Not to be semantically confused with : Hemlock (Conium maculatum) [another deadly poisonous umbellifer, but one which is not aquatic and is in a differing genus] nor to Dropwort (Filipendula vulgaris) [a plant belonging to the Rose Family]

Can be mistaken for : Parsley (Petroselinum crispum) because the leaves smell similarly, and for Celery (Apium graveolens) because the stems look similar and for the non-native Parsnip Pastinaca sativa because of a resemblance of the tubers.

Easily mis-identified as : River Water-dropwort (Oenanthe fluviatilis) but that has the lower leaves submerged in the water.

Slight resemblance to : Corky-Fruited Water-Dropwort (Oenanthe pimpinelloides), Narrow-leaved Water-dropwort (Oenanthe silaifolia), Parsley Water-Dropwort (Oenanthe lachenalii) but the pinnate leaves of all three of these have narrow linear leaflets on at least some of the pinnate leaves, but not all.

Superficial resemblance to : Fine-Leaved Water-Dropwort (Oenanthe aquatica) but the lower submerged leaves are 3- to 4-pinnate and the upper leaves 2- to 3-pinnate and finely divided leaflets.

The white tubular shaped fleshy tubers are the most toxic, especially in late winter and early spring. There are typically five or more which gives rise to their alternative name of 'Dead Mans Fingers' (not to be confused with Xylaria polymorpha, a fungus with the same common name). When cut they exude a yellowish liquid which stains the skin. It is one of the most toxic plants in the UK. Cattle may safely graze the leaves, providing they do not consume too much, but if they perchance get to eat tubers may well die. The tubers, of course, are normally below earth and inaccessible to cattle but may become exposed by pulling or dredging operations or by scouring of the stream/river bed during floods.

It is a perennial and a lowland species growing below 300m, flowering in June to July. Habitat is shallow freshwater streams and slower flowing rivers, lakes, ponds, canals, marshes, fens, flushes and wet woodlands. They are often found near the coast in more brackish waters, but never in the sea itself.

POLYYNES The principle toxin is a polyyne of which only very small amounts are sufficient to cause death. It is a convulsant poison, a neurotoxin; in horse and cattle the displayed symptoms include salivation, mydriasis, respiratory distress followed by spasmodic convulsions usually leading to death. Cattle that survive may develop diarrhoea for a couple of days and then slowly recover. Pigs vomit and die suddenly, but 50% of sheep may recover. The leaves will poison humans if eaten in mistake for parsley, and deaths have resulted, or the stems for celery or the tubers for parsnips. It was once used by farmers to control rats and moles. The leaves smell of parsley or celery but at least those are the least poisonous of the parts of this plant but folk have still died eating them. The stems contain less of the toxins than the tubers, but more than the leaves. The toxin seems able to enter the body through the skin, so care should be exercised when handling it, particularly any sap. The main highly toxic constituent is a polyyne called Oenanthotoxin, a poly-unsaturated higher alcohol, with two strained triple bonds. The highest concentration is contained within the tubers; less within the stem and less still within the leaves, although there have been fatalities with folk eating the leaves. The toxin is also reputedly able to infitrate the skin, so the casual examiner of the plant should be careful, especially with any sap. The dihydro derivative of Oenanthotoxin is present at approximately 10-times less concentration. DiHydroOenanthotoxin has one less double bond, two rather than three. This is also highly toxic. These two polyynes are instrumental in blocking the GABAergic responses in humans (and presumably most other animals?). Oenanthotoxin is extremely toxic; the LD50 for mice was found to be 0.58mg/kg. The LD50 of Oenantotoxin for man has not been determined (experimentation to find out would be unethical), but symptoms include nausea, diarrhoea, convulsions, seizures, tachycardia, mydriasis and rhabdomyolosis. Death is a distinct possibility for symptoms progress rapidly giving but an hour for any treatment to take effect. Oenanthotoxin is typically present in all Oenanthe species (Water-dropworts), but amounts may vary between species, time of year, the location within the plant studied (roots, stems, leaves, flowers, fruit, etc) and the growing conditions (both climatic and soil chemistry). The growing conditions may include the presence or absence of organisms detrimental to the plant, for if under attack it will then likely garner a greater quantity of toxins to help defend itself. There are very likely other toxic metabolites which the plant manufacturers, but presumably these pale into relative insignificance in the presence of the highly toxic polyynes. Hemlock Water-dropwort has typically been found to contain a higher amount of the polyyne toxins than other Oenanthe species, but don't count on it! Oenanthotoxin is isomeric with the polyyne Cicutoxin, where the third double bond has moved to another location on the molecule adjacent to the two others. Cicutoxin is the toxin present in Cowbane (Cicuta virosa) another convulsant poison, but is not present in Hemlock Water-dropwort.

The essential oil of Hemlock Water-dropwort contains mainly monoterpene hydrocarbons at 86%, the main compound being trans-β-Ocimene at 31%, followed by Sabinene at 29% and the cis-isomer of the first, cis-β-Ocimene at 12%. These presumably impart the parsley-like aroma to the plant.