Hardwood trees and shrubs without showy flowers: prune in the dormant season to easily visualize the structure of the tree, to maximize wound closure in the growing season after pruning, to reduce the chance of transmitting disease, and to discourage excessive sap flow from wounds. Recent wounds and the chemical scents they emit can actually attract insects that spread tree disease. In particular, wounded elm wood is known to attract bark beetles that harbor spores of the Dutch elm disease fungus, and open wound on oaks are known to attract beetles that spread the oak wilt fungus. Take care to prune these trees during the correct time of year to prevent spread of these fatal diseases. The best time is during the late winter.
Flowering Trees & Shrubs
Flowering trees and shrubs can also be pruned during the dormant season for the same reasons stated above. However, to preserve the current year's flower crop, prune according to the following schedule:
Trees and shrubs that flower in early spring (Redbud, Crabapple, etc.) should be pruned immediately after flowering (flower buds arise the year before they open, and will form on the new growth).
Trees and shrubs that flower in the summer or fall always should be pruned during the dormant season (flowers buds will form on new twigs during the next growing season, and the flowers will open normally).
Dead branches can be removed any time of the year.