Perennial plants are a gardening favorite—and with good reason. These plants come back year after year, sometimes for three years and often for 10 or even 20 years. That means you plant one and enjoy over and over again. Annual flowers, on the other hand, complete their growing cycle in one season and then die. A few popular perennial plants are daylilies, lilies, hostas, iris and coneflowers. When buying perennials for your garden, you have lots of choices. Some factors to consider when buying perennial plants for sale are hardiness zone, sunlight requirements, soil requirements, mature height and spread, foliage or flower color, bloom time and maintenance needs.