Capsella bursa-pastoris (L.) Medik., Shepherd's-purse

Account Summary

Growth form and preferred habitats

Although very variable in plant size, fruit and leaf-form, this small cosmopolitan annual, or occasionally biennial weed with its distinctive and unique triangular, flattened fruits has to be one of the most familiar and best recorded ruderal species in the flora of B & I. Its reproductive strategy and dispersal is so effective that C. bursa-pastoris has become one of the most ubiquitous weeds in the world and thus it features in Holm et al.'s 1977 book, The world's worst weeds. However, being a small, non-competitive annual, definitely pernicious, but seldom present in large quantity and certainly not noxious, it hardly deserves such an extreme degree of notoriety. Widespread human transport of the species is achieved by numerous methods, but Reynolds (2002) in her Cat Alien Pl Ir proposed that it arrived in Ireland with animal foodstuff.

C. bursa-pastoris is a stress-tolerant pioneer colonist, ± confined to disturbed or bare soil in open habitats where it can avoid competition. It prefers disturbed, fertile soil situations and, elsewhere in B & I, is most frequent as a weed of arable crops and gardens, particularly of broad-leaved vegetables, eg potatoes, cabbage, peas and sugar-beet (Holm et al. 1977). However, it is also very common in less fertile, disturbed habitats, including trampled or compacted ground, especially where the land is seasonally wet, eg around farm gateways, animal troughs, or in clayey deposits at the base of walls. Only rarely is it found in gaps in pastures, meadows or in woods.

The plant rapidly develops a rosette of basal leaves and a tough branched taproot. It is the latter that enables it to successfully colonise and survive in potentially dry, stony or calcium-rich soils, eg in building rubble, cliff crevices, or in trampled and compacted ground. While tolerant of some drought, essentially C. bursa-pastoris is a mesophyte, preferring near neutral, moderate to base-rich soils. It is therefore absent from very acid, very dry, or permanently wet sites.

Fermanagh occurrence

C. bursa-pastoris is common and widespread in disturbed, open habitats throughout Fermanagh, being recorded in 256 tetrads, 48.5% of those in the VC. As the tetrad map indicates, it is least frequent or absent on the wetter, more acid soils of the Western Plateau area of the county.

Flowering reproduction

The species is tetraploid, probably of hybrid origin

and reproduction is entirely by seed (Hurka & Neuffer 1997). Plants frequently branch from near the base and stems produce numerous long racemes of flowers. As is the case with other ruderal therophytes in this family, C. bursa-pastoris flowers readily and profusely. It also is predominantly a self-pollinating species, often self-fertilising before the flowers open. Under cloudy and rainy conditions flowers are mostly self-pollinated before they open, but dry, sunny weather favours outcrossing. At low temperatures (c 4-10°C) the duration of flowering is prolonged up to five-fold, but pre-conditions favouring cross-fertilisation are strongly reduced (Hurka et al. 1976).

The usually rapid fertilisation process enables C. bursa-pastoris to complete its life-cycle within six weeks with a guaranteed full seed set. Greenhouse experiments suggest outcrossing is rare under field conditions, but is somewhere between 0-20%, so that the breeding system is flexible (Hurka & Neuffer 1997).

The gradual shedding of ripe seed from the plant, together with a very wide range of phenotypic plasticity within the species in response to its environment, frequently allows two or more generations to complete their development within a single growing season. The number of potential offspring produced in a year in this inbred manner is extremely high (Salisbury 1964) and the abundant seeds are long-lived in the soil, persisting for at least 30 years (Salisbury 1964).

Seed germination

Cool temperatures below 10ºC break seed dormancy and since like other cosmopolitan weeds C. bursa-pastoris is indifferent to day-length and photo-period control, subsequent germination is intermittent and can occur during any month of the year. Germination is favoured by seasonally higher and preferably naturally fluctuating temperatures, plus exposure to light (Salisbury 1963, 1964; Popay & Roberts 1970). These particular germination conditions produce a major burst of growth in early spring, while subsequent tillage or disturbance of the soil during the summer brings to the light a fresh supply of non-dormant seed, which will then germinate given sufficient warmth and moisture.

Herbivory and ecology

The plant is eaten by stock and by wild animals, most notably rabbits (Crawley 1990), but slugs are also partial to the leaves, as are numerous insects (Aksoy et al. 1998). Being an insubstantial annual, C. bursa-pastoris is very much an opportunist colonist and pioneer species of open, bare ground conditions and it has very little competitive ability, so that even if abundant amongst crops, it has little effect on the ultimate yield.

European and world occurrence

C. bursa-pastoris is probably of Mediterranean origin and has spread far and wide as an agricultural weed to develop a worldwide distribution that avoids only the hot and the wet tropics (Hurka & Neuffer 1997). A related diploid taxon, C. rubella Reut. replaces it in southern parts of C Europe (Hultén & Fries 1986, Map 971).

Names

The genus name 'Capsella' is a diminutive of 'capsa' meaning 'a box', and refers to the small, notched-triangular or heart-shaped fruit capsule which opens downwards by means of two valves. The Latin specific epithet 'bursa-pastoris' translates as 'shepherd's purse' from 'pastor' meaning 'shepherd' and 'bursa', 'purse' (Gilbert-Carter 1964). This and the familiar English common name 'Shepherd's Purse' derive from the shape of the small triangular fruit which resembles a miniature mediaeval leather belt purse and, as the seeds are yellowish to golden brown and oval and flattish in shape, they resemble miniature coins and thus reinforce the allusion (Salisbury 1964). In Europe, in ancient times, it was used as a pot-herb and it is indeed quite nutritious, eg 100 g of the fresh basal leaves in the spring containing one and a half times the recommended daily human requirement of vitamin C (which is 60 mg/day) (Zennie & Ogzewalla 1977).

Threats

Possibly capable of further increase and spread in disturbed habitats, but unlikely to oust any but other ruderal species.

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