I have done the plastic edge or border only to have it pull up over time or have the turf settle exposing too much of the lip. My new neighbor is a landscaper and told me to just cut a trench with a spade (square). I have to say I think this looks the best. The trench I cut is 4 deep X 3 wide inches approx., I drive spade down then move over 3 inches drive spade at an angle back to my original cut. The mulch will fill in and does not move past. Over time you will have to clean up trench, I just use a trimmer or you can use a bed definer.