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Spring-Flowering Trees and Shrubs
Early-spring bloomers, such as lilac, forsythia, and rhododendron, bear flowers on wood formed the previous year. The best time to prune them is late spring -- immediately after they finish blooming. If you prune them later in the growing season or during winter, you'll remove flower buds and decrease the amount of spring bloom.
Test Garden Tip: To keep spring bloomers flowering vigorously, remove some of the oldest shoots all the way to the ground. This allows younger stems to grow and bloom.
Summer-Blooming Trees and Shrubs
Plants that bloom in summer, such as potentilla, butterfly bush, and crape myrtle, produce their flowers on new growth from the current season. Prune them in winter while they're dormant, or in early spring just before they push out their new growth. You can even cut them all the way to the ground in late winter, and they'll still bloom that same summer.
Test Garden Tip: Save time by using a pole pruner with a rotating head to remove stems all the way to the base of the plant. That way, you don't have to bend over for each cut, saving not only time, but also wear and tear on your back!
Deciduous Fruit Trees
Apples (including crabapples), peaches, pears, plums, and cherries should be pruned in midwinter. Although winter pruning removes some of their flower buds, the goal in pruning fruit trees is to open up the tree to allow in more light for a better crop of fruit, rather than to get maximum bloom.
Dormant pruning is especially important for apples, pears, and crabapples because pruning wounds during the growing season expose the trees to a bacterial disease called fireblight.
Test Garden Tip: To control the spread of diseases while pruning, dip your pruning shears in rubbing alcohol or a solution of one part bleach to nine parts water.
Original article and pictures take http://www.bhg.com/gardening/trees-shrubs-vines/care/what-to-prune-when/ site