Shrubs need some pruning every few years. Pruning promotes a more open plant with abundant flowering. Yearly pruning of some flowering shrubs promotes more flowers.
Renewal pruning is removing one-third of the oldest stems by cutting them off as low to the ground as possible. This is easiest to do in the winter months. Do not remove too many stems or the shrub will become leggy and the remaining growth will fall over.
Sometimes, a shrub needs to be pruned to reduce the size. Spring blooming shrubs like lilac and forsythia can be pruned just after blooming. Pruning later in the season will result in the loss of flower buds for the next year.
Shrubs that bloom later in the summer, such as Bumald and blue-mist spirea and hydrangea need to be pruned in the early spring. These shrubs bloom from the current year’s growth.
Shearing is faster than hand pruning and gives a very formal appearance. However, shearing can lead to plant health problems. In leafy shrubs, shearing results in weak and unnatural growth. In evergreens, shearing can enhance spider mite and aphid infestations. Do not shear evergreens in the late fall or winter. The foliage will brown because of water loss from drying winds.
For more information, see the following Colorado State University Extension fact sheet(s).