I've bought 20 cedar hedges to plant along my property line that is a 12"thick retaining Wall. but I'm second guessing the root system damaging the wall. The trench is 12" wide with one side of the trench actually being the retaining Wall. I only want these to be about 6 feet tall max. Once I trim the top will the root system not grow anymore? What do you think? Don't want to loose $350 tearing the trees up and cost of a new retaining Wall.