Perennials are plants that come back every years. In the case of garden flowers, the term refers to plants that appear to die with the first frost of autumn, but will either re-grow from seeds that are dropped on the ground, or grow back from the root each spring. Although perennials are often purchased from greenhouses or nurseries, it's easy, and much less expensive, to grow them from seed. Unlike annuals, perennials don't have to be replaced every year, and some varieties may last for decades.
Decide what perennials you're interested in planting, and research the needs of those particular plants. Some perennials can survive extremes in temperatures, while others require a mild climate. Some thrive in arid climates, while others do better in rainy, damp climates. Some perennials won't bloom without full sunlight, some prefer shade. The catalog listing or seed packet will include this information, or you can check the National Gardening Association website.
Choose a well-drained area. Don't choose a boggy area, or a space where rain puddles. Remove any weeds or grass with a hoe, then prepare the soil. Work in 4 to 6 inches of peat moss and organic matter such as compost or manure to enrich the soil and promote drainage.
Sow the seeds in rows, but check the seed packet for spacing instructions. Plant fairly generously; once the seedlings emerge, you can thin the weakest plants.
Water the seedlings immediately after planting, and don't let the soil dry out. Once the seedlings are established, most perennials require less water and can withstand drier conditions.
Spread 1 or 2 inches of mulch around the plants once they're established, but be careful not to pile too much on the plants. Mulch will help keep weeds down and moisture in.