Landscape timbers are, in many ways, the perfect material for building a border around a playground. They're bulky enough to create a formidable retaining wall that can hold playground mulch in place, long-lasting enough to be more or less-hassle free, and they add a natural look that plastic or rubber just can't reproduce. The trick to building a safe and pleasant play area with landscape timbers is doing the installation correctly.
By taking a few extra steps to make sure the border is stable and has access to proper drainage, you'll greatly increase the longevity of the structure and thus save yourself the headache of redoing the project a couple of years later.
Mark out the area that you intend to turn into a play area. Use spray chalk or paint to make temporary guide lines for yourself.
Use the shovel to dig a shallow trough around the perimeter of the play area. The trough should be slightly wider than the width of your landscaping timbers, and just 3 to 4 inches deep.
Fill the trough with your pebble rock or gravel. Tamp down slightly and make sure that it is level.
Drill holes through the landscaping timbers at either end of each piece.
Place the landscaping timbers on top of the gravel-filled trough. Wiggle them a bit to allow the weight of the timbers to settle into place.
Place the lengths of rebar through each pre-drilled hole. Use a mallet to drive them down into the ground until the top end of the rebar is flush with the landscaping timber.
Fill the interior of the play area with a suitable safety mulch or other material.