The evergreen Japanese yew (Taxus cuspidata) is a species of varied cultivars. Capitata, for example, reaches 40 feet, making it hard for you to prune a mature specimen. Densa, on the other hand, reaches a maximum height of 4 feet, and Nana stops growing at 15 feet. Unlike other evergreens, yews recover from hard pruning well, though their regrowth is slow. The ideal time for you to prune yours depends on your goal.
Japanese yews are susceptible to fungal diseases, such as sooty mold. Black vine weevils can also infest the plants. Besides, as the yew ages, stems die, while wind, snow and ice cause breakage. In case of disease, death and injury, trim the affected parts as you notice the problem, regardless of the season.
Japanese yews produce a fresh growth of leaves in spring and another halfway through summer. If you want to control the shape of your plants, trim them in early summer after the foliage becomes dark green. Repeat in mid-August. Cut the stems ¼ inch above a bud, at a 45-degree angle. This trimming is not necessary to maintain healthy yews; it is done for aesthetic reasons.
If you do not want your Japanese yew "Capitata" to get too tall, prune its top whenever new growth develops. Take care to maintain the pyramidal shape of this plant by positioning your shears at a slant. When trimming new evergreen wood, cut two-thirds of it, says the University of Wisconsin Cooperative Extension.
Sometimes, plant pathogens survive without a host. But as soon as they have the chance, they transfer to your live yew and interfere with its development. Thus, it is a good idea to sterilize your shears' blades before you cut into the wood. Fill a bucket with 9 parts water and 1 part bleach. Dip the blades in the liquid mix before you start trimming and after every cut. Alternatively, wipe the tool with a cloth moistened with alcohol.