Sagina nodosa (L.) Fenzl, Knotted Pearlwort
Account Summary
Native, occasional to locally frequent and quite widely scattered. Eurosiberian boreo-temperate, but also present in N America.
1860; Smith, T.O.; Co Fermanagh.
April to October.
Growth form and preferred habitats
Rosette forming, diffusely tufted perennial, taller than other Pearlworts with stems, ascending or procumbent, usually 10-15 cm tall. Plants sometimes flower in their first year of growth. The stem leaves of S. nodosa are linear-subulate as in other Pearlworts, with the leaf length diminishing upwards from 10-15 mm to 1-2 mm. The plant gets its characteristic 'knotted' appearance from the presence of bunches of shorter leaves (or undeveloped shoots) in the upper stem leaf-axils. The flowers, 0.5-1.0 cm in diameter, are not always present in great numbers, but are most attractive. They have pure white, entire petals nearly twice as long as the sepals, making the plant rather more showy and conspicuous than other Sagina species.
Knotted Pearlwort is a widespread, locally common, stress-tolerant, perennial colonist of open or disturbed, slightly damp to wet, muddy to gravelly, moderately fertile, base-rich lake shores, short fens, flushes, runnels and forest tracks. It is particularly associated with calcareous fens and moist to wet limestone grassland, including upland peat or clay areas flushed by seepage from preferably base-rich springs. S. nodosa also frequents open, moderately acid to calcareous, sandy ground in dune grassland and dune-slacks near the coast. In addition it is reported from ledges on coastal cliffs, less commonly in moist, open woods, and infrequently as a weed of arable cereal crops (Clapham et al. 1987).
Since it requires open habitats, S. nodosa tolerates, or possibly even demands, a moderate degree of grazing and trampling to keep the ground sufficiently open, through limiting and discouraging more vigorous, taller competitors (Sinker et al. 1985; Grime et al. 1988).
Variation and Taxonomy
The species is polymorphic to a considerable degree and varies widely in hairiness, habit (ie prostrate or erect), length of shoots and leaves, degree of leaf succulence, flower size and number of flowers and bulbils (Jonsell et al. 2001). A form, described as var. moniliformis (G.F.W. Meyer) Lange, has stems more or less procumbent and the upper axillary buds, with their characteristic bunches of dwarf stems and leaves readily become detached, forming bulbils that can disperse and vegetatively propagate the plant (Clapham et al. 1987; Sell & Murrell 2018).
The great variability of the plant is reflected in the difficulty taxonomists have faced in assigning the species to a genus: in the past Sagina nodosa has been listed and named within the genera Alsine, Arenaria, Moehringia and Spergularia (Jalas & Suominen 1983).
Reproduction
Flowers are produced from July to September. They are protandrous (male first), and S. nodosa has the highest rate of cross-fertilisation in the genus. Larger flowers are bisexual, but smaller ones which are rare, may be entirely female. Few insects visit the flowers which are automatically self-pollinated. The fruit capsule is short, ovoid, 4 mm and splits to the base to form four or five valves. Seeds are 0.4 mm, dark brown and tubercled. The seed is long-term persistent in the soil (ie survives more than five years) (Thompson et al. 1997).
Flowers are often sparse or absent, and when this occurs bulbils tend to be more numerous. In some areas, eg in seashore habitats, vegetative reproduction predominates (Jonsell et al. 2001).
Fermanagh occurrence

In Fermanagh, S. nodosa is occasional to locally frequent and has been recorded in 102 scattered tetrads, 19.3% of those in the VC. In nine of these tetrads, there are pre-1975 records only, which suggests there has been some local loss of suitable habitats. It is principally found around the Upper Lough Erne basin, which is partially fed by lime-rich waters arising in the hills to the SE. However, it is widely and more thinly scattered in the upland limestones of the Western Plateau, and also more rarely along damp forest tracks.
Irish occurrence
It is rather strongly and obviously associated with the wet, gravelly shores of the larger lakes in the west and centre of Ireland which are geologically underlain by limestone basement rocks. Elsewhere in Ireland, S. nodosa has a definite N, W & C pattern of occurrence at the hectad scale. It is frequent in open areas of sandy and gravelly seashores and in coastal grasslands, especially along the western Atlantic coastline.
British occurrence
In Britain, the species is also widespread, but in a more patchy and attenuated manner than in Ireland, displaying a definite, strong northern and rather weaker western predominance in its distribution (New Atlas). The current distribution of S. nodosa, in Britain at least, is partially imposed by major losses of suitable open, calcareous habitats, especially in S & E England. Analysis of the New Atlas Flora Database indicates that the decline affecting Knotted Pearlwort has occurred since 1950 (P.S. Lusby, in: Preston et al. 2002), pointing to changes in land use and agricultural intensification being most probably responsible.
European and world occurrence
Native in N & C Europe from Iceland to mainly coastal areas of N Spain and Portugal and stretching north-eastwards to northernmost Norway and C Russia. Not present in the Mediterranean basin, nor in Macaronesia (ie Canary Isles & Madeira) (Jalas & Suominen 1983, Map 910). S. nodosa also occurs in Asia, although the map provided by Hultén & Fries (1986, Map 757) shows it very thinly scattered there. The latter authors comment that the species is probably rare in Asia, and admit that their map of its Asian distribution may be incomplete. S. nodosa is also widespread in N America and therefore belongs to the amphi-Atlantic group of northern European plants that span the ocean (Hultén 1958, Map 105).
Names
The generic name 'Sagina' is Latin, meaning 'food crop' or 'fodder'. Spergula arvensis (Corn Spurrey), which was cultivated and much valued in Flanders as a fodder crop for dairy cows, or as human famine food in the 15th century, was previously called Sagina spergula (Gilbert-Carter 1964; Thirsk 1997, p. 17). The Latin specific epithet 'nodosa' means 'knotty' or 'many jointed' and is derived from 'nodus' meaning 'a knot'. In this instance, 'nodosa' refers to the characteristic bundles or fascicles of dwarf branches in the upper stem leaf axils.
The English common name 'Knotted Pearlwort' is a modern book name, but the 'Pearlwort' portion is of 17th century origin, a reference by John Ray (1660) either to the small fruit capsule, or to the pearl-like unopened flower (Grigson 1974).
Threats
Cultural eutrophication from intensive farming and nitrogen in road traffic pollution stimulating competition from taller, more vigorous species.