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How do I prune my flowering shrubs?
Shrubs can be divided into three general groups in regards to pruning. The first group include those shrubs that require minimal regular pruning. This group includes azaleas, flowering dogwood, potentilla, barberry, magnolias, and most viburnums and lilacs. These shrubs develop a basic branch framework from which the plant flowers on year after year. All that needs to be removed is dead and diseased wood, crossing branches and suckers and watersprouts. These shrubs can be damaged by excessive pruning.
The second group is the spring flowering shrubs that each year can produce new shoots from the base of the plant. This group includes forsythia, mock orange, kerria, beauty bush, weigela, and spirea. They are best pruned as soon as they finish flowering. Since these shrubs will flower next spring on the new growth they make this summer, it is important to encourage the plant to make new growth after flowering. For the first four or five years after planting it is not necessary to prune these plants much, but after that they need annual pruning. Each spring after flowering, remove up to one third of the old stems at ground level. This will encourage new growth from the base of the plant and retain the plant's normal habit. Do NOT give these plants an annual "hair cut" as so many people do. It only destroys the natural look of the plant and does not help replace older branches with new younger wood which will flower better.
The third group is summer flowering shrubs that make rampant growth and flower on new growth made each spring. Shrubs of this group include most roses, buddleia, Pee Gee hydrangea, beauty berry and dogwoods grown for their ornamental bark. They are best pruned in early spring before they begin to leaf out. These plants form a basic framework of branches from which they will flower each year. It is important to prune these shrubs when they are young to develop a good plant form. Subsequent pruning is then restricted to removing dead and diseased wood, removing thin, weak branches, removing old branches when new younger shoots have developed to replace them, and then shortening the remaining branches by 25% to 75%, depending upon the size of plant or flowers desired. Generally more drastic pruning will produce fewer, but generally result in larger flowers.
Good pruning is both an art and a science. On your next visit to the Missouri Botanical Garden pay attention to how the shrubs are being pruned. Several good books with specific recommendations on how to prune various shrubs are available in the Center for Home Gardening.