Draba muralis L., Wall Whitlowgrass

Account Summary

Growth form and preferred habitats

Thanks to its well-developed taproot and spreading fibrous root system, this small winter annual is tolerant of fairly dry soils in May and early June at what really is the end of its growing season. In terms of its ecological characteristics, D. muralis is essentially a warmth-loving, Mediterranean or sub-Mediterranean, spring flowering, calcicole annual of open, winter-moist, but shallow, immature, sometimes rather unstable soils of base-rich Carboniferous limestone geology. The preferred soil has plenty of lime, is often stony, free-draining, rich in humus and has a pH of 6.5 or above (Ratcliffe 1960).

D. muralis is a well-adapted winter annual and typical therophyte, very resistant to adverse growing conditions and thus avoiding competition. While often persistent in its existing stations, its distribution is much confined by its limited powers of natural dispersal, a situation further aggravated by its seed survival being transient. Only the fact that it is accidentally transported by man explains its current wide and still spreading distribution.

Flowering reproduction

Following vernalization by winter cold, from mid to late April onwards plants flower freely irrespective of their size. Flowering and fruiting continues throughout May and June, or even into July provided drought does not intervene by killing the plants (Ratcliffe 1960). The small white flowers are automatically selfed, each resultant fruit containing around ten or eleven small, light seed, although they can produce up to 16 in each oval, flattened pod. Salisbury (1964), who published these figures, reckoned the mean production per plant was over 500 seeds and Ratcliffe (1960) likewise estimated means of between 500 and 1500 per plant.

The species has no specialised mode of seed dispersal, wind probably carrying the seed little beyond one metre from the parent plant.

Germination

After seed dispersal, there is an after-ripening period of around two months which prevents premature germination in the summer months, even if conditions are mild and wet. The current author (RSF) has not located any information in the literature on seed longevity or persistence in the soil (eg in the survey by Thompson et al. 1997), but presumably it is transient (ie less than one year). Germination of this winter annual can be described as ± continuous once it has begun. It stretches over a period of around 25 days, beginning in the September after seed production when the soil becomes permanently moist, although the process may exhibit a slight intermittence towards the end of this period. Up to 65% of the seed germinates in the autumn and there is a hiatus of growth in the months around the turn of the year, with germination and rosette growth resuming in late February or March depending, upon the seasonal weather (Salisbury 1964).

Fermanagh occurrence

D. muralis has persisted, for instance, on the wall of St Margaret's Church in Clabby since at least July 1988. It has also been seen, along with Erophila verna (Common Whitlowgrass), on gravelly path sides on the Necarne Estate near Irvinestown in April 2000, but these two stations for the species, both discovered by RHN, are the only ones so far recorded in Fermanagh. The species is therefore not yet established in this VC, and is thus an adventive.

British occurrence

While D. muralis is a rare native with small, scattered colonies on lower-lying, warmed limestone rocks in SW England, the Peak District and the Pennines, it does manage to reach up to 490 m in the Craven Pennines in W Yorkshire − where the first discovery of the species in Britain was made by John Ray in 1670 (Ratcliffe 1960; Rich 1991; D.A. Pearman, in: Preston et al. 2002).

It addition to this very limited native distribution, D. muralis is also widely but thinly scattered elsewhere in Britain, mainly in mild, winter-wet, mild western districts, to which it "spread artificially", as it was quaintly put by Ratcliffe (1960).

The anthropogenic dissemination of the plant in Britain began early in the 19th century, for it was reported from the old botanic gardens in both Edinburgh and Glasgow by 1824 and 1865 respectively. Many other early county records beyond the native English range of the species, describe its discovery in horticultural nurseries and in botanic or private gardens, nearly always growing on old stone walls where the lime mortar provided the preferred nutrient conditions, along with little or no competition.

Irish occurrence

D. muralis is also a very thinly and widely scattered alien throughout Ireland, having been recorded in 20 of the 40 Irish VCs at any date (Reynolds 2002). The New Atlas map displays its occurrence in 21 hexad squares with 1987 or later dates (Preston et al. 2002). Here again, as in Britain, dissemination along with garden plants from particular horticultural nurseries is the most likely source of records (Brunker 1950) and the plant is always found in man-made habitats involving limestone rock, or on old walls with lime mortar.

The earliest Irish record appears to have been a single plant on the walls of Blarney Castle, County Cork, found by Mr James Drummond, a Scotsman who around 1809 was Curator of the short-lived Cork City Botanic Garden and who emigrated to W Australia in 1829 (Praeger 1949). His undated find was published in the Catalogue of the indigenous plants of Ireland (Mackay 1825). The second published Irish record was by another Scotsman, George Dickie, who reported it growing, "On old walls about Belfast", in a supplementary list in his Flora of Ulster, recognising it as an introduction (Dickie 1864).

It appears that D. muralis is a well adapted winter annual and typical therophyte, very resistant to adverse conditions, and thus avoiding competition. While it is very persistent in its existing stations, its distribution is much confined by its very limited powers of natural dispersal, a situation further aggravated by its seed survival being transient. Only the fact that it is accidentally or otherwise transported by man explains its current wider distribution.

European and world occurrence

D. muralis is considered native in temperate areas of W, S & C Europe, extending eastwards to Turkey and the Caucasus. It is also native in NW Africa and Madeira (Rich 1991). It is absent from much of NW France, the Netherlands and becomes more scattered northwards into southern Scandinavia (Hultén & Fries 1986, Map 963). It has been introduced in N America (Rich 1991).

Names

The genus name 'Draba' is from the Greek 'drabe', a Classical name given by Dioscorides to a plant of the Cabbage Family, possibly Lepidium draba (Hoary Cress) which was supposed to have value in poulticing whitlows, that is, wounds of the nails, and hence the English common name. The Latin specific epithet 'muralis' simply means 'growing on walls' (Stearn 1992).

Threats

Extreme rarity always represents a survival problem.

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