Prepare a soil bed that is friable at least 10" deep. Place the rhizome just beneath the surface with the roots spread out and deeper than the rhizome. Soil should be neutral to slightly alkaline.DEPTH TO PLANT: Place your rhizomes just below the surface of the ground with the roots well spread out underneath so the rhizome is within reach of the warmth of the sun's rays while the roots beneath are in moist (not soggy) soil. Be sure to firm the soil lightly around each rhizome when planting. (See figure.)
DISTANCE APART: Depends on your particular plans. Generally Iris are planted from 8 inches to 2 feet apart. (See figure.) Planting three of one kind in a triangle, single rhizomes 8-15" apart with the fans pointing outward, will create a massed effect but will need to be divided every 2 years. If the plants are set about 2 feet apart they will need dividing only every third or fourth year. The best planting time is July through September in the Northwest area. When dividing, dig the plant, remove and discard the old center rhizomes and replant the new fans. You may cut out the old centers with a knife while they are in the ground if you do not wish to lift the plant.
Care of the plants is relatively simple. Keep weeds and grass tufts out of the rhizome clumps. Cultivate shallowly, since the feeder roots are near the surface. Newly-set plants should be kept moist until the roots are growing well. Established plants rarely need watering except during prolonged dry spells; at such times, deep, infrequent watering is best.
Fertilizer should be applied as a side dressing in early spring, and then again after bloom. It can burn rhizomes, so apply it around, but not on, the plant. Too much nitrogen promotes soft, lush growth which is susceptible to rot, so a 5-10-10, super phosphate or similar formula is best.
Air circulation and sanitation are the best problem preventatives. Remove old iris leaves and other debris from around the base of the plant. Aphids, caterpillars, etc. may damage the flowers, but rarely do serious harm to the plant. Slugs love to nibble at new shoots and will even climb and attack the tall leaves of some varieties. Old bloom stalks should be cut or broken off at ground level - but healthy, green foliage should NOT be cut off. It needs to be left on the plant to foster development of new sprouts for the next season.
During the growing season, the plants may be sprayed, along with roses and other perennials, with a combination insecticide-fungicide spray at regular intervals - usually 10 days to 2 weeks. In some years, usually warm, wet ones, leaf spot can make the leaves unsightly. Cutting off the spotted leaves will improve the appearance of the garden and retard the spread of disease.
Mulching of bearded iris is to be avoided. If you desire to mulch, do not cover the rhizomes. The sun must reach them to facilitate development of next year's increase. Freezing weather will not harm the rhizomes, other than causing slight "heaving" out of the ground. If this occurs, simply cover the exposed roots with additional soil.