how deep should I dig for a wood garden border
3. Dig a trench at the edge of your line: The trench should be as deep as your plastic lawn edging PLUS 2.5 cm. For example, if you are using 4-inch deep lawn edging, the trench will be 5 inches deep on the grass side.ips. 1  The purpose of plastic lawn edging is to keep the grass from growing into your flower beds. 2  You can mark the line you want to cut by laying your garden hose in the shape you want and leaving it there for a few hours.