Coronopus didymus (L.) Sm., Lesser Swine-cress

Account Summary

Growth form and preferred habitats

A small, sprawling, soft, deeply pinnatisect, feathery-leaved, alien weedy ruderal annual or biennial is gradually spreading across these islands. In view of the colonising behaviour of the species in Europe and elsewhere, its gradual spread to all regions, while not spectacularly rapid, remains inexorable.

Flowering reproduction

C. didymus forms a disc-like mat of prostrate shoots spreading out around the original long central tap-root bearing finely divided pinnate leaves which smell strongly foetid or pungently cress-like when bruised. The very small, generally petal-less flowers are crowded on several inconspicuous inflorescences amid the leaves. However, the flowers are actually borne opposite the leaves, or less often in the leaf axils, or completely terminal. Flowering takes place from mid- to late-summer and automatic self-pollination is the norm, although it has been suggested that ants may sometimes be involved (Rich 1991). A study in India found that four species of ant plus two of aphids were attracted to the flowers by their unpleasant scent and by nectar (Chauhan 1979).

Salisbury (1964) estimated that, depending upon size, individual plants could produce between 1,600 and 18,000 seed per season. The small doubly notched, twin-valved fruits (which give the plant its specific name, Latin 'didymus' meaning 'double' or 'twin' (Gilbert-Carter 1964)) contain a single seed in each lobe. The two fruit halves split apart when mature and are probably distributed in mud by animals, including man. Dispersal may perhaps also involve ants as vectors.

Seed of Lesser Swinecress persists in the soil seed bank for more than five years and while germination occurs chiefly in April and September, flushes of emergence following soil disturbance can occur in any month except December and January (Roberts 1986). Growth of the plant occurs all year round in milder areas of New Zealand (Popay et al. 1995; Roy et al. 1998). Since the oceanic climate in our part of Ireland is rather similar to that in lowland New Zealand and as so many 'Kiwi' plants grow exceptionally well in our gardens, we may expect the same sort of year round growth and seed and weed population behaviour by C. didymus if or when it becomes fully established in Fermanagh and other parts of western Ireland.

Centre of origin unknown

The origin and native distribution of this small ruderal plant is unknown. There is disagreement among Flora writers as to whether it is from Eurasia (Hickman 1993), or S America (Clement & Foster 1994; Webb et al. 1996; Stace 1997). Whatever the truth of the matter, it is now a very successful cosmopolitan weed (Baker 1972).

Fermanagh occurrence

The first Fermanagh record for this species was in a planted urban flower tub as detailed above. There are just four additional records as follows: Lakeland Forum, Enniskillen town, 1993, I. McNeill; in great profusion in a garden at Magheranageeragh, 1 October 1994, RHN; beside Dunnes' store car park, Enniskillen town, 25 May 2001, RHN; in quantity at Brockagh Sandpit, to the W of roadway, 6 October 2001, RHN.

Although thankfully C. didymus is still only a very rare casual plant in Fermanagh, it seems very possible that like Arabidopsis thaliana (Thale Cress) and Cardamine hirsuta (Hairy Bitter-cress), this crucifer has the potential to become a common weed in horticultural nurseries and given the popularity of local garden centres it could then spread throughout the VC with purchased plants.

British and Irish occurrence

In some cases, as in Fermanagh, C. didymus appears to be spread by the horticultural trade but, of course, this is only one example of human assisted dispersal. Since its first introduction to Britain (probably as ships' ballast), from the early 18th century onwards, C. didymus has continued to spread slowly. In B & I, Lesser Swine-cress does appear to be more actively spreading in recent years. Salisbury noted this fact in 1964, stating then that, "at one time rather uncommon, it is now widespread in the southern counties [of England] and would seem to have become appreciably more abundant during the last thirty years".

Lesser Swine-cress has now become an established weed of disturbed situations, including in arable fields, open pastures, waysides, waste land and coastal shingle and the New Atlas map shows it to be very common in southern parts of B & I. Although present as far north as Inverness, C. didymus becomes much more scarce and coastal in its occurrence northwards from the English Midlands. Similarly, in Ireland, it declines above a line across the map between Dublin and Galway (New Atlas). The New Atlas editors reckon that the species is "now frequent in urban and industrial areas, and is still spreading into rural areas where it is widespread but scattered" (D.A. Pearman, in: Preston et al. 2002).

European and world occurrence

Represented as an introduction "probably from S America", Jalas et al. (1996) in Atlas Florae Europaeae 11 indicate that C. didymus is widespread, but not continuously so, in W Europe from S Spain to N Germany, becoming much rarer and strictly coastal further north into Norway and Sweden. The species also extends discontinuously eastwards along the Mediterranean reaching the Greek Isles and Crete.

Toxicity and grazing

The unpleasant smelling constituent of the feathery leaves has been identified as benzyl isothiocyanate, which is capable of tainting the milk of grazing cattle (Shimoda et al. 2000). Fortunately, the species is avoided by stock animals given adequate pasture and while tainting has been noted in New Zealand and studied in Japan, to date it is rare for the plant to be sufficiently abundant in grassland in B & I to create this problem (Cooper & Johnson 1998).

Names

The genus name 'Coronopus' is a name used by Theophrastus, possibly for Plantago coronopus (Buck's-horn Plantain), from two Greek words 'corone' meaning 'crow' and 'pous' or 'pus' meaning 'foot' (Gilbert-Carter 1964). One has to suppose that 'crowfoot' here refers to the divided leaves of the plant, as with the aquatic species of Ranunculus. The English common name 'Lesser Swinecress' is given in relation to C. squamatus, 'Swine's Cress', a 16th century derogatory name referring to the taproot of the plant which may be grubbed up by pigs, the plant being a cress fit only for pigs (Grigson 1974). The New Zealand common name 'Twin Cress' referring to the characteristic shape of the two-notched fruit capsule is a much more useful name in the current author's view (RSF). An alternative English name for C. didymus is 'Lesser Wart Cress', the fruit of C. squamatus, 'Wart Cress' being here regarded as wart-shaped (Prior 1879).

Threats

None as yet, but is likely to spread, increase and become a persistent weed.

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