Many people feel rather intimidated at the idea of pruning their flowering shrubs. In addition to not knowing where to cut, they often worry that it might be the wrong time to cut and they’ll lose all the flowers. In general, if it blooms before June 15, wait until the shrub flowers and the blooms fade. If it blooms after June 15, prune while it is dormant or in very early spring before the buds start to show.
As far as where to cut, if the shrub is multi-stemmed with canes that emerge from the ground, it should never be chopped or sheared at the ends of the branches.
Rather, up to one third of the oldest, largest canes should be cut back to the ground. Shrubs such as Red Twig Dogwoods and Forsythia should be pruned in this fashion, which is called renewal pruning. This will prevent them from developing unsightly ‘witches brooms’ at branch ends and will allow their natural, usually arched, growth habit to emerge.
The other method of pruning is for single-stemmed shrubs and is called ‘heading back.’ While any dead or unsightly branches can be cut out, the branches that should be focused on for chopping are those that point towards the center of the shrub (where they won’t get much light anyway) and any that cross or rub against another branch. The branch or twig should be cut back either to an outward-pointing side branch or node or all the way back to the main trunk. As with renewal pruning, this will also prevent ‘witches brooms’ from developing.
Sheared and shaped hedges, such as boxwood and privet, offer other pruning issues. They should be pruned so that they are wider at the bottom than the top so that sunlight can reach the lower foliage.
If pruned straight up and down or too wide at the top the lower leaves will gradually drop and the small branches at the bottom will die. Each time you shear a hedge, leave about ½ to 1 inch of previous growth. New growth will emerge at or near the cuts and the shrub needs this new growth to keep the plant healthy.