Hypericum tetrapterum Fr., Square-stalked St John's-wort

Account Summary

Growth form and preferred habitats

This is a hairless herb with stiffly erect, sharply defined square stems, the stem-lines being strong, forming definite wings. Stems are normally held erect from a decumbent base which produces slender creeping stolons, although they can sometimes be entirely procumbent (Sell & Murrell 2018). H. tetrapterum can grow up to 100 cm tall, but more usually it is smaller, between 10 and 60 cm tall. Leaves are oval (ovate to orbicular) and possess numerous small translucent glands and a few black ones. The pale yellow flowers are numerous, borne in ± flat-topped cymes and are 9-13 mm in diameter. The sepals are very acute and the petals slightly longer than the stamens.

Throughout lowland B & I, this is a widespread and occasional to frequent or locally common plant, occurring in a wide range of damp to wet, reasonably fertile habitats. These include the sides of ditches, streams and other damp to wet eutrophic situations, in woods, marshes, lakeshore meadows and pastures.

Fermanagh occurrence

In Fermanagh, it is recorded in 265 tetrads, 50.2% of those in the VC. Although widespread and commonly found here, it is never abundant and more often than not it is only sparse, forming small, isolated patches. In addition to the typical damp ground habitats, a substantial proportion of the Fermanagh records of H. tetrapterum (approximately 50 of them, representing 11% of the species total) are from drier, more ruderal, wayside and waste ground habitats, including some from quarries, roadsides, cliffs, rocky ground and grassland in districts of predominantly shallow limestone soils.

Locally, H. tetrapterum is absent only from the most acid or regularly waterlogged soils. There are even a few records of it from acidic bogland. Several of these habitats lie beyond the normal range expected for this species elsewhere in these islands. They are not readily explicable, except perhaps as errors for other similar Hypericum species. In the absence of vouchers we cannot check the identification of records from the exceptional habitats, but we feel that with just five records, H. maculatum subsp. obtusiusculum (Imperforate St John's-wort) appears decidedly under-recorded in Fermanagh (see the species account above).

Even if we discard around 50 of the Fermanagh H. tetrapterum records as possible errors, it still remains a very frequent St John's-wort, second only to H. pulchrum (Slender St John's-wort) in terms of frequency and distribution in the VC. This is a situation we could confidently expect, since there is no shortage of suitably damp ditches, stream-sides and marshy grasslands for it to colonise.

Reproduction

H. tetrapterum reproduces both vegetatively by stolons and sexually by insect pollinated flowers. The resultant fruit capsules of the sexual process release huge numbers of tiny, lightweight seed on the breeze in autumn. Seed production is vast. Salisbury (1942) estimated a mean seed production of between 23 and 32 thousand seeds per plant! Furthermore, estimates of seed longevity determined in NW Europe range from ephemeral to long-term persistent, ie from less than twelve months, to at least five years (Thompson et al. 1997).

Salisbury (1942, p. 108) concluded after reviewing the reproductive processes of ten members of the genus, "The species of Hypericum ... support the view that reproductive capacity is a positive asset to the species, and that for species having no special means of vegetative spread the higher the potential reproductive capacity, the commoner the species tends to be."

British and Irish occurrence

H. tetrapterum is widespread in B & I, but it becomes rarer or absent in northern Scotland, probably due to the mountainous ground and the huge extent of very wet, nutrient poor peatland. It is absent altogether (as a native) north of Ross & Cromarty (ie all VCs from 88-112, apart from 94 & 95). A comparative glance at the New Atlas maps of H. tetrapterum, H. pulchrum and H. perforatum (Perforate St John's-wort) indicates how very well distributed these three St John's-wort species are in these islands. H. tetrapterum is second only to H. pulchrum in respect of dispersal (measured in number of map hectads with symbols: H. tetrapterum 2959; H. pulchrum 3323), although H. perforatum, possessing a weedy form and having the most marked ability of vegetative propagation of these three species, is by far the most widespread Hypericum worldwide.

European and world occurrence

H. tetrapterum is a member of the European temperate element and is restricted to W, C and S Europe and adjacent parts of Africa and Asia. Like so many members of the genus, it has been introduced to New Zealand, where it occurs on both islands (Hultén & Fries 1986, Map 1315; Webb et al. 1988; Sell & Murrell 2018).

Threats

None.

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