PRUNING DECIDUOUS FLOWERING SHRUBS
How and when you prune deciduous flowering shrubs influences the size and quantity of blossoms, or whether they flower at all. The key is to observe when it flowers, and whether blossoms develop on the older twigs or newly grown ones.
Spring flowering shrubs blossom on twigs that matured the year before. These shrubs are pruned at the end of their flowering season, to encourage more abundant summer growth that will support next year’s crop of flowers. Examples include; Bridalwreath, Forsythia, and Lilac.
Summer flowering shrubs blossom on new twigs grown in the spring. These plants are pruned in winter while dormant, which encourages a flush of new stems in spring. These in turn bear more abundant flowers in mid to late summer. Examples include; Spirea, Potentilla, Itea, and Weigela.