Oenanthe crocata L., Hemlock Water-dropwort

Account Summary

Growth form and preferred habitats

A tall, robust and vigorous, tuberous, wintergreen perennial that produces lush amounts of its 3-4 pinnate, parsley-scented, basal leaves when growing in wet conditions, O. crocata is the largest and the most widespread of the seven species of the genus in B & I, four of which occur in Fermanagh. It often grows luxuriantly, forming extensive clonal patches, and its thick, hollow, ribbed stems can reach a height of 150 cm when inhabiting reliably wet, nutrient-enriched sites (Walls 1995). Stem leaves are 2-3 pinnate, with narrower leaflets than the basal leaves.

It is a lowland plant of wet, moderately acid to neutral, muddy soils and shallow waters, mainly in sun, but occasionally under partial shade of woodland canopy.

Although it is generally most frequently found in or on acidic water and soils and, therefore, often considered a calcifuge, O. crocata is tolerant of lime-rich waters and can also be found in a range of calcareous habitats throughout B & I, including wetlands in the karst Burren district, Co Clare (H9) and along many chalk-derived rivers in S England (Webb & Scannell 1983; Walls 1995). In coastal sites, it can also colonise stony storm beaches and boulders at the top of beaches provided there is flushing ground water and it can also grow on dripping or flushed parts of sea cliffs (Preston & Croft 1997; M. Southam & M.J. Wigginton, in: Preston et al. 2002).

The species can survive a limited amount of drainage and the influence of agricultural improvements and fertiliser or slurry run-off, but plants become tough and weedy if the water table drops too low (Walls 1995). The plant is sufficiently ruderal, weedy and stress tolerant to successfully colonise crevices in waterside masonry or those in rocky stream-sides in coastal parts of W Britain (Preston & Croft 1997). Further south, on the Isles of Scilly, O. crocata can even grow in very dry conditions on the tops of walls (Lousley 1971). The established strategy is categorised as C/CR by Grime et al. (1988, 2007), meaning it is ecologically intermediate between a Competitor and a Competitive Ruderal.

Fermanagh occurrence

Locally, O. crocata has been recorded from January to September in a total of 79 Fermanagh tetrads, 15% of those in the VC. Hemlock Water-dropwort is unusual amongst locally widespread large emergent plants in Fermanagh in being much more frequently recorded around the calcareous, open water shores of Lower Lough Erne and its wooded islands, than around the more acidic and more definitely eutrophic Upper Lough. Around Lower Lough Erne shore area, it has post-1975 records in 35 tetrads, compared with the very sheltered, dissected shores of Upper Lough Erne where it has been recorded in just eight tetrads during the same 35-year period (1975-2010). The species is also frequent along the banks and ditches linked with the Colebrooke and the Ballycassidy Rivers that flow into Lower Lough Erne and in the west of the VC with Lough Melvin and adjacent Upper and Lower Lough Macnean.

The Fermanagh populations of O. crocata are stable, or possibly even spreading somewhat within their well-defined area and there have been 25 additional records since 2010 (BSBI Database accessed RSF, 27 January 2022).

Toxicity

All species of the genus Oenanthe are toxic to man and livestock to some extent, as the 'dropwort' portion of their English common name implies. However, O. crocata is by far the most dangerous and lethal of them all. The active principle, 'oenanthetoxin', is chemically very similar to 'cicutoxin' found in Cicuta virosa (Cowbane). A convulsant poison, the toxin is unaffected by drying or storage, making it very dangerous, even when decomposing parts of the plant are left exposed on diggings after drain clearing. The roots tubers are the most poisonous part of the plant and ingestion of just a small portion of these is sufficient to kill a cow (Walls 1995). Cattle, horses, sheep and pigs have all been poisoned on occasions. Death is often sudden, sometimes without the appearance of any clinical signs. People mistakenly cooking the stems for Wild Celery (Apium graveolens), or making soup of the tubers, thinking it is Wild Parsnip (Pastinaca sativa), are occasionally poisoned (Cooper & Johnson 1998).

Flowering reproduction

In line with other Oenanthe species, effective reproduction in O. crocata appears heavily dependent on seed, dispersal being usually achieved by floatation in water flushing or draining the habitats it tends to occupy. Flowering occurs in June and July, the inflorescence, 5-10 cm in diameter is a compound umbel with 10-40 rays, numerous 2 mm white flowers and linear bracts. The terminal umbel has hermaphrodite, bisexual flowers, the lateral umbels with mainly male flowers. The outer flowers of each umbel have petals somewhat unequal, ie radiating, making the flowers more conspicuous and attractive to insect visitors (Sell & Murrell 2009).

As with all Oenanthe species, nectar is well exposed and pollination of the protandrous flowers (anthers maturing before the stigmas) is carried out by flies and other unspecified insect visitors (Proctor & Yeo 1973; Fitter 1987). The fruit is 4-6 mm long, cylindrical or barrel-shaped, the styles about half the length of the fruit. The pedicels are not thickened after fruiting, nor are they constricted at the top. The mericarps have slender ridges that possibly contain air-spaces that would aid dispersal by flotation, but this is not mentioned by any reference the current author (RSF) has accessed (Tutin 1980; Clapham et al. 1987). There does not appear to be any available information on seed longevity, but seeds of other Oenanthe species are transient, surviving for a maximum of one year.

After flowering and fruiting the stout, hollow flowering stems often collapse, rot and disappear (Preston & Croft 1997)

Vegetative reproduction

Finger-like, cylindrical-obovoid tubers that overwinter the species are formed from buds at the base of the plant at or close to the soil surface in the autumn. At the same time, the current tuber or rootstock that gave rise to the aerial shoot, withers and dies off. The overwintering tubers can readily detach and spread the species, especially in flood waters in the spring (Walls 1995; Preston & Croft 1997).

British and Irish occurrence

The New Atlas hectad map of B & I displays a strong southern and western and essentially lowland distribution. O. crocata is uncommon in areas with a mean January daytime temperature of less than 5ºC (Rich et al. 1996). Having said this, its distribution stretches beyond Inverness and it has colonised as far north as Orkney, where it was first recorded in 1988 (Walls 1995; New Atlas).

The Irish distribution appears rather curious, the plant being frequent (sometimes abundant) in the N, S & E, but scarce or absent in large areas of the lowland C & W. Although farmers would likely be keen to eradicate such a poisonous species, they rarely make the attempt, probably regarding the task as near impossible (Preston & Croft 1997).

European and world occurrence

Hemlock Water-dropwort is a member of the Suboceanic southern temperate phytogeographical element, its distribution being strongly oceanic (or Atlantic), western and southern in Europe and the Mediterranean basin. The most northerly station on the continental mainland is at Voorne, in Holland, where it was first noted in 1975. It extends south to Spain, Portugal and Morocco and eastwards in the W Mediterranean to reach Italy (Preston & Croft 1997). O. crocata appears to be increasing and spreading northwards from its warmer southern strongholds, probably as a result of recent and continuing global warming (Walls 1995).

Names

The Latin specific epithet 'crocata' means 'citrus yellow' or 'saffron-like', referring to the sap of the plant that turns yellow on exposure to the air when it is cut (Gilbert-Carter 1964; Gledhill 1985). The English common name 'Hemlock Water-dropwort' is another invented 'book name' that is less useless than most such names because it alerts people to the deadly poisonous nature of this species by making a connection to the related, extremely dangerous genuine Hemlock, Conium maculatum.

Threats

Stable, or possibly spreading within its well-defined area.

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