The "urbanite" material we used for the bed walls is a made from broken pieces of recycled sidewalks & driveways.  After forming the walls, we filled the beds with organic compost & intensively planted many varieties of fruit trees, pigeon peas, perennial vegetables, gingers & sweet potatoes as a summertime cover crop, or living mulch to keep the soil moist & cool.  In the cooler winter months, we interplanted brassicas, salad greens, root crops & nastursiums.