Viburnum shrubs tend to be inherently well-shaped and pleasantly sized without the need for much pruning at all. Both of the two different flowering forms of viburnum may be pruned, and it's done in the same manner for either one. The snowball-like, sterile white flower clusters occur on Japanese snowball viburnum, while the more lacy, ornate fertile white flowers surrounded by the white sterile blossoms occur on double-file viburnum. Tip-prune immediately after the flowering display to mildly shape shrubs if needed, but don't prune past mid-summer, as, at that point, the next year's flower buds are already formed.
Shorten any diseased or dead branches on the viburnum by making conservative pruning cuts with the hand pruner, 1/4 inch above a lower branch or dormant bud. Make crisp, even, one-motion snips of the pruning blades. Prune dead and diseased branches any time of year they are encountered.
Shape the overall shrub by reducing the length of errant branches to make the plant look uniform and attractive. Make pruning cuts 1/4 inch above a lower branch junction, leaf pair or dormant buds. Make these casual tip pruning cuts immediately after the springtime flower display ends. Michael Dirr, professor of horticulture at the University of Georgia, believes this is the best time to conduct any pruning on a viburnum plant.
Rejuvenate old, tired-looking Japanese snowball or double-file viburnum shrubs in late winter by cutting back all branches to a height of 12 to 18 inches above the ground. Employ this technique only on shrubs that are lopsided in shape and have lost vigor. Allow the new branches to sprout and grow the entire growing season, and then abundantly flower the next spring. Do not prune again for 10 years, if ever.