From The Scots Herbal by Tess Darwin:
Grows wild throughout Scotland. Sloe or blackthorn berry, might be classified as a food rather than a medicine, but has a bitter, acrid taste (perhaps more familiar to us today in sloe gin than in the fruit) and was eaten from prehistoric times for its vitamin and mineral content. It has occasionally been found in ancient graves.
Long associated with dark forces and sometimes used in rites of black witches. The hard, strong wood and shape of blackthorn made it ideal for walking sticks and weapons and may have led to the belief that, like other prickly plants (which would catch and hold malevolent spirits), the tree had protective powers against evil. It was planted around fields as much for this as for the thorns, and the protection more pleasantly absorbed through drinking sloe gin, also a remedy for diarrhea. A slightly purgative tea made from the leaves, flowers, and bark was used to reduce fever. The berries and bark were also used for dye.
The blackthorn is a widely distributed native tree of Britain, Europe and parts of Asia. It grows abundantly in hedgerows, thickets and on waste ground. It is unpopular with farmers because of its suckers and vicious black thorns which make it impenetrable. It is a small tree which grows to a height of around 13 ft. The black, thorn studded twigs carry alternate winter buds that are oval and purplish- black or reddish in colour. In April the blackthorn produces clouds of white blossom which are followed by the leaves, the first of the hedgerow trees to flower. They are small, pale green and oval shaped on opening but later become longer, narrower and dull green. The small, round fruits, known as sloes, ripen slowly throughout the year and only sweeten after the first frosts. It is the ancestor of the cultivated damson and plum. The bark is black and on old trees it becomes broken to form small square plates.
The white flowers can be collected during April and the sloes in autumn. They should not be collected until after the first frosts. It is considered unlucky to bring Blackthorn indoors while in blossom.
November 11th is recognized in Ireland as the day of the blackthorn sprites, the lunantishees, Otherworldly beings who guard the sacred blackthorn from any human foolhardy enough to profane the sacred tree by cutting the wood now. The blackthorn has an ominous image. The thorns of the blackthorn were used for pricking wax images for cursing. Witches were thought to carry black rods of blackthorn which could cause miscarriages. When witches were burned blackthorn sticks were thrown onto the fire. The sorcerer Major Weir was burned at the stake in 1670 with a blackthorn rod, which was described as the chief agent of his magic. Some traditions say that Christ's crown of thorns was made from blackthorn. The shillelagh, or Irish club is made from the dense, heavy blackthorn. The usher of the house of lords and the Order of the Garter is called Blackrod because he knocks on the doors of the house of parliament with a blackthorn rod to demand its opening.
On the other hand, the sloe could sometimes be an instrument of blessing. In the north of England people would make a blackthorn globe and gather round a bonfire to chant "Auld cider" to bless the apple trees on January 1st. On New Year’s morning a crown of blackthorn and mistletoe was hung up for luck. Also at New Year in Worcestershire a baked crown of blackthorn was scattered on the fields to bless them.
The blossoming of the blackthorn marks the time for sowing barley. In Scotland it is said:
‘When the slae tree is white as a sheet
Sow your barley, wither it be drey or wet.’
Which, roughly translated into Gloucestershire, meant:
‘When the blackthorn blossom’s white
Sow your barley day and night.’
For more on the medicinal uses of Blackthorn visit: http://www.herbs2000.com/herbs/herbs_blackthorn.htm