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Hypericum pulchrum L., Slender St John's-wort

Account Summary

Native, very common and widespread. Sub-oceanic temperate.

1881; Stewart, S.A.; Co Fermanagh.

Throughout the year.

Growth form, identification and preferred habitats

This slender, erect, hairless perennial with smooth stems can grow up to 60 cm tall, but very often it is much smaller (c 15-25 cm). Leaves are cordate or truncate at the base and petals and sepals are fringed with shortly stalked black glands and the backs of petals are sometimes red-tinged (Parnell & Curtis 2012). Young or sterile plants are trickier to identify. In H. pulchrum, young plants (and sometimes mature ones also) have decumbent to prostrate stems with oblong leaves, making them rather similar to H. humifusum (Trailing St John's-wort). However, the leaves are more leathery than the latter and they also lack the intramarginal black glands of H. humifusum, which should enable H. pulchrum to be distinguished. The absence of transparent pellucid glands from the basal part of the leaf nearest the stem is another helpful characteristic of H. pulchrum (Robson 1990).

In general, H. pulchrum occurs in habitats where a variety of stressful growing conditions severely limit biological competition from taller, more vigorous and more aggressive plant species. It tolerates a wide range of dry, shallow, stony, infertile and often leached, sandy or peaty soils in a long list of open, or semi-shaded habitats at almost all altitudes. Locally these include the margins and open areas of woods and scrub, hedgerows, dry, rough grazing grassy heaths or moors, coarse grassy banks, roadsides, quarries and gravel-pits.

Fermanagh occurrence

H. pulchrum is the most common and widespread Hypericum species in Fermanagh by a considerable margin. It has been commonly recorded in 310 tetrads, 58.7% of those in the VC. By comparison its nearest rival, H. tetrapterum (Square-stalked St John's-wort), has two thirds the record frequency of Slender St John's-wort and is distributed across 50.2% of the tetrads.

As the tetrad distribution map indicates, H. pulchrum occurs widely across the VC, but the dearth of records in lowland tetrads adjacent to the eastern shores of Lough Erne is rather surprising.

A possible limestone ecotype

While H. pulchrum more commonly grows on acid to neutral soils and elsewhere in B & I is often reckoned a definite calcifuge species (Robson 1990; Crawley 2005), in Fermanagh and other parts of western Ireland, H. pulchrum occurs in some profusion on often quite bare limestone habitats. These include stabilised screes, rock ledges on cliffs and knolls and both on the surface and in the deep crevices typical of limestone pavement.

On noticing this behaviour on the limestones in the Burren, Co Clare and in Connemara, Webb & Scannell (1983) (Flora of Connemara and the Burren) remarked on the possibility that a calcicole edaphic ecotype may have differentiated, it becoming more tolerant of alkaline, base-rich soils than the normal form of the species. If this is the case, then both ecotypes are present in Fermanagh, since H. pulchrum appears common on both acid soils and on alkaline, lime-rich ones. It thus appears indifferent to soil base-status, a situation also described in the Shropshire region of England by Sinker et al. (1985).

Other variation

H. pulchrum varies relatively little, but a dwarf, few-flowered, prostrate to procumbent form (f. procumbens (Rostrup) Beeby) is found rarely in exposed habitats at the NW extreme of the species range in the Faeröes, the Shetlands, Caithness, W. Ross and the Outer Hebrides in Scotland and on Clare Island and Achill Island off the W coast of Ireland (Robson 1990). This variant is said to breed true, but a complete series of intermediates links it with the typical plant (N.K.B. Robson, in: Tutin et al. 1968).

Reproduction

In common with H. perforatum (Perforate St John's-wort), H. pulchrum can increase its presence on a very local basis through vegetative reproduction by means of adventitious shoot buds produced on its most superficial spreading lateral roots. The mean annual seed production of H. pulchrum is approximately 4 to 5 thousand, considerably less than the other two most widespread St John's-wort species in B & I, H. tetrapterum and H. perforatum (Salisbury 1942). In common with these latter species, however, buried seed of H. pulchrum survives for at least five years in soil (Thompson et al. 1997).

British and Irish occurrence

Although it is often regarded as a 'locally common' species in most of B & I, on the evidence provided by the hectad maps in the New Atlas, H. pulchrum is the most widespread Hypericum species by quite a wide margin. The species map has record symbols in 3323 hectads throughout these islands, representing 85% of the total area. This is 364 hectads more than the nearest rival in the genus, H. tetrapterum (Preston et al. 2002). Having said this, H. pulchrum is absent or rare in the low-lying English Wash and in the limestone belt that lies to the SW of it. In Ireland, it is similarly absent from around the Dublin conurbation (H21) and from the far W of Co Mayo and W Galway (H16 & H27) where completely unsuitable wet, peaty soil conditions are very common.

European occurrence

H. pulchrum is a suboceanic temperate species confined to NW Europe. It occurs in the Faeröes, coastal S Scandinavia and the region west and north of a line through Germany, Switzerland, France, Spain and C Portugal. There are also outlying stations further east from SW Poland to NW Italy (Hultén & Fries 1986, Map 1312; Robson 1990; Pignatti 1997, 1; Sell & Murrell 2018).