Plastic Landscape Rocks from Topp Industries are constructed out of weather-resistant polyethylene resin. The standard bottom opening of 91⁄2-inches ID can easily be increased to 18 inches ID at the site. The molded-in weight tray prevents movement, and fill material such as brick, sand, concrete, dirt and gravel can be used. Available in different styles and colors, these landscape rocks also have optional plastic vents and hold-down stakes. Topp Industries Inc.