Most landscape trees and shrubs can be pruned in late winter or early spring while they are dormant. Exceptions are trees that produce sap, such as maple and birch, and some flowering shrubs that bloom on the last season's growth.
Flowering shrubs that should not be pruned until after they have flowered include forsythia, lilac, viburnum and spirea. Evergreens generally have a much longer pruning season than deciduous trees and can be pruned even after new growth has hardened in late summer.
Never climb or work near power lines to prune trees; hire a professional. Local USDA Extension agents can provide recommendations.