Manure can be used as a substitute fertilizer. Work the fertilizer or manure shallowly into the soil. If the weather in June before the plants bloom is dry, liberal watering is advisable. Cultivation is essential to keep down weeds although the danger of disease is apparently not so important as with the bearded iris. The growth of the Japanese iris clumps is relatively slow, but they should be divided occasionally as they become crowded.
Siberian iris are perhaps the easiest to grow of all.
Once established they persist in spite of competition from weeds and grass. They do, however, respond to good garden culture, and clumps that become over-size should be divided and the divisions reset. Fertilize them in the same manner as the Japanese iris. Propagation Iris, except the bulbous types, are propagated by simple division of the rhizomes. The divisions of the bearded iris are called fans and consist of one of the fan-like clusters of leaves with a section of the attached rhizomes about 2 to 3 inches long.