Rorippa palustris (L.) Besser, Marsh Yellow-cress

Account Summary

Identification

The short fruits (only up to 9 mm long) and small flowers with tiny yellow petals, which are as long as, or just shorter than the sepals, clearly distinguish the R. palustris from all other species in the genus except the very much rarer R. islandica sens. str. Previously these two morphologically very similar taxa were combined in the R. islandica s. lat. group until Jonsell (1968) split them on grounds of differing ploidy level; the tetraploid taxon now recognised as R. palustris became by far the commoner and more widespread construct in comparison with the diploid plant, R. islandica sens.str. In B & I the form present is subsp. palustris (Sell & Murrell 2014).

Discrimination of R. palustris from R. islandica sens. str. is difficult, and while plants with an upright habit, sepals longer than 1.6 mm, and with fruits less than twice as long as their pedicels and arranged all round the stem (rather than held to one side, ie secund), are most probably R. palustris, the only absolutely certain method of distinguishing the two species involves laboratory study: either chromosome counting, or a microscopic examination (x 25-50) of the seed surface and its comparison with verified reference material (see Rich 1991, p. 48-49; Chater & Rich 1995).

Growth form and preferred habitats

This distinctive summer annual sometimes biennial or, occasionally, a short-lived monocarpic perennial is a pioneer coloniser of open, unshaded, marshy, or damp, muddy bare ground, exposed in summer around lake and pond shores and river banks. Various floras consulted differ considerably with respect to the life-form and perennating ability of the species: e.g., Grime et al. (1988) describe it in England as a 'summer-annual, semi-rosette therophyte', whilst in New Zealand, Webb et al. (1988) feature it as a 'perennial, rarely annual, taprooted herb.' Rich (1991) regards it as 'annual (rarely perennial)', and Clapham et al. (1968) describe it (albeit as part of R. islandica s. lat), as 'an annual or biennial herb with a pale slender tap-root'. There clearly is a little project needed to clarify the definitive position on this matter, or to measure the range of variation it encompasses.

Marsh Yellow-cress spreads from the natural habitats listed above, to other more disturbed sites associated with man, eg beside lakeshore jetties, ditches and canal banks, although again, generally on ground that is seasonally flooded (Chater & Rich 1995). In drier wayside areas, and in other sites where the mud hardens early in the growing season, the species shows considerable phenotypic plasticity. Plants in these drier conditions are often greatly dwarfed, flowering sooner, but then only managing to produce comparatively few seeds (Grime et al. 1988).

Marsh Yellow-cress is often found in association with other typical pioneer colonists of bare mud habitats, eg Agrostis stolonifera (Creeping Bent), Alopecurus geniculatus (Marsh Foxtail), Chenopodium rubrum (Red Goosefoot), Littorella uniflora (Shoreweed) and Ranuculus scleratus (Celery-leaved Buttercup). A moderate degree of trampling, grazing, or other disturbance may be required to keep growing conditions sufficiently open for plants of low competitive ability like R. palustris to thrive (Grime et al. 1988; Rodwell et al. 2000, p. 428).

Fermanagh occurrence

In Fermanagh, R. palustris has been quite commonly recorded in 107 tetrads, 20.3% of those in the VC. It is particularly common in muddy, marshy, often seasonally or periodically flooded, mesotrophic to eutrophic sites around both Upper and Lower Lough Erne, although as the tetrad map shows, it does occur in a few other similar sites scattered across the VC.

Isolated plants or small populations also occur uncommonly or occasionally as a weed of winter-wet, moderately fertile ground along other artificial, man-made habitats more remote from water bodies, including old disused railway lines, on waste ground, roadsides and even sometimes in damp areas in gardens.

Irish occurrence

In Ireland, R. palustris is recorded as common and widespread particularly in NI, but also in Mid and East Cork (H4 & H5). It appears to be very scarce or absent down most of the W coast where strongly acidic peat soils predominate, and likewise in the drier, warmer, more agriculturally favoured SE of the island. This rather peculiar distribution pattern fails to match any of the 23 categories of vascular plant diversity in the flora of B & I examined by Arnold et al. (2002) in Chapter 5 and mapped in Chapter 6 of the New Atlas. It somewhat resembles the map for the number of hybrids recorded in each hectad in Chapter 5. It is possible that the Irish distribution of this species may at the present time simply reflect local recorder effort and field skills.

British occurrence

In Britain, R. palustris is common and widespread in most lowland damp mud in waterside areas, becoming rare or absent only in N Scotland, although it is also uncommon in Wales and SW England. On account of the identification difficulty with respect to R. islandica s. str., R. palustris might eventually prove to be somewhat under-recorded (Garrard & Streeter 1983; Rich 1991; Preston et al. 2002).

Flowering reproduction

R. palustris flowers from May to October and, while it attracts small bees and flies, generally it is automatically self-pollinated so that even isolated plants achieve a good fruit set (Clapham et al. 1962; Rich 1991). The average sized plant bears around 200 fruits each of which contains around 65 seeds, making a total seed production of 13,000 (Salisbury 1942, pp. 202-3).

Seed dispersal, longevity and germination

Seed release is passive after the pod splits, and the propagules are buoyant and remain afloat in water for up to twelve days. Like many other members of the Brassicaceae, the seedcoat may become gelatinous and sticky when wet, or the seed adheres externally in mud to waterfowl and other waterside animals. This feature enables at least the very occasional long distance jump-dispersal event to take place between different water systems, and also the colonisation of isolated ponds (Ridley 1930, pp. 201-2 & 545).

Seed persists in mud for up to six years (Roberts 1986; Thompson et al. 1997). Germination is promoted by light and fluctuating temperatures and it has twin peaks in March and September. In winter-flooded water margin situations, only the former is significant (Grime et al. 1981, 1988; Roberts 1986).

Vegetative reproduction

Effectively reproduction is entirely by seed, but detached portions of vegetative shoot or dislodged whole plants can rapidly re-root in wet mud, thus enabling broken or uprooted plants to re-anchor after transport by flood water, so that a modicum of vegetative reproduction and dispersal may occasionally supplement the sexual process. This could therefore permit a very limited amount of clonal development of the species to arise (Grime et al. 1988).

Hybridization

Being a comparatively short-lived plant, even when behaving as a perennial, and highly autogamous in its reproductive strategy, R. palustris is not much involved in hybridization (Jonsell 1975). However, a very rare sterile triploid hybrid is known to occur with R. amphibia on the banks of the Thames around Kew, but it has never been reported anywhere in Ireland (Rich 1991).

European and world occurrence

The distribution of R. palustris beyond B & I is widespread throughout Europe except in the Mediterranean basin where it becomes very much scarcer and scattered (Jalas & Suominen 1994, Map 2324). Elsewhere, the species is more or less cosmopolitan, being considered native in most of the world, including New Zealand where it was first collected by Banks and Solander in 1769 or 1770 (Garnock-Jones 1978), but other populations are also probably introduced there (Webb et al. 1988). The same or a similar situation may well apply in S America and in other parts of Australasia (Rich 1991).

Names

The genus name 'Rorippa' originates as a Latinised form of an old Saxon (East German) vernacular or common name 'Rorippen', mentioned by Euricius Cordus (Stearn 1992). The Latin specific epithet 'palustris' is derived from 'palus' meaning a bog or swamp and thus indicates a plant that lives in swampy places (Gilbert-Carter 1964). The modern English common name 'Yellow Marsh-cress' is a book name that tells us just a little about the flower colour and habitat.

Threats

None.

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