium.org/imagecollection/taxon.php?Taxon=Sium%20suave
15. Welch KD, Stonecipher CA, Lee ST, Cook D. (2020). The acute toxicity of water hemlock (Cicuta douglasii) in a goat model — animal. Toxicon. https://pubmed.ncbi.nlm.nih.gov/32103796/
16. Moerman DE. Native American Ethnobotany Database: Cicuta douglasii (entries from Turner et al. 1990, Thompson; Turner & Bell 1971, 1973; Hart 1992; Mahar 1953; Turner, Bouchard & Kennedy 1980; Steedman 1928). Botanical Research Institute of Texas. http://naeb.brit.org/uses/search/?string=Cicuta+douglasii

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