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Centaurium erythraea Rafn, Common Centaury

Account Summary

Native, occasional. European southern-temperate, but also widely naturalised in N America, S Australia and New Zealand.

1882; Stewart, S.A.; Co Fermanagh.

May to December.

Growth form and preferred habitats

This glabrous, rosette-forming, tap-rooted biennial or winter-annual has 4-6 pairs of opposite, ovate stem leaves that are more than 5 mm broad, clustered pink flowers, usually in at least five cymes, and produces one (or rarely more) erect flowering stem(s) that can vary greatly in height, from 9-30 cm tall (Ubsdell 1976a; Garrard & Streeter 1983).

C. erythraea is a mobile, small-seeded, locally frequent, grassland colonising species that typically occupies relatively infertile, unproductive, lowland soils that are characterised by a moderate degree of disturbance. These conditions together create an open or semi-open habitat with little competition and a near absence of grazing since the plant contains a number of very bitter substances that deter most browsers. As an example of this, C. erythraea has persisted in a paddock heavily grazed by two horses for 24 years on shallow-soiled land owned by the current author (RSF) and his wife.

The species is rather variable and ecologically wide-ranging and it also occurs thinly scattered on lakeshores, riverbanks, woodland glades and margins, open scrub, roadsides and quarries, where it grows on suitably drained, open sandy soils of a somewhat more acidic nature. It can tolerate trampling, compacted soils and light grazing, but cannot compete successfully with vigorous grasses (Sinker et al. 1985). The established strategy of C. erythraea is thus well categorized as a Stress-tolerant Ruderal species (Grime et al. 1988, 2007).

In coastal areas of B & I, C. erythraea is very frequent on sheltered, dry or well-drained, mildly acid to calcareous dune grasslands. Studies in the Sheffield area showed C. erythraea occurring on soils down to pH 4.8, but no lower, and the majority of sites were above pH 5.5 (Ubsdell 1976a; Grime et al. 1988, 2007).

Variation

Phenotypic variation within C. erythraea with respect to the environment in B & I is considerable and no less than five varieties have been proposed (Ubsdell 1976a; Sell & Murrell 2009). Only one of these, var. erythraea, occurs inland as well as on sheltered coasts (An Irish Flora 1996).

Fermanagh occurrence

C. erythraea has been recorded in 67 Fermanagh tetrads, 12.7% of those in the VC. There are 16 Fermanagh tetrads where the species has pre-1976 records only, which reflects the transitory nature of some of the more open habitats it colonises. The well-drained but not overly dry, infertile soils of a neutral to mildly acidic reaction that Common Centaury most typically frequents are common in limestone districts of Fermanagh.

British and Irish occurrence

In terms of frequency, C. erythraea is chiefly, yet far from exclusively, a coastal plant in the north of Ireland (FNEI 3). Further south in Ireland, it is more generally distributed inland, occupying relatively dry, but essentially well-drained, short, unproductive pastures with quite high levels of bare soil. Although it has been recorded in every Irish VC, C. erythraea is still much more abundant at the coast (Cen Cat Fl Ir 2; An Irish Flora 1996).

In Britain, the New Atlas map shows the distribution is widespread and perfectly stable, or actually increasing in frequency in lowland areas south of a line between Sunderland and Lancaster, as shown in the 1987-2004 monitoring survey (Braithwaite et al. 2006). It becomes scarce and much more coastal further north beyond this line and it only very rarely occurs inland in Scotland where it has been recorded as far north as the isle of Lewis on the W coast.

Phenology and ecological separation from a closely related species

Seed of C. erythraea germinates in the autumn, produces a taproot and overwinters as a tight rosette of leaves flattened close to the ground. Whether C. erythraea is regarded as a winter-annual or a biennial, certainly it is monocarpic (ie plants fruit only once and then die), so the distinction between these two life forms rests purely on the length of time the individual plant needs to achieve the size and accumulate the photosynthetic resources needed to produce flowers, fruit and seed. After seeding in the late autumn the plant dies. Shed seed germinates in vegetation gaps during the autumn (Grime et al. 1988, 2007).

After winter, growth resumes in the spring, producing an erect stem up to 30 cm tall. Height is very variable, however, and in the north of Ireland the plant is frequently only around 15 cm tall. It develops lower stem leaves which are ovate and more than 5 mm broad, a feature which distinguishes this species from C. littorale (Seaside Centaury, or sometimes known as Dwarf-tufted Centaury), which has a shorter stem and narrower leaves. The latter is a closely related species which rather rarely occurs intermingled with C. erythraea on sheltered coasts of Britain and N Europe (Ubsdell 1979), but it is confined to a narrow portion of the Co Antrim coast in Ireland (H39). Both these species are tetraploid (2n=40) with regular meiosis (Ubsdell 1976b). While there are no effective barriers to gene exchange between the two species, ecological isolation operates through slight differences in flowering times and the tendency for C. erythraea to inbreed (Ubsdell 1979).

Since Fermanagh lacks a coastline, C. littorale has not appeared and is not likely to arise in this area (Ubsdell 1979; Garrard & Streeter 1983; Webb et al. 1996; Stace 1997).

Flowering reproduction

C. erythraea flowers from June to October, small individuals having only a few flowers (usually less than 10), while larger plants produce a profusely branched, laxly spreading or densely crowded cymose inflorescence, with many pale pink, star-like flowers. The anthers and stigma are exserted beyond the floral tube, so the flowers are open to many insect types. They produce no free nectar, but are insect-pollinated, most likely by small black thrips. They open early in the day, between 5 am and 7 am and begin to close around noon, or earlier if the sky becomes overcast. Failing insect pollination, in dull weather when the flowers close up for example, self-pollination is highly effective and plants are highly self-fertile with 77-100% seed set (Melderis & Bangerter 1955; Ubsdell 1979). Flower number and seeds/capsule measurement means found the average individual produced around 10,800 seed (Ubsdell 1979). The fruit is a two-valved capsule that splits to release numerous seeds. All seed set in experimental measurements by Ubsdell (1979) showed a high percentage germination.

There is no specific adaptation enabling seed dispersal but presumably, as with other plentifully produced small seed of species of semi-open habitats, it relies upon wind, rain wash and transfer in mud (Ridley 1930). Seeds are short-term persistent in the soil seed bank, ie present for at least one year, but less than five years (Thompson et al. 1997).

In common with other members of the Gentian Family, C. erythraea roots have a mycorrhizal fungal association. As the small, lightweight seed incorporates minimal food reserves, the presence of the fungal partner, open habitats featuring little or no competition for scarce nutrients and for light, are all essential, required factors for successful seedling establishment (Brown & Oosterhuis 1981).

C. erythraea shares many features of its biology and ecology with the related annual Blackstonia perfoliata (Yellow-wort) and, in many regions of B & I, they frequently occur together. At first glance this might suggest B. perfoliata is an obvious 'absentee species' not yet recorded in Fermanagh. However, the species distribution shown in the two BSBI atlases (Perring & Walters 1962; Preston et al. 2002) clearly demonstrates that B. perfoliata has a very much more restricted B & I distribution than C. erythraea and the former does not occur anywhere across the whole six counties of NI.

Toxicity

C. erythraea contains a bitter principle, erythro-centaurin and a bitter glucocide, erytaurin, which appear effective in discouraging most browsing animals. Of all the bitter appetising wild herbs, Common Centaury is regarded the most efficacious, sharing some of the antiseptic properties of Field Gentian (Gentianella campestris) and Bogbean (Menyanthes trifoliata) (Grieve 1931, referred to as Erythraea centaurium (Pers.)).

European and world occurrence

C. erythraea s.l. is present throughout Europe from S Sweden southwards to the Mediterranean basin, Asia Minor, N Africa, Arabia and the Azores. It stretches eastwards to SW Asia and is introduced and naturalised in eastern and western N America, Chile, S Australia and New Zealand (Hultén & Fries 1986, Map 1492; Clapham et al. 1987).

Uses

It has a herbal reputation for ailments of the liver and kidneys and has been used to treat fevers and bleeding. Being bitter in flavour, it was said to "purify the blood" and be an excellent tonic (Grieve 1931). Used as a tonic, C. erythraea is less liable to upset digestion than most other vegetable bitters and so was a preferable alternative to Yellow Gentian (Gentiana lutea) from the continent. It has also been used to treat colic, coughs, jaundice and as a wound healer or blood stauncher (Allen & Hatfield 2004).

Names

The scientific genus name 'Centaurium' and the English 'Centaury' are both derived (like 'Centaurea') from the Greek 'kentaur', referring to the legend of Chiron the centaur, one of mythology's divine healers, who was reputed to have a wide knowledge of herbs. The Latin specific epithet 'erythraea', previously was the name of the genus and is Greek for 'reddish', which really does not describe the flower colour accurately (Gilbert-Carter 1964; Grigson 1974).

Grigson (1955, 1987) list ten English common names, some of which refer to the herbal uses mentioned, for instance 'Bloodwort', 'Feverfew' and 'Gentian', the latter since it was a cheap substitute for imported continental Gentiana lutea (Yellow Gentian), the extremely bitter roots of which had many herbal uses, especially as a strong tonic. Culpeper (1653) approved of Centaury's use for a whole raft of medicinal problems, but aware of its rather bitter taste he wrote, "It is very wholesome, but not very toothsome.", which, in the view of the current author (RSF), expressed it rather well (Grigson 1955, 1987).

In Irish and Manx folklore, C. erythraea is a 'blessed herb', bringing good luck, since legend says it grew where Jesus Christ trod on the way to Calvary (Grigson 1955, 1987).

Threats: None.