BASIC CULTURE FOR JAPANESE IRIS
Japanese iris will grow successfully in ordinary garden conditions or the perennial border. They require ample moisture, especially up to bloom time. They will do very well beside a stream or a pond but in cold climates they may suffocate under ice in the winter. Perhaps no other irises are influenced to as great a degree by culture as are the Japanese. Good culture will increase height, branching, flower size and quantity of bloom. They will grow and bloom better if planted in full sun.
Soil Requirements: Japanese iris prefer a heavy, rich soil with ample organic matter, especially manure or peat. If the soil is clay, the addition of the organic matter will help to loosen it; if it is more sandy, the organic material will help in the retention of water and add nutrients. Depending on what is available, till in an ample amount of manure, hay, straw, peat, etc. If using hay or straw, add a high nitrogen fertilizer after the iris are growing to compensate for the nitrogen tied up in the decomposition process. The soil pH should be acid, ideally between 5.0 to 6.5. There is evidence that Japanese iris will tolerate a wider pH range, but growth and bloom will not be as good. If the pH is too high, the leaves will yellow. To lower the pH add ferrous sulfate or agricultural sulfur.
Planting: Plant strong divisions of two to three fans. Small divisions take longer to get started and are more subject to loss. The roots should not be allowed to dry out during transplanting. Soaking the rhizomes and roots in water overnight before transplanting is beneficial. The rhizome should be planted 1 to 3 inches deep, depending on the heft of the soil. If planted in a depression of 3 to 4 inches, the depression will help to catch and hold more moisture. Since new roots form above the old roots, planting in a depression will permit the gradual filling in of more soil and compost and help to maintain the plant's vigor for a longer period of time. After planting, keep the soil moist until the plant is established. In the spring, if rainfall is not sufficient, give extra water until bloom time. The equivalent of 1" of rain per week will keep them doing well in most soils. Do not fertilize until established after a growing season.
Time of Planting: Japanese iris can be planted almost anytime from spring until fall, but shortly after bloom period is probably best because it gives the plants sufficient time to establish new roots for good bloom the next season. Of course in very hot areas, transplanting should be done in cooler months.
Mulching: After planting, a heavy mulch of 2 to 3 inches is beneficial. Oat straw is recommended. The mulch helps to conserve moisture as well as reduce weeds. If plants are set out in the fall a mulch is needed to prevent heaving over the winter.
Fertilizing: Japanese iris are heavy feeders. A liberal application of a balanced fertilizer, such as 12-12-12, in spring and just before blooming is beneficial. Weak plants showing light green foliage will be invigorated with a bi-weekly foliar and drench application of a water soluble acid fertilizer.
Pests: Iris borers and other insects can be controlled with the application of a systemic insecticide such as Cygon 2E mixed at a rate of one tablespoon per gallon of water. Thoroughly wet the foliage when the spring growth is about 6" high and again a few weeks later. Thrips live between the leaves and cause a rusty color to develop on the leaves and can devastate a plant. Pull the leaves apart to see if these tiny slender insects are present. Juveniles are white and adults are black. Use a systemic insecticide to kill them. if adults were present a second application must be made 10 days later when eggs have hatched. Check again in 10 days until they are exterminated.