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    Japanese Iris

When to Plant:  Japanese iris can be planted and transplanted almost anytime from spring until fall if the transplants are kept wet for the rest of the year and the temperatures are below 90 degrees F for a month afterwards. For regions with hot and dryer summers, the fall will be a better planting time.

Where to Plant:  Japanese iris need six hours of full sun to bloom properly. In the hottest areas, afternoon shade is suggested. Soil

Preparation:  Japanese iris prefers a rich soil with ample organic matter to help in water retention as well as adding nutrients. The soil pH should be slightly acid (5.0 – 6.5). Attention must be given to the pH of your water, which can gradually raise the pH of your soil. An indication of too high pH is the gradual yellowing of the leaves. The addition of granular ferrous sulfate (iron sulfate) or agricultural sulfur can lower the soil pH. The preparation of your iris bed with compost or manure will be a good start for the JI bed, but do not use granular fertilizer until the plants are established. Leaves, pine needles, grass clippings, straw or sawdust are all good soil amendments. Basic Planting

Steps:  Plant the divisions 12-18 inches apart. Plant the rhizome 2-3 inches deep. They can be planted in a depression that will help catch and hold more water; fill the depression with mulch.  Heavy mulching (2-3 inches) is recommended year round.

Watering:  Japanese iris require large amounts of water during their growth and flowering period; however, the plant should not be allowed to stand in water, especially in winter.



Quality of water (pH and / or salts) is the greatest influence upon the Japanese Iris plant. A lack of moisture will stunt the plants and produce miniature blooms. An abundance of water and manure can produce four to five foot tall bloom stalks. JI must be kept watered all summer long. Never let the soil dry out. Mulch the plantings. Depending on the soil, 1-2 inches of water per week is recommended. Older clumps need more water than new divisions. They do very well beside a stream or pond.

One good method of watering is to form a basin around the plant. Slightly elevate the plant with the crown one-inch below the soil. Plastic or aluminum lawn edging works well for a basin. Flood the basin two to three times each watering. Applying aquarium water can serve to keep rhizomes wet and to keep pH lowered.

Japanese irises are NOT bog plants in northern zones (zones 3-6). One of the biggest misconceptions about Japanese Iris come from seeing photos of flooded fields in Japan. The Japanese flood the fields at bloom time for the esthetic setting and the beauty of the blooms reflected in the water. When JI are planted in a bog and if the water freezes over the top of the crowns the plants may suffocate and die. Artificial, plastic lined bogs have produced mixed results. If using plastic lined bogs, it is best to put some holes in the bottom and allow the water drainage to be retarded, but not prevented. If JI are grown in pots in ponds, the pots need to be lifted after frost, foliage cut off and the pots buried in the garden. These pots can be returned to the pond the next spring.

Fertilizing:  Japanese iris are heavy feeders and fertilizing three times a year is suggested. A liberal application of fertilizer in the spring and just before or after bloom is beneficial. A third application in the fall is also beneficial. JI like nitrogen. Water-soluble acid fertilizer, such as Miracid (acid type that is used for azaleas and camellias) is good for quick action but only lasts for 2-3 weeks. An all purpose fertilizer such as 5-10-10 can be used and the acid condition provided by the addition of soil sulfur, aluminum sulfate or small pieces of aluminum foil buried just under the soil surface. One half of one pound of soil sulfur per 100 square feet of planting area applied yearly in early fall is ample. Oak leaves, pine needles and peat moss help to supply the acid condition these plants favor, and the more leaves which are used, the less sulfur is required. Do not let the plants dry out after fertilizing because this will quickly burn the plant roots.

Bloom Season:  Successfully growing JI requires a lot of patience. Under the best of conditions, spectacular display is not attained until the second or third year. JI will bloom shortly after the Tall Bearded Iris.

General Garden Care:  Remove the old foliage after the first frosts. Destroy the old foliage that may contain borer eggs or foliage thrips. The two main pests of JI can be controlled, where warranted with systemic insecticide such as Cygon 2E or Isotox.

Moving & Thinning:  Try not to replant Japanese Iris back in the same soil where JI have grown for three or more years. New roots form above the old roots each year, by the time the crown grow to the surface and the roots can be seen, it is time to dig and divide the plant. Plants under good culture require division every 3-4 years. When bloom sizes and plant height decrease, it is time to divide. When dividing, cut back ¾ of the foliage and plant 2-4 fan divisions. Keep the transplants well watered until they are well established. Do NOT let the rhizomes or the roots dry out during transplanting – soak in a bucket of water up to 48 hours.