Iris bloom profusely in early summer.
image by kconnors/morguefile
Transplanting iris bulbs not only increases the size of your iris bed, it revitalizes old iris plants as well. After several years of growing in the same location, rhizomes multiply and roots begin to crowd, reducing bloom size and producing pale blooms. Bigger, brighter blooms will reward an afternoon's work of digging and transplanting the bulbs the following year.
Prepare a new location for iris in a sunny location. Till the soil to a depth of 6-8 inches. Remove rocks or roots and amend the soil with compost or well-rotted manure. Work amendments into the existing soil.
Dig iris bulbs once the foliage begin to die back in late summer. Although gardeners commonly refer to these as bulbs, many common irises actually produce rhizomes, a thickened root with roots along the bottom and sides of the tuber. Rhizomes grow near the surface of the soil and produce new rhizomes each year. Lift the bulbs free of soil with a garden fork or spade.
Trim foliage to a height of 4--6 inches for easy handling. Gently tug the rhizomes to pull them apart. Cut individual rhizomes away from the main bulb with a sharp knife if necessary. Each section must have several white roots.
Inspect rhizomes for any signs of disease or insect damage. Healthy rhizomes are firm and plump. Discard any mushy or discolored bulbs.
Plant rhizomes so that the top one-third of the bulb is above the surface of the soil. Firm soil down around the rhizome and water thoroughly. Keep soil moist throughout the fall to help your iris develop strong root systems. Growth resumes in spring, rewarding you with robust growth and large, showy blooms.