During July and August Iris growth slows, and the plant becomes somewhat dormant. It is during this time of year that they are best dug, divided and replanted. The German Iris grows from a thickened underground stem called a rhizome. This structure usually grows parallel to the soil surface and just slightly beneath it. The true roots of the plants extend from these rhizomes into the soil beneath.
A good loam soil with a pH near neutral is most suitable for irises in areas where soil is excessively sandy, plants benefit from additions of organic matter. Acid soils should be limed to bring acidity closer to a neutral condition. A soil test may be helpful to establish conditions in new beds. Good soils preparation at planting time is important, since iris plants should be allowed to stay in place for three to four years.
In selecting a location for an iris planting, look for plenty of sunshine and good drainage. In heavy soils where drainage may be a problem, raised beds might be considered. Do not locate iris close to trees or shrubs that produce prolific, shallow roots. Other plants, as well as grass competition, can weaken growth and reduce flowering. When irises are dug for replanting, they may be divided. Plants that have been healthy and undisturbed for a number of years often form a large clump or tangled rhizomes. Lift the entire clump and separate the individual rhizomes by breaking them apart.
Allow healthy leaves to remain attached, but remove any dead or diseased leaves. After the rhizomes have been separated, discard the oldest positions, leaving only current growth and the growth form the past season. Cut leaves back to about six inches before replanting. Keep roots attached to the rhizome complete and moist. Make sure that all rhizomes to be replanted are firm and healthy. Soft rhizomes may contain iris borer or soft rot.
In resetting iris rhizomes, they should be spaced eight to 12 inches apart. Place the roots deeply into the soil with the rhizome horizontal to the soil surface and just barely covered. Iris rhizomes may be placed in row, or grouped to threes to give the illusion of a clump as they develop and grow together. The young plants will move in the direction of the new growth.
In grouping the rhizomes to form clumps, place two rhizomes facing out and on e facing in, so that an open center does not develop. Healthy irises spread rapidly, and such groupings should be placed 15 to 18 inches apart. Irises planted in August should become well established before cold weather arrives. If set too late in the fall, they are sometimes heaved out by freezing and thawing. A winter mulch can help prevent this.
It also helps to keep weeds down during the fall. Irises should be kept weed free, and a herbicide such as Dacthal may be used to keep weed seeds from germinating. Varieties that repeat bloom in the fall, known as remontant irises, should not be allowed to lapse into a rest period. They should be watered and fertilized throughout the summer to stimulate growth.
http://en.wikipedia.org/wiki/Iris
