Prunus spinosa

Prunus spinosa is a large deciduous shrub or small tree growing to 5 metres (16 feet) tall, with blackish bark and dense, stiff, spiny branches. The leaves are oval, 2–4.5 centimetres (3⁄4–1+3⁄4 inches) long and 1.2–2 cm (1⁄2–3⁄4 in) broad, with a serrated margin. The flowers are about 1.5 cm (1⁄2 in) in diameter, with five creamy-white petals; they are produced shortly before the leaves in early spring,[3] and are hermaphroditic and insect-pollinated. The fruit, called a "sloe", is a drupe 10–12 millimetres (3⁄8–1⁄2 in) in diameter, black with a purple-blue waxy bloom, ripening in autumn and harvested – traditionally, at least in the UK – in October or November after the first frosts. Sloes are thin-fleshed, with a very strongly astringent flavour when fresh.[4]

Blackthorn usually grows as a bush but can grow to become a tree to a height of 6 m. Its branches usually grow forming a tangle.[5][6]

Prunus spinosa is frequently confused with the related P. cerasifera (cherry plum), particularly in early spring when the latter starts flowering somewhat earlier than P. spinosa. They can be distinguished by flower colour, pure white in P. spinosa, creamy white in P. cerasifera. In addition, the sepals are bent backwards in P. cerasifera, but not in P. spinosa.[7] They can be distinguished in winter by the shrubbier habit with stiffer, wider-angled branches of P. spinosa; in summer by the relatively narrower leaves of P. spinosa, more than twice as long as broad;[4][8] and in autumn by the colour of the fruit skin purplish black in P. spinosa and yellow or red in P. cerasifera.[9]^: 207

Prunus spinosa has a tetraploid (2n=4x=32) set of chromosomes.[10]

Like many other fruits with pits, the pit of the sloe contains trace amounts of hydrogen cyanide.[11]

The specific name spinosa is a Latin term indicating the pointed and thornlike spur shoots characteristic of this species. The common name "blackthorn" is due to the thorny nature of the shrub, and possibly its very dark bark: it has a much darker bark than the white-thorn (hawthorn), to which it is contrasted.[12]

The word commonly used for the fruit, "sloe", comes from Old English slāh, cognate with Old High German slēha, slēwa, and Modern German Schlehe.[13] Other cognate forms are Frisian and Middle Low German[lower-alpha 1] slē, Middle Dutch slee, slie, sleeu; Modern Dutch slee; Modern Low German slee/slē, slī;[13][14] Danish slåen.[13]

The names related to 'sloe' come from the common Germanic root slaihwō. Compare Old Slavic, Bulgarian, Macedonian, Ukrainian and Russian слива (sliva, Ukr. slyva),[14][13] West Slavic / Polish śliwa; plum of any species, including sloe śliwa tarnina—root present in other Slavic languages, e.g. Bosnian, Croatian, Montenegrin and Serbian šljiva / шљива.

The species is native to Europe, western Asia, and locally in northwest Africa.[4][15] It is also locally naturalized in New Zealand, Tasmania and eastern North America.[15]

Pocket plum gall on blackthorn, caused by the fungus Taphrina pruni

The foliage is sometimes eaten by the larvae of Lepidoptera, including the small eggar moth, emperor moth, willow beauty, white-pinion spotted, common emerald, November moth, pale November moth, mottled pug, green pug, brimstone moth, feathered thorn, brown-tail, yellow-tail, short-cloaked moth, lesser yellow underwing, lesser broad-bordered yellow underwing, double square-spot, black hairstreak, brown hairstreak, hawthorn moth (Scythropia crataegella) and the case-bearer moth Coleophora anatipennella. Dead blackthorn wood provides food for the caterpillars of the concealer moth Esperia oliviella.

Global plum and sloe output in 2005

The shrub, with its long, sharp thorns, is traditionally used in Britain and other parts of northern Europe to make a cattle-proof hedge.[16]

The fruit is similar to a small damson or plum, suitable for preserves, but rather tart and astringent for eating, unless it is picked after the first few days of autumn frost. This effect can be reproduced by freezing harvested sloes.[17]

Since the plant is hardy, and grows in a wide range of conditions, it is used as a rootstock for many other species of plum, as well as some other fruit species.

In Middle English, slō has been used to denote something of trifling value.[33][14]

The expression "sloe-eyed" for a person with dark eyes comes from the fruit, and is first attested in A. J. Wilson's 1867 novel Vashti.[34]

The flowering of the blackthorn may have been associated with the ancient Celtic celebration of Imbolc, traditionally celebrated on February 1 in Ireland, Scotland and the Isle of Man.[35]

The name of the dark-coloured cloth prunella was derived from the French word prunelle, meaning sloe.[36]