Healthy, well-maintained shrubs should require only light to moderate pruning.
Anytime, Renewal pruning in spring: Elderberry: Early spring, Severely prune: Forsythia: After bloom in spring: F. Bronx & Citrus Swizzle: Anytime: Fothergilla: Prune after flowering to shape: Hazelnut: Early spring, Renewal prune: Honeysuckle. Spring-flowering shrubs: Spring-flowering shrubs produce flower buds on one-year-old wood (wood produced the preceding summer).
May-summer is best.
Therefore, you should prune these shrubs AFTER they have flowered in spring, but before the next year’s flower buds are set. If you prune these shrubs in winter or early spring, you will remove many of the flower buds. Aug 13, Late fall/winter pruning is especially important for oak trees to help prevent spread of the fungus that causes oak wilt (see University of Wisconsin Garden Facts XHT). Late spring and summer are usually not good times of year to prune because disease pathogens are present and wound closure is slower.
Deciduous Trees February through March is generally regarded as the best time to prune most deciduous trees.
Evergreen Shrubs. Prune evergreen shrubs, such as juniper and yew, in late March or early April before new growth begins. Light pruning may also be done in mid-summer. Avoid pruning evergreen shrubs in the fall. Fall pruned evergreens are more susceptible to winter injury. Deciduous Trees.