Botanical illustration of Prunus spinosa. Original book source: Flora von Deutschland, Österreich und der Schweiz 1885. Permission granted to use under GFDL rmanyby Kurt Stueber
Blackthorn Prunus spinosa is one of the first of several white-flowered shrubs to bloom in the spring. This year, I found it coming into flower on 25th March, a few days after the equinox. It is usually beaten in the race to flower by the closely related and very similar Prunus cerasifera, the Cherry Plum. More about Cherry Plum later.
Prunus spinosa, the mass of buds and flowers seen in late March. Image: John Grace.
Prunus spinosa is thorny, as its name suggests. The thorns are very sharp and rather long (2-8 cm), protruding at right-angles to the stem and repelling any four- or two-legged beast that comes to raid. The thorns are formed from the short-shoots, which in most members of the genus Prunus are blunt and have leaves and flowers. They are strong too, and in past times were used by country people for sewing. Even today, the thorns have sometimes been used by restorers of vintage metal-work such as jewelry, as the points get into the corners of old brooches and coronets without actual scratching.
P. spinosa thorns. Note that they have buds that will develop into leaves and flowers. Photo: John Grace.
The flower is typical of the Rosaceae: bowl-shaped with five white petals, five sepals, many stamens, and a single carpel. Flowers appear before the leaves, and there are usually 1-3 together. Flowers are visited by flying insects and butterflies, who refuel with small volumes of highly concentrated sugar (Gyan and Woodell 1987). The fruit is a blue-black plum, coated with a frost-like bloom which wears away to reveal shiny almost-black skin as winter approaches. This plum is called a ‘sloe’. The fruit-stone is called a ‘pit’, looking just like a smaller version of its edible and much-loved relatives the cherries, peaches, nectarines, apricots, and almonds. Usually, the diameter of the fruit is 10-15 mm, larger than a pea but smaller than a cherry. The flesh is extremely bitter, quite inedible for humans.
Flowers and buds of Prunus spinosa. Image: John Grace.
I wonder how evolution has produced such formidable thorns. I like to think of Brown Bears roaming amongst thickets of blackthorn across the Euro-Siberian plains, looking to lunch on any leaves, fruits or bark. Bears in Britain became extinct 1,500 years ago, but the are plenty of other animals that are interested in blackthorn. Birds feed on the sloes: the British Trust for Ornithology lists Blackbirds and Thrushes as the main feeders. Bullfinches eat the flower buds. Researchers at the Tekirdağ Namık Kemal University in Turkey have shown that goats like to eat sloes, and are agents of dispersal. I suppose their taste buds and olfactory sensors are quite unlike ours, and I suspect that the fruits become less astringent after the onset of winter. Deer are repelled by blackthorn hedges. My overall conclusion about the spines is that they evolved to protect leaves and the sugar-rich bark from several species of hungry herbivores.
Blackthorn look-alike, the alien species Prunus cerasifera, the Cherry Plum. Note the reflexed sepals. Image: Chris Jeffree.
Prunus spinosa and other shrubs may have been more common in pre-historic times. Keith Kirby and many others have challenged the idea that lowland Britain was formerly covered with dense woodland. He points out that the pollen record would selectively reveal pollen from wind-pollinated trees but under-record insect-pollinated shrubs. He postulates a herbivore-driven cycle of forest, grassland and scrub, in which the scrub was composed of spiny species including P. spinosa and Hawthorn (Crataegus monogyna); the large grazing animals might have then been aurochs.
Comparison of P. spinosa (upper) and P. cerasifera (lower) showing differences in the colour of bark (green in P. cerasifera) and in the sepals (reflexed in P. cerasifera). Taken March 28th, when P. spinosa was still opening its flowers but P. cerasifera had already lost most of its petals. Photos: John Grace
The blackthorn is found throughout Europe in forest margins and openings, often on poor and rocky ground, sometimes in ravines, waste land, meadows and in hedges. I see a lot on motorway verges and embankments, almost certainly planted. The species has a tough root system which includes lateral branches that sprout new plants (gardeners call them ‘suckers’ ). Thus, an initial planting of a single sapling can be expected to form a dense network of roots to stabilise the soil and aerial shoots (they annoy the gardener). On road verges, the plants never grow tall enough to pose wind-throw threats to motorists and the dense mass of branches can serve to cushion out-of-control vehicles that happen to veer off the carriageway. The white flowers in Spring are a bonus.
P. spinosa in contrasting habitats. Left: a wild location at a coastal site, Bennane Head in Ayrshire, where the plant has colonised the base of a cliff (the white flowers are P. spinosa, the foreground is a mass of brambles, and ivy is climbing the cliff face). The plants are less than 2 metres tall. Right: by the Edinburgh ring-road near to Dreghorn, planted and spreading, up to 3 metres tall.
Also seen on motorway verges is Prunus cerasifera, the Cherry Plum. Not only does it flower earlier but it is less spiny, its flowers are larger, on somewhat longer stalks and it grows taller (up to 12 metres). It is native to Southeast Europe and Western Asia having been introduced to Britain in 1597, and quickly accepted as a fine ornamental garden plant. However, it was not recorded in the wild until the early 20th Century, perhaps because it was mistaken for P. spinosa. Various cultivars are for sale. Its flowers are quite similar in structure to those of P. spinosa, but in some forms they are tinged with a very delicate pink. Key differences, other than those just mentioned are: young twigs are green not brown, the sepals are reflexed, the fruits are yellow or red.
Members of the Prunus genus often form hybrids. It is believed that the Wild Plum P. domestica, ancestor of our modern varieties of sweet dessert plums (and possibly the more astringent damson Prunus insititia), was the result of a cross between P. spinosa and P. cerasifera. The chromosome numbers support this hypothesis: P. spinosa has 2n=32, P. cerasifera has 2n=16 and P. domestica has 2n=48. Moreover, in the Caucasus where the two species and a hybrid grow together it was claimed that the fertile hybrid had been synthesised by making controlled crosses. However, some authorities disagree with this simple view (Zohary 1992), and seek more evidence. Irrespective of it origin, we can be fairly sure from Archaeological data (well-preserved discarded fruit-stones) that P. domestica has provided sustenance for humans for thousands of years, at least since 5,879 BC (Ucchesu et al 2017).
The fruits of P. spinosa, some not quite ripe. Note the blue-grey ‘bloom’. The bloom is caused by minute scales of wax; it reduces loss of water. Image: Chris Jeffree.
For the Irish, the Blackthorn is the traditional wood for making the shillelagh, a short stick that can be used as a weapon or more likely, just to wave in a threatening manner. Blackthorn is a readily available heavy wood, and knobbly, but for shillelagh-making the stem must be treated in various ways to make sure it doesn’t split. It can be placed up a chimney for several months or buried in a dung-heap. An Irish person would make his/her own, but I’ve discovered you can buy them from Amazon.
Shillelaghs of different shapes and sizes. Attribution: Samuraiantiqueworld, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
For British people, Prunus spinosa is the source of the sloes used in the home to make sloe gin. The sloes are collected in October/November, pricked with a cocktail stick, then put to soak for three months in good-quality gin which has been sweetened with much sugar. After that time the clear liquid is poured into elegant bottles labelled ‘sloe gin’. Possibly more sugar needs to be added. The colour and taste is wonderful.
Sloe Gin we made in 2015, from sloes at Glen Luce, Dumfries and Galloway. Photo: John Grace.
In the Basque region of Spain they make something similar called patxaran but instead of gin they use anise with added coffee beans and a vanilla pod. Perhaps this year I’ll try to make some.
References
Gyan KY & Woodell SRJ (1987) Nectar production, sugar content, amino acids and potassium in Prunus spinosa L., Crataegus monogyna Jacq. and Rubus fruticosus L. at Wytham, Oxfordshire. Functional Ecology 1, 251-259.
Kirby KJ (2004) A model of a natural wooded landscape in Britain as influenced by large herbivore activity. Forestry 77, 405-420.
Ucchesu M et al (2017). First finds of Prunus domestica L. in Italy from the Phoenician and Punic periods (6th–2nd centuries BC) Vegetation History and Archaeobotany 26 DOI:10.1007/s00334-017-0622-2
Zohary D (1992) Is the European plum, Prunus domestica L., a P. cerasifera EHRH. x P. spinosa L. allo-polyploid?. Euphytica 60, 75–77. https://doi.org/10.1007/BF00022260
John Grace
Botany in Scotland 