Iris rhizomes (rye-zomes) are a long, fleshy modified root. The rhizome is planted horizontally in the ground with the bumpier side up. Planting iris rhizomes can get complicated as there are several varieties all with different cultivation needs. Gardeners are most familiar with bearded and Siberian iris. Dutch and Japanese iris are gaining popularity for their tolerance of wet, boggy soils. All iris benefit from fertile soils containing a lot of organic matter. Siberian and bearded iris need well drained soils or they will rot. Dutch and Japanese iris will thrive in both boggy and well drained soils.
Prepare planting areas 2 to 3 weeks prior to planting iris rhizomes. You can plant all iris varieties in mid to late spring and some, like the Dutch and Japanese, can also be planted in early fall.
Choose a spot in full sun to part shade depending on the requirements of the iris variety you are planting. The site should have well drained soil rich in organic matter for all iris varieties.
Use a garden fork to loosen the top 10 inches of soil in the area you will be planting iris.
Spread 3 to 4 inches of compost over the loosened soil. Use the garden fork to work the compost into the top 5 inches of soil.
Place iris rhizomes horizontally in the ground. You should see buds (where growth emerges) on one side and small roots on the other. The bud side is planted facing up.
Plant Dutch, Japanese and Siberian iris rhizomes 3 to 4 inches deep. Spacing will vary depending on variety. For bearded iris dig a hole twice as big as the rhizome. Build a mound of soil in the middle of the hole tall enough to place the rhizome just under the soil's surface. Back fill the hole with soil just barely (less than 1/4 inch) covering the rhizome with soil.
Firm the soil tightly around the rhizomes and water well.