Trees may be pruned and trained to control their shape and size, maintain their vigour and in some cases, to improve ornamental qualities such as stem colour or foliage size. Young trees in particular benefit from regular training and pruning to ensure that they develop a well balanced framework of branches. However, in general, once mature, the majority of trees only need pruning if any of their branches are dead, dying, diseased, inconveniently placed or sprouting too low down the trunk.
Any pruning work carried out on deciduous trees should take place in the dormant season (late autumn or winter). As their sap pressure is high during late winter and early spring, you should avoid making any cuts during this period, as they will tend to 'bleed'. This loss of sap can weaken a young tree. Some trees such as horse chestnuts (Aesculus), birches (Betula), cherries (Prunus) and maples (Acer) have a tendency to bleed extensively and so should be pruned in mid-summer after new growth has matured.
Other than the removal of dead, diseased or awkward branches, evergreens need little pruning. Any such work should be carried out in spring or summer once the risk of frost is past.
More extensive tree surgery, such as felling, removing large branches and stump chipping, is often best left to a professional arborist. However, with care, you should be able to remove any easily reached dead or diseased branches less than 30 cm (12 in) in diameter yourself. However, do bear in mind that there can be official constraints on cutting certain trees, so always make sure that you check the legal position before undertaking any major work on mature trees.