Polygala serpyllifolia Hosé, Heath Milkwort

Account Summary

Native, frequent. Suboceanic temperate, but also present in Greenland.

1881-2; Barrington, R.M.; Co Fermanagh.

April to January.

Growth form and preferred habitats

This is a variable, low-growing, 6-25 cm, decumbent to ascending, slender stemmed, few-flowered (3-10), perennial of wet to dry, acidic, pastures and often more peaty habitats in upland grassland, heaths, bogs and moorland. Its predominant occurrence on higher altitude, acidic habitats compared to those favoured by the similar P. vulgaris (Common Milkwort), together with its lack of a woody base and the opposite or slightly sub-opposite leaves on the lower stem of this plant, readily allow P. serpyllifolia to be distinguished from P. vulgaris which has all of its leaves alternate (Sell & Murrell 2009). However, since quite often a number of the significant lower leaves drop off early in the growing season, in order to absolutely confirm the identification of this species, it then becomes necessary to examine the relative positions of the leaf-scars on the nodes, or to examine other more critical characters (see R. FitzGerald et al., in: Rich & Jermy 1998).

Apart from the distinctions regarding lime-tolerance (calcicole) or lime-avoidance (calcifuge) and a preference for base-poor, acid conditions in the case of P. serpyllifolia, the ecology and biology of these two Milkwort species is really remarkably similar. Both species frequent and characterise the species-rich vegetation of old, grazed, but otherwise relatively undisturbed, nutrient-impoverished grasslands. Both Milkwort species are stress-tolerant competition avoiders. Conducive circumstances are more often located on higher ground in the case of P. serpyllifolia, for instance on sheep-grazed, hilly moors, heaths and blanket bogland, but it can also occur on damp hummocks in lowland raised and valley bogs, so it is not confined to the uplands.

Since P. serpyllifolia is a mainly upland species, reaching almost to the summits of the highest hills, its habitats are less severely affected by agricultural management grassland 'improvements' than those of P. vulgaris (see RSF's species account on this website).

Flowering reproduction

Both these Milkwort species demonstrate a similar complete dependence on seed for their reproduction. They flower from May to August or September and produce a continuous spectrum of flower colour ranging from the typical and most common deep purple-blue, through pale blue to almost white, to pink and a strong purple. Heath Milkwort regularly demonstrates this colour range on a single hill slope, with the frequency emphasis on the deeper colours rather than among the paler hues. The raceme inflorescence of P. serpyllifolia has significantly fewer flowers (3-10) than that of P. vulgaris (10-40). The individual flowers are also slightly shorter, the petaloid sepals (wings) being 4.5-5.5 mm long, whereas those of P. vulgaris measure 4.0-8.5 mm. The flowers of both species are cross-pollinated by bees rewarded by nectar, but they may also be selfed (Hickey & King 1981) (See RSF's P. vulgaris account on this website for a fuller description of Polygala pollination).

Seed production is not sizeable in both Milkworts, each flower producing just two relatively large seeds. The fact that P. serpyllifolia has many fewer flowers than P. vulgaris normally does, obviously affects its relative level of seed production. The fruit capsule splits to release the seeds, each of which possess a nutritive oil body or elaiosome attractive to ants, which help to disperse them locally, thus minimising seed predation (Ridley 1930). There does not appear to be any mechanism of long distance dispersal, but since these two Polygala species are widespread, it must remain to be discovered. Seed of P. serpyllifolia can persist in the soil for more than a year and perhaps for up to five (Thompson et al. 1997).

While these two Polygala species can very occasionally occur near one another on flushed heaths and moors (see RSF's P. vulgaris species account on this website), no hybridization results (Stace 1975; Stace et al. 2015).

Variation

In B & I, three varieties are recognised and named in their critical Flora by Sell & Murrell (2009): var. vincoides (Chidat ex Davy) P.D. Sell, with leaves on the upper stem ± opposite, elliptical and shortly pointed; var. serpyllifolia, upper stem leaves obviously alternate, lanceolate, rounded to pointed; and var. decora C.E. Salmon, with upper leaves obviously alternate, oblong-elliptical, pointed at apex and flowers usually larger than the other two varieties.

Fermanagh occurrence

In Fermanagh, these two milkwort species are almost equally common in terms of both numbers of records and tetrads. Heath Milkwort has just a slight edge on the lime-tolerant P. vulgaris with records in 133 tetrads (25.2% of the total), compared with 112 tetrads for P. vulgaris. However, in common with other local Flora writers, RHN and the current author (RSF) believe P. vulgaris is probably somewhat over-recorded, some field workers having missed the opposite basal leaves which distinguish P. serpyllifolia.

There are large areas of strongly acidic, wet, peaty and podsolized ground in Fermanagh, especially on higher ground, but also in specific areas of bogland and, more generally, anywhere a layer of glacial clays impedes drainage. Even within the limestone areas in the VC, leached peaty soils sufficiently acidic to exclude P. vulgaris do occur. This situation led RHN and the current author (RSF) to expect Heath Milkwort to be the commoner of the two species by a greater margin than the existing Fermanagh data demonstrates.

British and Irish occurrence

The New Atlas map shows that P. serpyllifolia has a definite, quite marked, northern and western tendency in its B & I distribution, somewhat less pronounced in Ireland than in Britain, but present nevertheless. There has been a substantial loss of suitable habitat in S England and inspection of the record database shows this to have occurred from 1950 onwards (M.J.Y. Foley, in: Preston et al. 2002). Of the three varieties mentioned above, var. serpyllifolia is the widespread plant, var. vincoides occurs in Cornwall (VCs 1 & 2) and Staffordshire (VC 39), and var. decora is recorded from mountainous areas of all three countries of Britain, plus Ireland (Sell & Murrell 2009).

European and world occurrence

P. serpyllifolia belongs to the Suboceanic temperate phytogeographic element and is mainly confined to W Europe, although also present in C Europe to a much lesser extent. It is poorly represented in the Mediterranean basin but is present in Corsica. It is reported from SE Greenland (Böcher et al. 1968) and the Faeroes, where it is said to be, "common, especially on the lowlands [ie heaths]" (Ostenfeld & Gröntved 1934), although it is completely absent from Iceland (Hultén & Fries 1986, Map 1289). It is considered native on Greenland by Hultén (1958) and, therefore (if this position is agreed), it belongs to the Amphi-Atlantic plants that bridge the Atlantic Ocean.