The dormant season is an excellent time to prune many deciduous shrubs. Some shrubs like 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 repeated cut with hedge trimmers, eventually develop a very shallow layer of foliage. When hand pruning, cut back branches to shoots that are at least 1/3 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 2-3 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.
Rejuvenation helps control plant height and since young shoots bloom better than old ones, it also ensures the greatest flowering each year.
When working with an older plant that has not been on a regular pruning schedule, plan to prune the plant gradually over a three year period. The first year start by pruning out 1/3 of the oldest, woodiest stems. Repeat the process for the next two years. At the end of three years the plant will be slightly shorter, fuller and have more flowers.
Renovation of a shrub involves cutting down the entire shrub to within several inches of the soil. Renovation removes all the old, woody branches that are susceptible to boring insect damage. It also removes any dead or diseased branches. Renovation allows the plant to re-grow in its natural form, and forces the plant to send out new, vigorous growth that is often healthier than the older growth that was removed.
However, renovation usually results in a lack of heavy flowering for a season while the plant re-grows. Renovation pruning can be done during the dormant period. Some shrubs, like evergreens, will not tolerate renovation pruning, so be sure to do a little research before cutting!