Blackthorn (Prunus spinosa) is a deciduous shrub or small tree that is a common constituent of open woodland and hedgerows, which turn white with blossom in March and early April.  It can also form dense thickets on scree and coastal slopes, where it has benefited from the cessation of scrub management.  This is particularly true in the south of Wales, where blackthorn can form large impenetrable thickets, sometimes at the expense of species-rich limestone grassland.