Hydrangea shrubs thrive in climates of USDA Zone 6 and above. This makes them ideal landscape plants for nearly all regions of California, except for very small areas on the northeast tip of the state bordering Nevada. Hydrangeas are deciduous and benefit from annual pruning to encourage new growth, to remove damage and to control the size and shape of the shrub when needed. Annual pruning for hydrangea shrubs growing in California should generally be conducted in the fall or early winter once all of the flower heads have died back or when they begin to look shabby in the landscape.
Remove any dead flower heads down to the parent stem, being the green or wood stem from which the flower stem emanates. Place all cuts on the bias with clean, sharp pruning shears. Cut back any dead, diseased, damaged or otherwise compromised branches and foliage, discarding them.
Prune away a third of the oldest branches each year to spur production of new flowering branches. Distribute the pruning cuts evenly throughout the shrub to maintain the natural architecture of the hydrangea. Place cut down to the main trunk or to the ground, depending on your species of hydrangea.
Reduce the size or alter the shape of the shrub if desired and as needed, by cutting back the branch tips to the desired length. Prune macrophylla and oakleaf hydrangea species for size in the late summer immediately after bloom. Prune panicle, climbing and smooth leaf hydrangea species for size in the fall or winter.