I would ask at a local nursery about low growing perenials for your area, Then each spring you could plant around the edges some petunias or marigolds (petunias are usually in the pink/purple color range and marigolds tend to be yellow/gold. They both will bloom colorfully all summer as long as they get watered, are not susceptible to pests, and won't spread where you don't want them. Shrub roses like the Knockout type need almost no care, do not grow terribly large, and bloom all summer. There are groundcover varieties called Drift that only get about two feet high and 2 or 3 feet wide. Water twice a week or so, mulch thick when planted and your garden will be colorful and attractive.