Tips for Pruning Your Hedge
Yew (Taxus species) can tolerate more severe pruning than most evergreens. You can rejuvenate or reduce the size of yew hedges by shearing twigs back into old wood.
Do not allow vigorous shrubs to grow to their final size before pruning them. To form a dense hedge, prune them as they grow. Although it may take years longer to achieve the desired height, the result is more satisfactory.
Article provided by the National Gardening Association