Sep 21, Prune viburnum bushes in the spring as soon as the last flowers fade.
Snowball bushes bloom on old wood or last season's growth. Pruning any time past late summer will risk removing precious bloom buds. Apr 10, If you're training your Snowball Viburnum Shrub to provide a lot of blooms, prune it back in spring after it has begun blooming to encourage more growth.
On the other hand, if the branches have become too long and you're more concerned with shaping a mature bush, cut back one-third of the canes in fall to thin out the base. Depending on the species or cultivar, snowball plants can bloom as early as March and into June, so you'll have to deadhead the plant throughout the blooming period.