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Rhamnus cathartica L., Buckthorn

Account Summary

Native, locally frequent. Eurosiberian temperate, but widely naturalised.

1825; Mackay, J.T.; islands in Lough Erne, near Enniskillen.

May to October.

Growth form and preferred habitats

A dioecious, deciduous, rather variable small tree or shrub (usually 5-10 m in height), with branches in opposite or sub-opposite pairs and small greenish flowers in axillary clusters. The prominent, impressed, curved veins on the opposite, finely-toothed, oval leaves of 'Purging Buckthorn', together with the spiny branches and, in autumn, small clusters of shiny black 'berries' on short stalks persisting after the leaves fall, make this an easy species to identify. It is the only tree (or shrub in wild habitats) in B & I that has both paired buds and thorns. A closer examination shows that R. cathartica has three sorts of twigs: long shoots that extend growth; short shoots that bear leaves and flowers but remain short; and shoots modified into sharp woody thorns (Edlin 1964).

R. cathartica demands base-rich or calcareous soils. On alkaline, lime-rich, lakeshore fen peat, R. cathartica is an undershrub of the more open areas of damp to wet, seasonally flooded, woody fen-carr scrub dominated by ash and/or alder and accompanied by various willows. Young bushes are very shade tolerant (Godwin 1943), but R. cathartica is not usually a woodland plant in B & I, being much more confined to scrub and hedgerows (Rackham 1980). When it does appear in woodland, it is generally in more open areas of Beech and Oak. Buckthorn often associates with Euonymus europaeus (Spindle), Prunus spinosa (Blackthorn) and, occasionally, Ligustrum vulgare (Wild Privet), although the latter is generally planted.

History and status

The fact that prehistoric R. cathartica fossils or sub-fossils (eg charcoal remains) first become recognised in the current Flandrian (or Irish, Littletonian) interglacial, during the Atlantic period (7500-5200 BP), and always from archaeological sites, is probably because the species is insect pollinated (see below). As Neolithic farmers are not known to have been actively involved in importing plants apart from cereal crop seeds and root vegetables, the late date of recognised fossil material does not necessarily suggest this woody species was introduced, although this could yet be the case (Godwin 1975; Rackham 1980). All B & I Floras assume R. cathartica is a native species in England, Wales and Ireland (eg Sell & Murrell 2009) and, since it is bird dispersed, this appears reasonable.

Flowering reproduction

Reproduction is entirely by seed and trees and bushes flower early for a woody species, certainly by the age of eleven (Godwin 1943). Tiny male and female flowers (3-7 mm across) appear from May to June on separate trees or bushes. The sex ratio is highly skewed towards fruit-bearing female bushes; the ratio can be as high as 1:6 or 1:7 (Godwin 1943). Flowers are borne either solitary, or in dense clusters of 2-5 on short shoots. Flower parts are in fours: the yellow-green sepals are the most conspicuous element, while the greenish-white petals are so tiny (1 mm) they are often considered absent. The calyx tube is adnate (fused) to the base of the ovary and its inner surface forms a nectary. Depending on gender, the flowers have either four stamens, or a pistil with a solitary style, a 4-branched stigma, and a 4-celled ovary (Sell & Murrell 2009). Sepals and petals are shorter and narrower in female flowers than in the male (Godwin 1943). The flowers are, as Edlin (1964) comments, "seldom noticed", but nevertheless they are strongly honey-scented and are clearly insect-pollinated, mainly by Honey Bees. Although insect visitors are said to be abundant, they are little recorded (Godwin 1943; Lang 1987).

The sexual process is efficient as most female bushes fruit heavily from September and the species is most prominent when it is festooned with shiny ovoid black berries, each c 6 × 8 mm, with a small depressed central scar at the apex. Ripe fruits often persist on the tree or bush well into December or even later. Bushes usually fruit heavily, for instance one bush 2.1 m high at Wicken Fen in Cambridgeshire produced 1,455 'berries' (Godwin 1936). Botanically, the fruit is a drupe, since each 'berry' contains three or four hard-layered stones or pyrenes (Godwin 1943).

The strong association of R. cathartica with hedgerows very probably reflects the frequency of bird dispersal of the seed. Blackbirds, other thrushes, Starlings, Robins and Great Tits are the main fruit consumers and agents of seed transport, while the Bullfinch is a seed predator (Godwin 1943; Snow & Snow 1988, p. 80). The observation that fruits remain available on bushes for several months after they ripen suggests birds prefer other items of diet and really only consume Buckthorn drupes when they run out of other hedge fruit, or bad weather induces near starvation (Godwin 1943). Seed can survive in soil for at least two years, but germination usually takes place the spring after seed is released (Godwin 1943).

Variation

The species is rather variable and Sell & Murrell (2009) list no less than six varieties recorded in B & I, one of which, var. prostrata Druce, is a prostrate plant! Var. cathartica and var. ambigua J. Murr appear to be widespread within the range of the species; var. hydrensis (Hacq.) DC. is in scattered localities and var. schroeteri DC is in the north of England. The distribution and ecology of these four varieties is described as, "not understood". Var. longipetiolata Grubov is an introduction planted by motorways and probably elsewhere and seems to have been imported from E Europe and Russia (Sell & Murrell 2009). No hybids are known (Godwin 1943).

Irish occurrence

R. cathartica has a rather scattered distribution in Ireland. The strongly expressed calcicolous ecological requirements result in it being restricted to a wide belt across the central plain where it can be frequent, but it is absent from southern counties and rather rare or absent in NI. R. cathartica is close to the northern limit of its natural range in these islands and, elsewhere in NI, it is a rare, sparse and decreasing species. In fact, beyond Fermanagh it is confined to just three hectads around Lough Neagh and the adjacent Lough Beg (Flora of Lough Neagh), plus one isolated hectad in north Co Down (H38) (NI Vascular Plant Database).

Fermanagh occurrence

In Fermanagh, R. cathartica is locally frequent in damp wood copses, scrub and bushy hedgerows around both parts of Lough Erne and, as the map indicates, is seldom seen beyond the Lough. There are records in the Fermanagh Flora Database from 50 tetrads, representing 9.5% of those in the VC. The two most isolated sites shown on the tetrad map are Druminiskill Td, 2 km SE of Arney Bridge, where it was recorded in July 1992 by an RSPB survey, and in a wood just N of Summerhill Lough, where it was found in August 1995 by RHN and HJN.

In addition to its more usual damp to wet ground habitats listed above, Buckthorn is also found in Fermanagh to a lesser extent in dry, sunny, sheltered areas of scrub on rock outcrops and in hedgerows of limestone districts.

Although R. catharctica has declined in other parts of its NI distribution over the last half-century or so (Hackney et al. 1992), it seems perfectly stable in Fermanagh. Buckthorn is the obligatory larvae food plant of the Brimstone butterfly caterpillar (Gonepteryx rhamni), a conspicuous bright yellow adult butterfly species. The insect was first recorded in Fermanagh in 1918 and has been seen sporadically since then. The population's best years in the VC were possibly 1983-1985, from which the Brimstone has declined markedly to extinction as a resident species in NI (Nash et al. 2012). The fact that the shrub population of R. catharctica has not declined in this period in Fermanagh, makes the insect's fluctuation, and the reasons for its apparent local decline, something of a mystery.

British occurrence

In England, it is frequent to locally common in the Midlands and S & SE where calcareous rock outcrops and soils occur. It is absent from Cornwall, N Devon and Northumberland, rare and not native anywhere in Scotland; it is also absent from most of Wales except parts of the S & NW (New Atlas). It has been planted along motorways and elsewhere.

European and world occurrence

Originally restricted mainly to Europe and W Asia, it belongs to the Eurosiberian temperate element. R. cathartica is widespread across most of Europe to 62oN in Norway & Sweden and eastwards across Russia into W Asia, Afghanistan and Turkestan. In S Europe, it stretches to Mid & E Spain and east to Italy, Sicily and Macedonia, but it is absent from the Balearics, Corsica, Sardinia and Greece. In N Africa, it is present on high mountains in Morocco and Algeria. See also the section on Variation above.

It was introduced to N America and is quite widespread in E and Midland regions (Hultén & Fries 1986, Map 1298).

Names and uses

The purging 'catharsis' in the name derives from the herbal and veterinary use of the pea-sized fleshy fruits. These are very poisonous when both ripe and unripe, so that the juice of ripe drupes (described as acrid, nauseous and bitter) must be extracted, sweetened, aromatics added (eg aniseed, cinnamon and nutmeg) and the resultant syrup diluted six-fold before its use as a very strong laxative for severe constipation (Grieve 1931; Lang 1987). Perhaps on account of this powerful medicinal effect and the 'cross-like' (crucifix) shape of the twig arrangement, the branches were also wielded as a folk charm against witches!

'Buckthorn' was well known to the Anglo-Saxon herbalists and was named in their medicinal writings as 'Hart's-thorn'. Both of these names are thought to refer to the wide-spreading branching of the shrub twigs, that is like a Deer's anthers. Another common name is 'Way-thorn', ie highway-thorn, translated from the German 'wegedorn' (Prior 1879). Other English names include 'Rainberry Thorn' and 'Rhine-berry' – the latter a translation of the German name on account of, "there being much of them alongst the River Rhene" (Lyte 1578, quoted by Prior 1879). It is also possible that the derivation of 'Rainberry Thorn' comes from 'rain', meaning 'a boundary', the usual place of growth of the shrub (Prior 1879; Britten & Holland 1886). Gerard (1597) referred to it as 'Laxative Ram', suggesting the fierce strength of the drug. It remained official in the British Pharmacopœia until 1867 but was discontinued due to its severity, being considered more suitable for veterinary practice treating dogs and cows (Grieve 1931; Allen & Hatfield 2004).

Grigson (1955 & 1987) and Grieve (1931) list another use in dyeing. Dried 'berries' provide a range of rich but fugitive colours. The fruit used to be sold under the name 'French berries' and imported along with those of R. infectorius from the Middle-east (Britten & Holland 1886). If gathered green and unripe, the berries give a saffron yellow dye that was fixed with alum and used to tint paper and also leather, especially for making gloves (Grigson 1955 & 1987). Ripe berries mixed with gum-arabic and limewater formed the pigment called 'Sap- or Bladder-green', said to be well known to water-colourists. The bark of the plant also provides a yellow dye (Grieve 1931).

Threats

None.