River rock offers an alternative to the use of mulch in a garden. Strategically placed, the rock—like mulch—helps retain moisture. River rock, however, has very smooth surfaces with no edges, without which the rock may quickly spread out if not retained. The use of a retaining wall will work, but it requires a substantial investment of time and money. A simple, black plastic landscape edging works well to hold river rocks in place.
Till the soil around the perimeter of the target area to a depth of at least 6 inches. The width of the tilled area is not a factor since the edging is very thin. A small flowerbed tiller works best and disturbs less topsoil. If you do not have a tiller, use a spade to turn up the soil and a hoe to break clods.
Place connectors that accompany the edging into the rounded lip on the top of the edging if more than 20 feet of edging is required. Quality edging comes in 20-foot sections. Insert the connector only halfway into the first piece of edging to ensure a sufficient amount of connector to fit into the adjoining piece of edging.
Insert the edging into the soil to a depth of 5.5 inches with the V groove in the edging facing the garden—not the lawn. Leave only one-third to one-half of the rounded lip visible above the surface of the ground.
Tamp the soil inside the rock garden. Ensure the surface of the soil inside the edging is lower than the surface outside the edging. This will ensure the trim holds the river rock in place.