Most viburnums don’t need hard pruning and it can ruin their shape. However, when plants have been neglected they are likely to grow out of shape and their old wood will flower less.
When this happens, simply remove the oldest stems over a three to four year period, so the plant maintains its shape and continues to flower on the younger growth.
By the end of the process you will be left with a healthy, flowering and shapely tree made up of new wood.