Plant your focal plant in the center of your planter. The focal plant should be the tallest or largest plant in the planter and ideally reach a height of 18 to 24 inches.
Plant your filler plants surrounding the focal plant. The filler plants should be grouped according to size, taller next to the focal plant to shorter closer to the edge of the planter. Filler plants can be flowering plants or plants with colorful foliage, like coleus, or both. It is important that, as you group these plants, you vary the leaf textures, leaf size and flowers for a more interesting display.
Add plants that spill over the side of the planter. Good plants for this are petunias, baby's breath, sweet alyssum and blue lobelia. Spillover plants are use to tie and group all the plants together, fill in any bare spots within your grouping of plants and add visual appeal to the entire planter.