Composite lumber does not appear to leach anything toxic into the soil it surrounds when using it in a container bed. The plastics used in composite lumber, usually HDPE or recycled materials are non-leaching, making it a suitable option for edging container garden beds, notes George Weigel, a certified horticulturist, on PennLive.com. Other benefits to using composite lumber in container beds, as opposed to wood, is that it does not rot, lasts indefinitely and costs less over its lifetime.
Till a 4-by-4-foot section of garden area to 8 inches deep. Remove all rocks and weeds from the area. Rake the area level.
Dig a trench with a shovel, 1 to 2 inches deep, around the 4-foot section of tilled garden area.
Place one piece of composite lumber beam on each side of the garden area. Position each piece in the trench so it rests upright.
Connect each end of each piece of composite lumber to the end of the nearest piece with a connecting joint, usually secured by placing the base of the hinge where the edges meet and screwing the sides of the hinge with a drill into the composite piece.
Fill the bed with a mixture of one-third compost, one-third peat moss and one-third vermiculite. Rake level before planting.