Minecraft wiki entry for Tree_farming, associated text: It is recommended that the perimeter walkway and all blocks with a torch underneath be a different material, such as cobblestone. This allows for quick visual identification during re-planting, of which blocks get saplings and which get torches that may have been inadvertently knocked out during harvest. It is recommended to do the same for torches on the wall, as these may get knocked off by growing trees. If you dig down two blocks instead, and place the torches under glass blocks, the trees still receive the proper light level, and you are far less likely to inadvertently break your light structure during harvesting or replanting.