Answer: I'm afraid the only way to stop the vertical growth of your holly is to cut the main leader. This will bush out your holly a bit and keep it shorter. Prune all the other side leaders at the same time to keep the shape you like. Viburnums are best pruned after flowering in early summer. I'd only do structural purning to remove dead, diseased or crossing branches or to open up the shrub to more light. They don't require heavy pruning unless you are trying to keep them very small. They look best however, with their natural shape.
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