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Mar 25, Hydrangea paniculata, panicle hydrangea. Prune back stems to just above a fat bud - called a heading cut - in fall, late winter or spring. These plants have conical-shaped flower heads. I recommend leaving the dry, tan flower heads on the plant to provide some winter interest in your landscape, so I wait to prune these until late winter or spring. If those buds are killed or damaged over the winter, the hydrangea’s flowering potential is reduced (although lower buds along the stem can develop flowers too).
Wait to prune your bigleaf hydrangeas until new growth appears in the spring. Make pruning cuts one quarter inch above the first set of live buds. Aug 18, Smooth and panicle hydrangea (blooms on new wood): Prune in fall or winter, when plants are dormant (before new growth begins). Thin out excessive growth or prune the whole plant back to the ground. Heavy early pruning will produce smaller plants with larger blooms, not pruning will produce larger.
The Limelight hydrangea blooms on new growth, so it should be pruned in late winter, before new growth begins. Although the florets look similar to those of bigleaf hydrangeas, Limelight has teardrop-shaped bloom clusters and blooming starts later in the summer. For more on this topic, read: Early Season Pruning If it's blue, or blooms in summer.