I have a 20 year old cedar hedge that has been very healthy and vigourous until the last year or so. It has brown spots on it I suspect from truck exhaust but also it has started to show bare holes between the trees. I have discovered that the hedge branches have an accumulation of dead leaves throughout the hedge and at the bottom. I didn't know I should have been clearing out the dead leaves so am now worried that the hedge will not regrow into its healthy green state it had before. Is there anything I can do to remedy this?