The simplest and most subtle landscape borders that effectively separate your lawn from a garden are 4-in.deep strips of steel, aluminum or plastic.
The metal lawn edging bends easily into smooth, graceful curves and stops the spread of grass roots. However, painted aluminum and steel offer the sleekest, most refined garden edging look because they almost disappear against the grass and garden bed.
The plastic types of garden edging have a prominent black bulge along the top edge.
All work best on fairly even terrain. If you have a lot of dips and rises, it’s easier to install a paver border.
Additionally, you may want to consider flower bed fencing, if there are pests you need to keep out.