Q. In what month should I severely cut back my yew shrub hedges?
Fear not! Yew has a good supply of dormant growing points and reacts well to regular and even drastic pruning.
You can give an established yew hedge a good shearing just before growth begins in the late winter. Yews will begin to grow at about the time that nights stop dipping below the freezing mark and the ground begins to warm up. That can be any time from April into early May in our area, so there isn't a set right time to trim, but March should be fine. Your pruning will take off many of the brown buds that were formed last growing season but will stimulate the growth of the smaller dormant buds all over the branches.
While you can do your cut back during the winter as well, by waiting you increase the rate of healing, reduce the delay in stimulating the new buds and your yew will revive more quickly. You can shear more gently to keep the shape one or two more times during the growing season if you wish.
If your yews have not been trimmed properly for some time, it can be more difficult to reestablish a good growing habit. Some plants simply may not grow back into the shape you desire. Try to keep your hedge trained to a gentle "A" shape so that the lower branches get plenty of sun and don't die back.
Courtesy of NYBG Plant Information