Blackthorn, or sloe (Prunus spinosa), is a common hedgerow plant. Depending on how close you get to it and the time of year, you will know it by its thorns, its white blossom or the dark rich colour of its globular fruit.
The fruit is prized by makers of sloe gin and is found in many other recipes, but is also worth knowing for an unusual navigation clue.
Blackthorn encroaches on spaces that are not regularly used by people or cattle. If a footpath is little used, it may be very clear on your map, but is hard to find in reality. Blackthorn is one of a group of plants, known as colonisers, that show little respect for lines on maps.
If you are confident that a footpath is supposed to be in a certain place, but can find no sign of it, then keep an eye out for blackthorn plants. They like to hide paths, masking the place in a hedge where there was once a break for a footpath, but where nobody has passed recently.
I took the picture at the bottom during a walk in Rutland. This must be a little-used gate and you can see the blackthorn trying to seal it off.
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