We have built a 1.5ft wall at rear of garden behind which are a cotoneaster, 2 mt ash & 1 azara. Will it suffice to put pipes of some kind around trunks to protect from soil & water or are we going to kill the trees over time by putting 1.5ft of topsoil over existing roots in this new raised bed? Would it be better to try to replant the trees higher in the autumn?