Hi and Welcome to the group,
Our local tree planting group put newspaper down then sacking on top, tucked in round the edges with a spade.
Sacking came from a packaging mill, comes from sacks coffee etc was imported in (smelled(sp?) lovely). that may give you some ideas anyway, even If you can't get the sacking. Maybe grass mowings on top of the newspaper? local people give the group newspapers when there is a planting session in the area.