While diseased, dying or wood which could be a hazard on otherwise healthy older trees can be removed at any time, the appropriate timing of pruning trees depends on the species and the type of pruning carried out. Dead wood is best removed in winter, and formative pruning of healthy young trees carried out from early/mid-summer through to winter.
Some trees are best pruned when in full leaf. Reasons for this vary, but are mainly to avoid bleeding, disease or over vigorous new growth.
Due to the increased pressure of rising sap some trees bleed when pruned in spring. These trees are best pruned from mid-summer to before mid-winter. The timing and individual requirements of each tree should be checked in a pruning guide. Trees prone to bleeding include: Acer, Betula, Carpinus, Carya, Juglans, Laburnum, spring-flowering deciduous Magnolia, Morus, Populus (some species), Sophora, Tilia.
Summer pruning can be useful in checking over-vigorous growth, for example in suckering species of Populus, or restricted forms of fruit such as apples. This pruning is generally light and carried out late enough not to promote new growth.
In some species disease risk is reduced or avoided by summer pruning. Prunus species are at a lower risk of infection from silver leaf in summer, and some poplars are at lower risk of canker because their wounds heal quicker in summer.
Healthy trees will tolerate minor pruning in the summer months. This may include corrective pruning to raise hanging branches, or removing weak growth, which can be easier to assess in full leaf.