Of course, you won't always want to employ contrast in your plantings....

By employing repetition -- i.e., installing plants that bear the same form or other characteristic -- you can help achieve unity in your design. Perhaps the ultimate example is the classic hedge (composed of one plant), as in the photo above of a privet hedge. Formal hedges can have such a unified look that they essentially function as "living walls" in a landscape design. The formal privet hedge in the picture above serves double-duty, disguising an eyesore: the chain-link fence.