BLACKTHORN/SLOE (Prunus spinosa)
Family: Rosaceae
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Blackthorn in flower.
(Hailsham, Cuckoo Trail, April 2010)
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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)
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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.
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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)
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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)
|
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Frosted sloes.
(Michelham, December 2006)
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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 15th century Worcester barrel (Greig 1981), at the 12th
century farm site at Gouda-Oostpolder, Netherlands (Bakels et al. 2000), in 15th
and 16th century cesspits at Göttingen, Lower Saxony (Hellwig 1997),
in refuse pits in early Medieval (8th-11th century) Douai,
northern France (van Zeist et al. 1994) and in the ‘Mirror Pit’ (14th-15th
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)
|
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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.
