It depends on when it blooms. Prune spring bloomers, like lilac, weigela and viburnum, right after the flowers fade, because they set next year's buds soon after. Prune summer bloomers, like potentilla, spirea, rose of Sharon, bluebeard and Russian sage, in late winter or early spring, before new growth starts. Panicle hydrangeas get a late-winter trim, and bigleaf hydrangeas only a light one right after flowering.