Plums are a diverse group of species. The commercially important plum trees are medium-sized, usually pruned to 5–6 metres (16–20 ft) height. The tree is of medium hardiness. Without pruning, the trees can reach 12 metres (39 ft) in height and spread across 10 metres (33 ft). They blossom in different months in different parts of the world. Fruits are usually of medium size, between 2–7 centimetres (0.79–2.76 in) in diameter, globose to oval. The flesh is firm and juicy. The fruit's peel is smooth, with a natural waxy surface that adheres to the flesh. The plum is a drupe, meaning its fleshy fruit surrounds a single hard seed.