Answer: Yews are slow growing plants and they put out much of their new growth energy in the springtime. So, to take advantage of that, do your pruning in the spring, as soon as you notice new growth on the plant. To maintain the health of your yew (and most other landscape plants), don't remove more than about one-third of the live material in any one pruning session. If your shrubs are quite overgrown, you'll want to reduce their size over a 2-3 year period.
Best wishes with your landscape.
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