The late dormant season in our area—late winter, before bud break—is the optimal time of year to prune summer-blooming ornamental shrubs and trees, as well as shade trees. The second best time is the middle of summer. Do not wait until the spring flush of growth to prune, and don’t prune in autumn. A rule of thumb to follow is not to prune when leaves are forming or falling. Of course, if you have a branch that is dead, diseased, or dangerous, it may be removed at any time. Hand pruning shears, loppers, or saws are recommended for pruning; hedge shears are not.
Late winter is not the time to prune spring-blooming woody plants, because the flower buds of spring-bloomers, like rhododendrons, azaleas (both of which normally require little pruning), and lilacs, are set in the previous growing season; in other words, these plants bloom on old wood. The better time to prune them is directly after they finish blooming, if you want to preserve this year’s show. However, it is desirable to winter-prune spring-bloomers that have become straggly through long neglect. Cut long branches to encourage new bud development early in the next growing season. If such pruning is postponed until summer, dormant buds already present may remain so until the following spring when several will break near each pruning wound. In the interim, the plant will appear unattractive.
To thin out most deciduous shrubs and trees, cut off a branch or twig at its point of origin from the parent stem, or to a lateral side branch, to the Y of a branch junction, or at ground level. Never simply shear the ends of branches off, and never top trees. To rejuvenate an old, overgrown shrub, remove one-third of the oldest, tallest branches at or slightly above ground level.
Proper pruning practices will preserve the natural shape of the plant, will protect it from many pathogens, and will promote vigorous growth of the plant.