Blackthorn/Sloe (Prunus spinosa)

BLACKTHORN/SLOE (Prunus spinosa)

Family: Rosaceae

Blackthorn in flower.

(Hailsham, Cuckoo Trail, April 2010)

Blackthorn is one of the most common shrubs/small trees in Britain (for distribution maps see Online Atlas of the British and Irish Flora; see also: Popescu and Caudullo 2016). Its prolific suckering and tangles of black, spine-clad branches produce impenetrable thickets in and around our woodland fringes and dominate many hedgerows. In winter Blackthorn bushes are bare, black and appear to be dead, but with the first hints of spring buds burst, and soon they are smothered with white blossom. Leaves usually start to appear after several weeks of flowering, unlike on Hawthorn shrubs/trees (Crataegus species), which produce leaves first.

Blackthorn blossom.

(Hailsham, Cuckoo Trail, April 2010)

Blackthorn spines  

The 1-3 inch long spines are modified side branches. This is apparent, even in winter, because of the flower- and leaf-buds arrayed along the length of each spine. Once dead, the spines remain sharp but become very brittle, and if they penetrate the flesh and break the spine tips can become deeply embedded. Such wounds inflicted by Blackthorn are notorious for becoming septic, and cases have been recorded where hedge-layers and those ‘hedging and ditching’ with the traditional billhook, slasher and maul, have contracted septicaemia, despite the fact that they were wearing thick leather, protective mitts, spats and aprons (cf. Strömqvist et al. 1985). So those gathering the flowers and fruits should work with care and avoid handling the dead branches. The toughness of Blackthorn wood has led to its being used to make items subjected to heavy wear such as walking sticks (see Chouinard n.d.) and the tines of rakes. 

Unripe sloes.

Blackthorn fruits: sloes           

Sloes resemble miniature plums. In the British Isles, sloes generally ripen in late October or early November. Gathering them by plucking from the bush is very inefficient and the sharp, brittle spines severely hamper the process. It has been found that beating the sloes from the bushes using a forked stick and catching them in a long-handled basket accelerates harvesting and also allows collection of the fruits from some of the higher branches.

Basket of nearly ripe sloes.

(Michelham, September 2005)

After November, most of the sloes that remain on the trees lose their juiciness and become rather fibrous and chewy. A majority of the freshly plucked sloes are inedible: they are not only intensely sour, but they are also highly astringent and often somewhat bitter too. However, they can easily be made sweet and palatable, while retaining just enough sourness to make them deliciously tangy. The edibility of sloes is dependent on the degree of sweetness (or the lack of it), sourness, astringency and bitterness, and each of the taste sensations is a product of a different suite of chemical compounds.

Field experiments have been conducted over several years to identify methods for making fruits edible (Hillman unpublished field notes):

i) heating by roasting sloes on hot rocks: this has a sweetening effect and seems to reduce the bitterness; however, it barely reduces the astringency at all; 

               

Sloes roasting on a fire.

ii) exposing the sloes to frost: if harvesting the fruits is delayed until after the first hard frosts they become conspicuously sweet and the tart (acid) flavour survives sufficiently to maintain the delicious tanginess (Łuczaj 2012: 250; see also Tardío et al. 2006: 36, who describe sloes being stored for several months in hay or grain to sweeten them). The same effect is achieved if the sloes are bulk-harvested earlier (around mid-October), before too many of the early-ripening fruits have dropped, and stored somewhere cool and moist until the first hard frosts (in early-mid November), then spread on mats for 2-3 frosty nights to get thoroughly frozen. The fruits that become the sweetest are those that are still juicy and not too wrinkled. However, it is of interest that frosting the fruits fails to adequately eliminate their astringency; 

Sloes ready for frosting – half wrinkled.

(Valley north of Etchingham, November 2005)

Frosted sloes.

(Michelham, December 2006)

iii) crushing the sloes: if sloes are crushed with a wooden pestle and mortar to create a ‘mash’ and then left for two days to ‘cure’, astringency is eliminated, regardless of what time of year this takes place; however, if the process is carried out before frosting it fails to sweeten the sloes comprehensively. Any fruit-stones not picked out during crushing can be spat out when the mash is eaten. The mash is absolutely delicious and if prepared after the first frosts it is sweet and free of astringency but retains a ‘zingy’ tartness. It can be eaten in considerable quantities as a seasonal staple;

iv) storing sloe cakes: prior to storing the sloe mash, and particularly if it includes a high proportion of wrinkled, sour fruits, it needs to be heated. In this case the mash is squeezed into small flat ‘cakes’ and heated on hot rocks by the fire, rather like a thick, textured form of fruit-leather. Heating the cakes also kills fungal spores and helps seal the surface against new attack by spoilage micro-organisms.

Other ways of preparing sloes

Sloes can be used to make jellies, syrups, jams, and liqueurs (Plants For A Future [PFAF] database; Popescu and Caudullo 2016). Sloe gin is a favourite in the UK and sloe-flavoured alcoholic drinks are also common in other countries in Europe (e.g., bargnolino in Italy; pacharán in Spain: Tardío et al. 2006: 36; épine or épinette or troussepinette in France; ‘plum’ brandy in Slovakia: Łuczaj 2012: 250).

Prehistoric and historic usage

Large numbers of sloe stones are regularly recovered from archaeological sites in circumstances 

that suggest they were eaten (for references see Kroll 1998: 41; Ucchesu et al. 2017: table 1). 

Amongst the earliest reported finds are from Iron Age Glastonbury Lake Village where a huge 

hoard is described by Bulleid and Gray:

“The objects discovered in or around this dwelling-mound were very numerous, and exceeded those of any other mound in the Village. Fragments of pottery were above the average number, and quantities of wheat and peas were found throughout the whole mound. Embedded in the peat immediately outside the border-palisading at the w. side of the mound, two wheelbarrows full of sloe-stones were met with, and from the peat s. of the mound the bones of both pelican and swan were procured.” (Bulleid and Gray 1911: 73).

There are also references to the use of sloes on much earlier sites in Europe. At the late Mesolithic 

site of Bökeberg III in southern Sweden the use of Prunus spinosa fruits is inferred on from the 

presence of pollen and wood charcoal (Regnell et al. 1995). Less equivocal evidence in the form 

of charred endocarp fragments were found at the Mesolithic site of Roc del Migdia in Catalonia 

(Holden et al. 1995). 

Finds of sloe stones (German: ‘Schlehe’) are mentioned in Oswald Heer’s early account of the plant remains preserved at Swiss lakeside pile-dwelling settlements (‘Pfahlbauten’; Heer 1865: 27). At these sites preservation is usually by waterlogging; for example, in their study of southern German and Swiss late Neolithic lakeside settlements, where a majority of the archaeobotanical remains represent food waste that was discarded and subsequent decay was prevented because of submersion in water, Colledge and Conolly note that all finds of Prunus spinosa were found in waterlogged form and none were charred (Colledge and Conolly 2014, table 4; see also Karg and Märkle 2002: figure 4, Jacomet 2006: table 3). At the late Neolithic pile-dwelling site of Pfäffikon-Burg large numbers of waterlogged sloe stones (n=213) and blackthorn buds are recorded (Zibulski 2010: table 1). Fruit flesh/pulp has also been found at the middle Neolithic sites of Schipluiden-Harnaschpolder (Netherlands), where large numbers of fragments (all waterlogged) were retrieved from a well context (Kubiak-Martens 2006), and Giribaldi (France) at which charred remains were found in several of the structures excavated (Thiébault et al. 2000). A single sloe[i] was found in association with the Neolithic mummified body of the iceman ‘Ötzi’, which was found in 1991 in the Ötztal Alps along the Austrian-Italian border, and although tenuous is possibly evidence of use of the fruit in his diet (Oeggl 2009: 5).

In the Medieval period the remains of sloes are commonly preserved in large numbers in latrines and rubbish pits[ii], for example in the 15^th century Worcester barrel (Greig 1981), at the 12^th century farm site at Gouda-Oostpolder, Netherlands (Bakels et al. 2000), in 15^th and 16^th century cesspits at Göttingen, Lower Saxony (Hellwig 1997), in refuse pits in early Medieval (8^th-11^th century) Douai, northern France (van Zeist et al. 1994) and in the ‘Mirror Pit’ (14^th-15^th century) in Ferrara, northern Italy (Mazzanti et al. 2005).

Other uses of Blackthorn

Blackthorn bark

The bark of Blackthorn was traditionally used as source of green or brown dye (Doğan et al. 2003: 448) and ink (e.g., in Medieval manuscripts, Fellows-Jensen and Springborg 2003: 160).

Blackthorn flowers

The freshly-picked flower-buds offer a unique wayside snack, and their almond flavour adds something special to a salad (Plants For A Future [PFAF] database). They’re at their tastiest when the buds are just bursting, but caution is strongly advised before eating because of the high toxicity levels. The almond aroma comes firstly from a compound called amygdalin, which is toxic, and secondly from the hydrogen cyanide (HCN) that amygdalin releases when the flowers are crushed between the teeth, and this is extremely poisonous (see Plants for A Future entry on toxicity). The flowers have laxative properties (for other uses see: Marchelak et al. 2017).

Sloe harvest: Ray Mears with beater.

(Michelham, November 2005)

Bibliography

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Bulleid, A. and Gray, H.St.G. 1911. The Glastonbury Lake Village: a full description of the excavations and the relics discovered 1892-1907. Volume 1. The Glastonbury Antiquarian Society.

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van Zeist, W., Woldring, H. and Neef, R. 1994. Plant husbandry and vegetation in early medieval Douai, northern France. Vegetation History and Archaeobotany 3: 191-218.

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[i] No mention is made of what part of the sloe was found.

[ii] See Greig 1995/6 for more references and for taphonomic issues relating to the deposition of edible fruits in latrine deposits.