Perhaps no other iris is as influenced by good culture as Japanese iris, properly known as Iris ensata. Demanding in their needs but if met they will reward you with tall robust plants and larger blooms. Hardy in zones 4-9.
Newly received plants that are bare root should be soaked in cool water for a few hours or overnight. Plant individual rhizomes 2-3 inches deep, 6 to 8 inches apart. Give a good 18 to 24 inch spacing between different rhizomes. Water in well to start the root development, do not let new transplant dry out. Depending on your soils and weather a daily watering may be needed for the first week or two. New transplants may not bloom the first year. Expect your best bloom on 2 and 3 year old plants.
Japanese irises prefer a rich, loose soil with ample organic matter. An addition of well aged manure and compost will help in water retention as well as adding nutrients. The soil pH should beslightlyacid, ideally between 5.5 to 6.5. Do not use bone meal or lime, as this will raise the soil pH too much and can kill Japanese irises. Also, do not fertilize newly planted or transplanted Japanese iris as this can burn the roots.
Japanese irises do not like to dry out. Constant moisture will reward you with healthy plants and larger blooms.Wetin the spring and keep moistallsummer. They do very well near water, as this is where they naturally grow, or where the water table is high. The crown of the plant needs to be above the water line.
These plants like full sun, with a minimum of 6 hours to bloom properly. In hot climate, some afternoon shade will benefit the bloom.
Japanese iris are heavy feeders. Depending on your soil a liberal application of balanced fertilizer for acid loving plants (Rhododendron, Camellia) in the spring just before or after bloom is beneficial. Most soils with heavy watering will usually need more nitrogen applied. Do not apply fertilizers to new transplants.
Japanese iris can be transplanted almost anytime from spring until fall if you keep the plant wet, and the temperatures are below 90 F and above 32 F for a month afterwards. Dig and divide your plants every 3 to 4 years to maintain plant vigor. New roots form above the old roots each year forcing the crown up and out of the soil. It is critical to transplant every 3 to 4 years.