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Cultivation of Japanese Irises

By John Coble and Bob Bauer

Japanese iris (Iris ensata) produce some of the largest iris blooms at 6-8 inches in diameter on 3-4 foot branched stalks when grown properly. They bloom about a month after the Tall Bearded iris in USDA Zones 4-9. They grow best in an organic-rich, slightly acid soil that can be kept damp-to-moist year round. Mulching, 2-3 inches, is also recommended year round. Six hours of full sun is needed for good bloom.

No other iris is influenced to as great a degree by culture as are the Japanese iris. Good culture will increase height, branching, flower size, and quantity and quality of bloom. No other factor will be a greater influence than the amount of water and its quality (pH and/or salts.) A lack of moisture will stunt the plants and produce miniature blooms. An abundance of water and manure can produce 4-5 foot tall bloomstalks! Depending on your soil and climate, 1 -2 inches of water and/or rain per week is required. Older clumps form dense root systems and will need more water than new divisions. But Japanese iris are not water plants. They should not be planted in the water where the water freezes over in the winter. Beside a stream or pond can be ideal where the roots can always reach moist soil with water borne nutrients.

JI prefer a rich soil with ample organic matter to help in water retention as well as adding nutrients and some aeration. The soil pH should be slightly acid, 5.0 to 6.5. Peat moss will help lower the soil pH but add few nutrients. Manure and/or compost will be beneficial in pH and adding nutrients. Attention must also be given to the pH of your irrigation water, which can gradually raise the pH of your soil. An indication of too high a pH is the gradual yellowing of the leaves. The soil pH can be lowered by the addition of granular ferrous sulfate (iron sulfate) or agricultural sulfur. Be cautious about using mushroom compost as most have lime added. Lime or bone meal can be harmful to Japanese irises.

Japanese irises are heavy feeders. Depending on your soil, you may want to apply a balanced fertilizer or one that is a little higher in nitrogen in early spring and again just after bloom, when increase fans are maturing and new rhizomes are forming. Composted manure is an excellent alternative to chemical fertilizers. This is also a time of active new root growth and the plants will benefit from a deep cultivation at the same time as fertilization, and then apply new mulch if needed and water the plants. Water soluble acid fertilizer, such as Miracid, is good for quick action but only lasts for 2-3 weeks. The preparation of your iris bed with compost or composted manure will be a good start for your JI, but do not use granular fertilizer on new transplants until they have new roots established, 4-6 weeks. If you wish to incorporate fertilizer into your new iris beds, prepare the beds

a month or two before planting, or in the fall for spring planting. Also be careful not to let your plants dry out after fertilizing, as this will quickly burn plant roots.

Virgin soil (virgin to JI) will produce your best plants. Try not to replant JI divisions back in the same soil where JI have grown for three or more years. Plant divisions 12-18" apart, 18-24" if you don't want them crowded in three years. The rhizome should be planted 2-3 inches deep. New roots form above the old roots each year and by the time the crown grows to the surface and roots can be seen, it is time to dig and divide the plant. Plants under good culture require division every 3 to 4 years. Your best bloom will be on 2 and 3 year old clumps. When bloom size or plant height decline, its time to divide.

Japanese iris can be transplanted almost any time from spring until fall if you can keep the transplants wet for the rest of the year, and the temperatures are below 90 degrees for a month after transplanting. Early spring to right after bloom is the best time for most northern climates. Hot and/or dryer regions may have better luck with fall planting (cooler and more moist.) The best time for you is the combination of your climate and your gardening practices.

When dividing, cut off 1/2 to 3/4 of the foliage (except for early spring dividing), and tear or cut the clump into natural rhizome divisions of 4-6 fans. Roots may be trimmed to 6-8 inches long, making them a manageable size for planting in the new hole. Do not let the roots dry out during transplanting, put your divisions in a bucket of water as you divide the plant. Soaking these new divisions in water for a couple of hours is beneficial and they can be held in the water for a few days if the new bed is not fully prepared. If you need to hold the divisions (or your new purchases) for more than a few days while the new bed is being prepared (or if the weather turns hot), it may be better to pot up the divisions so that new roots can quickly form in an environment that you have more control over.

In early winter, remove the old foliage after it turns yellow to brown with a serrated knife. One reason is to help discourage voles (field mice) from building winter nests under the canopy of old foliage and eating the crown of rhizomes. Cut the foliage down to 2-3 inches

above the mulch and let the stubs collect leaves for further winter mulch. Destroy the old foliage which may contain iris borer eggs or foliage thrips and their eggs. These two main pests of JI can be controlled, where warranted, with systemic insecticides in mid-spring.

Japanese iris are available in singles (3 falls and 3 standards), doubles (6 falls and no standards), and multi-petal peony forms (9-12 falls). Forms range from the classic tailored, smooth petal edges to exaggerated ruffled and billowy edges with multiple style arms with large serrated crests. Color patterns range from solids to bicolors and bitones to strongly veined contrasting colors and shadings


-- Main.RPries - 2010-07-06
Topic revision: r2 - 23 Jan 2016, BobPries
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