While bearded iris are relatively durable and easy to maintain, there are a few things you can do to get them off to a good start. Here are some tips for planting iris.
These flowers' popularity is explained in part by their being widely adaptable and easy to grow throughout most of North America. The only exception is the humid subtropics. Generally, iris thrive in USDA Hardiness Zones 3 through 10 in the dry-summer West and in 3 through 8 in the rainy-summer East.
Most iris need at least 6 hours of full sun, and good drainage. In deeply cultivated, rich soil, they respond with superior growth and flowers. A soil pH near neutral is best. Sand or clay soils are okay, but you need to pay more attention to drainage in heavy clay soils. If water in your garden pools for more than half an hour after a rain, plant in raised beds or mounds.
In cool maritime regions like Seattle and Boston, day-long full sun is required, but in hot regions, such as Phoenix or Dallas, locate plants so they receive afternoon shade.
Whether you buy plants by mail or are the beneficiary of a generous neighbor, what you put into the ground looks the same: a 1/2-inch-thick, 2- to 3-inch-long fleshy rhizome with short, spaghetti-like roots spreading from its sides, and a short fan of leaves at one end. The rhizomes are tough enough to stay out of the ground for a week or two without serious harm, but the sooner they are planted, the better.
Before planting, amend the soil with generous amounts of compost. If your soil tends to be acidic, add ground dolomitic limestone to raise the pH to nearly neutral. (Use a soil test to determine your soil's pH and the type and amounts of amendments necessary.)
Dig shallow hole large enough to accommodate the rhizome and the attached fibrous roots, then create a mound in the center. Place the rhizome on the mound, adjusting the height of the mound so that the top of the rhizome will be at the soil surface and spreading the roots down the sides of the mound. In sandy soils, the rhizome can be covered by 1/2 inch of soil. In clay soils, the top surface of the rhizome should be exposed.
Space rhizomes 1 to 2 feet apart, or closer for a more immediate effect and for dwarf varieties. However, close planting usually means you'll need to divide them sooner. Because new plants will emerge from the fan end and sides of the rhizome, orient the leaf end in the direction you want to growth to take.
If you're planting several iris of the same variety, arrange them in drifts with the fans all pointing in the same direction, or plant in a circular clump with fans pointing outward.
Water the newly-planted rhizomes to settle the soil and start growth, then water sparingly until new growth indicates that roots are growing.