The easiest and best way to combine plants is to choose a "filler" plant, a "spiller" plant, and a "thriller" plant. A filler plant which mounds to form a bridge between the spiller and thriller plants. One common filler plant is geraniums or begonias. A spiller falls down the sides of the planter to create the look of luxury. Common spillers include English ivy and Creeping Jenny. Thrillers are tall plants that add interest to the container with bold color or style. Ornamental grasses are stunning thriller plants.