Nearly all shrubs that bloom in very early spring do so on wood grown the previous summer. Therefore, the best time to prune shrubs like azaleas and rhododendrons in Minnesota or elsewhere is immediately after they finish blooming. If you prune them later in the season, you cut off their newly forming flower buds, eliminating flowers the following year. Depending on the species and variety of the shrub, prune them during April or early May in Minnesota.
Shrubs that bloom later in spring, such as lilac, bridal's wreath and forsythia, should also be pruned immediately after they finish blooming. Very few shrubs that bloom later in spring flower on the current years' wood. Accordingly, prune them right after their flowers fade, unless you wish to cultivate and harvest their fruits. In that case, prune them in late winter, but only to remove dead or diseased branches, and to remove older growth making room for newer, more fruitful branches to develop.
Although only a few varieties of shrubs bloom in summer, such as roses, hibiscus or hydrangeas, those that do bloom in summer produce their flowers on branches that grew during the current growing season. These shrubs are best pruned in late winter or very early spring. Ensure that you prune them before the sap begins to rise and before the buds begin to open.
Make the pruning cut just beyond the point where the branch grows out of its originating branch, also called the crotch of the branch. There is a slightly raised ridge encircling the branch near the crotch. Make the pruning cut just beyond that ridge. Pruning cuts made this way heal faster and do not require sealing with products specifically designed to seal pruning wounds.
Remove all dead or diseased branches, cutting back to live, healthy wood. Also remove one of any pair of branches that are crossing or growing too close together; remove the weaker of the two.
Whether you are pruning in early spring, summer or fall, never remove more than one-third of the shrub's branches, to ensure the remaining branches can manufacture enough food to keep the shrub alive.
Some shrubs are prolific growers, sending up "suckers" around their bases. These suckers can -- and should -- be cut off at ground level any time during the growing season. They sap energy from the shrub and will reduce the number of flowers the following year.