Elegantly beautiful, irises are hardy perennial plants that start, not with a bulb but a rhizome, which is a thick stem from which roots grow. Irises are easy to grow in zones 3 to 10. The ideal time to plant irises is mid to late summer, preferably the first part of August.
The best locations are in areas that receive a lot of sunlight and have good drainage. To determine the depth of the hole, estimate the thickness of the rhizome and length of the shorter roots. The hole should be the sum of those two measurements plus another 1 to 1.5 inch. Dig the hole as deep as your calculations indicate and wide enough to accommodate the rhizome and roots comfortably. In the center of the hole, build a dirt mound that is as tall as the length of the shorter roots. Place the rhizome on top of the mound and spread the roots down the outside of the mound. Fill the hole with soil, covering the rhizome so it is about 1 to 1.5 inches below the surface. Water thoroughly.
To Cover or Not
Generally it is a good idea to plant your iris rhizome with an inch or so of soil coverage. This is particularly true in colder climates that experience deep freezes. In mild, warmer areas, or if you must plant in heavier clay soil, it is recommended that the rhizome be left partially exposed on the surface. There still needs to be a shallow hole dug, with a mound inside on which you can drape the roots to help them get established. Cover the roots with soil. Planting too deep in these conditions may cause the rhizome to rot.