I am trying to rejuvenate my deceased father's cotoneaster hedge. There was a lot of dead branches and the live ones had little growth on the main stalks with a few spindley side branches. Where I had evened the top of the hedge it has bulked up a bit but the lower part of the hedge is thin and sparse. I would like to prune it back a few times this year to stimulate the thickening process. It is May 16th. How soon can I tackle the pruning and how many times in the season can I cut the hedge back?