The dormant season is an excellent time to prune many deciduous shrubs. Some shrubs such as lilac, weigela, forsythia, spirea and viburnum and flowering quince are planted for their wonderful flowers, so it is important to know when the shrub blooms before deciding what time of year is best for pruning.
When to prune
For blooming shrubs, keep this rule of thumb in mind — it is best to prune a blooming shrub right after it has finished blooming. So spring bloomers like forsythia and weigela can be pruned after blooming in spring, and summer bloomers like viburnum and spirea are best pruned after blooming in the summer.
It also important to know the goal of the pruning — size reduction, rejuvenation or renovation.
Size reduction or heading back cuts are used to give a plant shape or prevent it from overgrowing its bounds. Just keep a few things in mind. Hand pruning, although it takes more time, will result in a healthier, more attractive plant with more foliage. Plants that are repeatedly cut with hedge trimmers, eventually develop a very shallow layer of foliage. When hand pruning, cut back branches to shoots that are at least one-third the diameter of the branch being removed. This minimizes suckering and helps prevent fungal rots from starting in the wound.
Rejuvenation pruning is a great way to keep shrubs healthy and growing vigorously. It involves completely removing the old, woody shoots. This is easiest to do if started when the plant is only 2 or 3 years old. Each year, simply prune out any shoots that have gotten thick and woody. This forces the plant to continue to send out new, vigorous shoots to replace those lost and makes the plant much less susceptible to attack by insect borers.