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Blackthorn, Prunus spinosa, is a shrubby plant often found growing wild in the British countryside. It is also frequently incorporated into field boundaries as a hedging plant, because the dense woody stems that are covered with long thorns form an effective, stock-proof barrier. It's a member of the rose family and is thought to be the wild form of the plant from which cultivated plums are descended.
The shrub is deciduous and when the leaves are lost in winter, the dark grey-brown bark of the plant make it look black from a distance - hence the name. The creamy white flowers of blackthorn, characteristically, appear before the leaves unfurl, and for a few weeks very early in spring the blossom-covered, bare-stemmed shrub becomes very distinctive in the countryside. This flowering period of blackthorn, which typically occurs between mid February / early March is so often accompanied by a cold snap in the weather in Britain that it's traditionally known as the 'Blackthorn winter'. From the flowers develop sloes - marble-sized, round, dark purple wild plums, covered with a distinctive lighter 'bloom' like the coating on a grape. These are ripe by the end of autumn, but for incorporation into the famous drink sloe gin, some cookery writers advise that they should not be picked until after the first frosts, when the tough-skinned fruit begins to soften (fresh, unsoftened sloes need to be pricked with a skewer or darning needle if you're using them to flavour gin). You can still quite often see people picking wild sloes in the countryside, and though the fruit is so intensely sour and acrid-tasting that it's virtually inedible in its raw state, sloes can also be made into a quite passable jam or jelly, either alone or with other wild fruits. Though sloe thorns are tough enough to reputedly, be capable of spearing through car tyres, at up to a couple of inches long, they are large enough to be avoided relatively easily if you are harvesting the wild fruit.
Sloe has a couple of drawbacks as a garden plant - one so serious that personally, I would never consider incorporating it into any garden of mine (though if I owned a field I wanted to hedge round, that would be a different matter). Firstly, the dead stems of blackthorn tend to remain on the plant. As blackthorn grows larger, the older growth round of the bottom of the plant tends to die back, quite naturally, and these older branches are retained on the main stems, drying out and becoming brittle with age. Perhaps because blackthorn forms such an impenetrable mass of branches, this old growth for some reason doesn't tend to fall off the plant and drop to the ground - so, if you come into contact with a blackthorn bush, say if you're trying to prune it or tidy up under it you soon find the old dead wood dropping onto you - and as the old stems are thorny, and tend to disintegrate at the surface into unpleasant powdery, material, this is not a great characteristic.
The second, and I would say more serious flaw that blackthorn has involves a particular type of algae that frequently grows on its branches. This algae is a known hazard to people working with blackthorn in the countryside - as I know from personal experience it's a pretty serious one. If any of the algal material gets into a person's bloodstream, a severely painful allergic reaction can result - and as blackthorn is such a spiny plant, puncture wounds from blackthorn can occur fairly frequently. I once tripped and fell onto a blackthorn bush and was quite deeply 'spined' by one of the thorns, around the area of the first knuckle joining from the palm on the ring-finger of my left hand. The puncture wound was tiny; not much more than a pin-prick - but within a couple of hours my finger, and then later much of my left hand became swollen and stiff. The pain steadily spread up my arm to my wrist and was absolutely excruciating: not knowing about the blackthorn algae problem at that time, I thought I must have broken at least my finger if not my hand, or even my whole wrist at the time I fell, and that I was experiencing a delayed reaction from a broken bone, the discomfort was so severe. I went to the doctor the next morning, and he gave me antibiotics (which understandably, didn't help as the blackthorn algae problem isn't a bacterial infection) and put my arm in a sling for the next few days. Although the pain and swelling subsided, my fourth finger (the bottom knuckle of which where the miniscule injury had occurred), was effectively paralysed after that so I couldn't bend it. This problem didn't resolve itself over the next couple of months and so I was booked in for exploratory surgery(!) on my finger at the local hospital. Happily, as this wasn't considered a serious problem (there was no pain - just a complete loss of movement in the affected digit) and NHS waiting lists are so long, I wasn't called back to the hospital for well over a year - by which time a tiny 'dot' of dark-coloured material had worked its way on its own out the finger-joint; after which my ability to move the finger returned, until these days it's as good as it's ever been.
I do pick sloes from blackthorn every autumn these days, but have an I think healthy aversion to the plant, and try to keep away from it. I wouldn't want it in any garden of mine, I don't even know if you can buy it from a garden centre and would advise anyone considering growing it as an ornamental plant in an area where anyone is likely to come into contact with it to definitely choose something else instead.