The soil should also have a good nitrogen content. Prepare the bed with compost and manure, but avoid granular fertilizers until the plants are established. Japanese iris rhizomes must not be allowed to dry out before planting. Soak them in water for up to 48 hours before planting. Plant the rhizomes 2-3 inches deep, depending on the heaviness of the soil.
Lighter soils require deeper planting.
Plant rhiz omes in a depression and fill the depression with mulch. The depression will help catch and retain more water, the Japanese iris's most important nutrient. Keep the transplants wet for the rest of the season. Use 2 to 3 inches of mulch all year. After the first frosts, remove and destroy old foliage.