I have a driveway about 250-300 feet long. One side is a hill of medium steepness, with about 3 dogwoods among other things. Those trees are in a bed on that hill, the bed is about 8 feet wide and extends the length of the entire driveway. The previous owner (from what I see on google maps satellite view) previously had red mulch under the trees. Now that is gone and the roots are so exposed, not only does it look bad it looks quite unhealthy. I'd rather plant some good groundcover under there like maybe pachysandra. Because any mulch is going to slide off in the rain as it apparently already did easily because there is no trace of it. My questions are: how deep can I put soil/compost etc on those roots in order to plant some groundcover plants? Wouldn't want to smother them and wouldn't want to dig down into the root system (clay soil) and damage roots. Thanks!