I'll assume you are talking about the popular bearded iris, which bloom in early spring. Plant so the rhizomes (horizontal stems) are just below the soil surface. In very sandy soil, the rhizomes may be planted a little deeper, but no more than 1 inch below the surface. Improve the soil before replanting, working in a generous amount of organic matter such as rotted manure or compost. Keep the soil moist, but not wet, until the ground freezes. The best time to plant or transplant bearded iris is late summer, from about early August to mid-September. Siberian and Japanese iris, cousins of the bearded iris, grow from fibrous roots instead of rhizomes. These iris are best divided in early spring, but they can also be divided in early fall. Plant the newly divided crowns at the same depth they were growing at originally and water well.