Formative pruning is generally completed when a tree is very young (generally up to 5-10 years), although it can be carried out on more mature specimens. It encourages the tree to grow into an attractive and healthy structural form. Ideally, this pruning should be started when the tree is in the nursery in order to influence the structure, shape or size of its crown. Formative pruning is used to remove any future weaknesses in the tree and to give a desired shape. Potential weaknesses are caused by crossing branches or any part of the tree that has grown in an unusual way.