What trees and shrubs need spring pruning?
Prune most deciduous trees and needle-leaf evergreens in the late winter or early spring when they are dormant and before new growth begins. Certain deciduous trees such as beech, birch, dogwood, elm, maple and sycamore will bleed excessively if pruned at this time, wait until summer to prune them.
Early-spring flowering shrubs like Azalea, Forsythia, Rhododendron and Spirea can be pruned just after flowering.
Summer-flowering shrubs that bloom such as herbaceas Hydrangea don't need pruning, in fact pruning will remove buds of flowers, but H. paniculata (tree form) form flowers on new woody growth, so can be pruned a lot. Pruning butterfly bush (Buddleja davidii) to within one foot of the ground annually enhances the flowering. This should be done before new growth begins in the spring since they flower on new growth. Do not prune in fall or winter, may force new growth, which increases the risk of cold damage on butterfly bush.
Prune evergreen hedges such as Buxus, Yew and Tsuga in the early spring, just before new growth begins.
Finally, prune all roses in early spring except climbers and ramblers; they need pruning only after they flower.
More here: http://www.clemson.edu/extension/hgic/plants/landscape/shrubs/hgic1084.html
Courtesy of the NYBG Plant Information Service