The optimum time to heavily trim a yew for size maintenance is in late winter or early spring before the shrub begins new growth. Follow-up trimming is done from mid-June until August. Avoid autumn shearing because it stimulates late-season growth, making the yew susceptible to winter injury.
Unlike most evergreens, yews develop new growth on old wood, making them very tolerant of heavy shearing or pruning. Disinfect tools with rubbing alcohol to avoid spreading disease or fungus among plants, and wear gloves to avoid exposure to the toxins in yew bark and foliage. Use pruning shears for branches with a diameter of less than 1/2 inch, loppers for 1/2-inch to 2-inch diameter branches and a pruning saw for branches with a diameter of 2 1/2 to 3 inches. Selectively remove thicker branches and ones that grow at sharp angles or cross through the shrub. Make pruning cuts at a 45-degree angle to help the pruning wounds shed water.
After pruning, water the yew well, and continue to give the plant at least an inch of water each week until new growth begins to emerge from the old bark. Apply a slow-release fertilizer to stimulate new growth, and add a 2- to 3-inch layer of mulch around the base of the yew to retain moisture.
When new growth slows in mid-summer, trim back any remaining short stubs of woody stem to within 1/2 inch above the base of the new growth. Shape the new growth by shearing off the top few inches with a hedge trimmer. Round off the top to decrease winter snow loads, and keep the shrub narrower on top than on the bottom, allowing sunlight to stimulate thicker growth on the lower parts of the yew.