After fertilisation of the flowers, they develop into medium sized berries, which ripen in Autumn, to a purplish blue colour. The seed is inside the berry. Birds see the colourful berries, get attracted to them, eat them, then disperse the seeds in their droppings. The seeds usually germinate in the second spring after ripening, because of the seed's hard coating which is designed to protect it in a bird's digestive system
Blackthorn grows to 5 m tall, with blackish bark and dense, stiff, spiny branches. The leaves are oval, 2–4.5 cm long and 1.2–2 cm broad, with a serrated margin. The flowers are 1.5 cm diameter, with five slightly creamy-white petals; they are produced shortly before the leaves in early spring, and are hermaphroditic and insect-pollinated. The fruit, called a "sloe", is a drupe 10–12 mm in diameter, black with a pale purple-blue waxy bloom, ripening in autumn, and harvested — traditionally, at least in the UK, in October or November after the first frosts. Sloes are thin-fleshed, with a very strongly astringent flavour when fresh.