Perhaps the most common type of purple iris is the bearded iris, commonly called German iris in the past. Bearded irises range from the dwarf iris, which can grow from 3 to 11 inches in height, to the tall bearded iris, which can reach heights of 2 to 4 feet. With orchid-like flowers, the iris produces about nine to 12 buds on the side branches of each iris stem. Irises are often planted in borders or near buildings. The University of North Dakota recommends planting irises in mid to late summer.
Select a location for your purple iris in full sun with well-drained soil. If possible, avoid locations with rich, heavy soil, as this tends to encourage root rot in irises. Till the ground to a depth of 8 inches or more. If soil pH is not within the preferred range of 6 to 7, amend accordingly. Allow the soil to settle for about a week prior to planting.
Dig a hole for the iris that is large enough for the roots of the plant. Put the iris in and cover the top of the rhizome, the thick fleshy underground stem, with just a 1/2 inch of soil. Plant up to six iris plants in a group, spacing them each about 12 to 15 inches apart. If you want to plant more than one group, allow 4 feet between groups, recommends the University of North Dakota, to prevent the plants from crowding too quickly.
Water the plant thoroughly, and apply fertilizer at a rate of 1/2 lb. per 50 square feet just after transplanting.
Water the iris deeply when weather is dry. Weed regularly to rid the area of weeds that try to compete with the plants for nutrients and moisture.
Fertilize each spring with a low nitrogen fertilizer, such as 5-10-10.