Pruning requirements of trees and shrubs will not only vary according to species, it will also depend on the purpose of pruning. If pruning is necessary because branches are dead and the tree or shrub causes a safety hazard, pruning can be performed at any time. However, the overall health of the plant should always be taken into consideration before addressing pruning issues. It is important to know that detrimental diseases can easily be spread if trees and shrubs are pruned at the wrong time of the year. For example, oak trees should only be pruned in the winter months when the trees are dormant to prevent the spread of a common fungal disease called oak wilt.
As a general rule, a light summer pruning can be performed on most deciduous trees and shrubs. Heavier pruning should be performed when the tree is dormant, preferably in late winter before active growth begins. Trees such as maple trees bleed sap heavily and should be pruned in winter while the trees are dormant. Spring flowering shrubs such as lilac and forsythia bloom on the previous season’s growth and should be pruned within two weeks after flowering. Pruning at any other time will reduce or eliminate the flower display.
Most conifers require minimal pruning that will also vary with species. Needled evergreens such as spruce and fir are best pruned in late winter before growth begins. Arborvitae and yew can be pruned during spring and early summer. Pruning of any kind can encourage new growth; therefore, pruning should not be performed late in the season to avoid the risk of cold temperatures damaging tender new growth.