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Apium nodiflorum (L.) Lag., Fool's Water-cress

Account Summary

Native, common. Eurosiberian southern-temperate.

1881; Stewart, S.A.; Co Fermanagh.

January to November.

Growth form, identification and preferred habitats

A. nodiflorum is a glabrous, polycarpic, patch-forming perennial with much-branched, slender, hollow, low-growing, trailing stems 30-90 cm long (not tall!). The basal roots of the plant are shallow and vegetative stems are mostly creeping and procumbent, while flowering stems are more ascending, reaching around 30-100 cm tall. The flowering stems also produce adventitious roots at the nodes near the ground. The simply pinnate leaves are sheathed at the base and divide into several pairs of opposite, toothed, stalk-less leaflets.

The leaves of A. nodiflorum are very variable and can easily be confused with those of Berula erecta (Lesser Water-Parsnip), the two species sometimes occurring together. The true Water-cress, Rorippa nasturtium-aquaticum s.l. is also ecologically very similar to A. nodiflorum and all three species can occur together or in pairs. The similarity between A. nodiflorum and R. nasturtium-aquaticum, and the fact that they can occur intermingled, means Fool's Water-cress can readily be mistaken for genuine Water-cress. Although A. nodiflorum is not really poisonous, it is rather unpalatable and consequently it is called 'Fool's Water-cress'. The Plant Crib 1998 provides a useful table to distinguish Apium nodiflorum, Berula erecta and another umbellifer, Sium latifolium (Greater Water-Parsnip) (Rich & Jermy 1998, p. 220). Apium species do not have the node-like ring-mark towards the base of the petioles of the lower leaves that B. erecta and S. latifolium both have, the ring-mark denoting a pair of leaflets that have not developed. The ring-mark is not always present in B. erecta, however, as it may sometimes be replaced by a pair of small leaflets.

A. nodiflorum is a wetland and waterside species that can develop dense, sometimes pure clonal patches and may dominate shallow, still or slow-flowing, moderately nutrient-rich (mesotrophic to eutrophic), often calcareous waters. It grows in ditches, streams, ponds and in marshes and fen swamps beside seasonally reduced rivers and lakes, especially where there is sufficient disturbance to limit the growth of taller herbs. Colonies can be either emergent on bare mud in more shallow situations, or largely submerged in deeper, faster flowing waters, although the latter is seldom or rarely the case in Fermanagh. Occasionally the plant spreads out from the waterside environment to invade or engulf adjacent, drier, marshy grassland vegetation (Preston & Croft 1997).

Although the taste of the plant is described as nauseous, stock animals will still browse on it and reduce its presence accordingly.

Shoots die down completely in the autumn and reappear in late spring (Grime et al. 1988, 2007).

Flowering reproduction

In July and August, plants produce more erect flowering stems up to 100 cm tall. The inflorescences are compound umbels that look as if they are arising on short peduncles in the axils of leaves, but on closer examination they are actually leaf-opposed. The flowers are minute, greenish-white, almost sessile (pedicels just 1-2 mm) and they are borne on 5-8 rays in short-stalked umbellules (partial umbels, subdivisions of the compound umbel) (Tutin 1980). Pollination is by insects and is sometimes described as 'promiscuous' since the pollen and nectar are openly displayed and freely available to a wide range of unspecialised flower visitors, including beetles, flies and bees (Fitter 1983; Knees 1989). However, as is also usual in this family, the flowers are self-fertile (Knees 1989).

Fruiting takes place in September and October. The fruit is small (2-2.5 mm), rounded, the mericarps having five slender ridges and with a single vitta (ie resin canal) between each pair of ridges (Tutin 1980). Germination occurs in spring and early summer and seedlings may become established in mud or submerged in water. It is believed that they rarely survive, except in very open areas, being very vulnerable to competition from other established wetland plants (Thommen & Westlake 1981). Thus the most reliable means of reproduction and dispersal in A. nodiflorum is probably vegetative.

Vegetative reproduction

This is achieved in part by creeping growth and rooting of horizontal stems. Stem fragmentation and dispersal by flotation is also prevalent and appears a significantly more reliable means of increase compared with the probably very rare establishment achieved by seedlings (Thommen & Westlake 1981; Grime et al. 1988, 2007).

Fermanagh occurrence

In Fermanagh, A. nodiflorum is quite common and widespread, being recorded in 120 tetrads, 22.7% of those in the VC. Having said this, it is chiefly associated with the shores of Upper and Lower Lough Erne, both of which water bodies are fed by lime-rich rivers and streams and they are becoming increasingly nutrient-enriched and eutrophic in recent years as a result of agrochemical and sewage run-off. However, A. nodiflorum is known to be very tolerant of both eutrophication and fluctuating water levels. The incredibly dissected, sheltered, muddy shores of Upper Lough Erne in particular, provide very many suitable sites for this species, but it is also well represented on the Rivers Finn, Ballycassidy and Swanlinbar that flow into the lake and in their feeder streams and ditches.

Irish occurrence

In NI, A. nodiflorum is common and widespread in Cos Fermanagh (H33), Armagh (H37), Down (H38) and Antrim (H39), but apparently less prevalent in the other two northern VCs (Cos Tyrone (H36) and Londonderry (H40)).

In the RoI, A. nodiflorum is generally widespread, but it becomes scarce or absent in the acidic inland, more upland areas of Co Donegal (H34 & H35), becoming decidedly coastal both there and in other equally wet, western counties.

British occurrence

In Britain, the species is widely distributed in lowland England and Wales, showing no decline over the last 40 years. However, it becomes much more local and scarce northwards and especially so across the border in Scotland, where it very quickly becomes rare and coastal (Preston et al. 2002). The distribution is particularly odd in the western Scottish isles, where A. nodiflorum and, indeed, A. inundatum (Lesser Marshwort) too, are absent from Mull and Skye, but not uncommon in the Outer Hebrides (Pankhurst & Mullin 1991). As in Co Donegal (H34 & H35), much of this distribution may be easily explained by the prevalence of strongly acidic, peaty soils and high ground.

European and world occurrence

A. nodiflorum is distributed throughout W, C & S Europe, although it is most frequent in the west of the Continent and reaches its northern limit in Scotland. The species is also found in SW & C Asia and in N Africa. It is a naturalised and still actively spreading introduction in N & S America (Preston & Croft 1997; Sell & Murrell 2009).

Threats

None.