Leaf Strip

Culture of Iris in New England: Japanese and Siberian Iris

Japanese Iris Culture

Exuburant Chanty, a 3 petaled japanese iris. Note the japanese iris foliage behind the blossom. Japanese iris have no beards and bloom is from late June to July in New England. Japanese iris leaves are taller and thinner than bearded iris leaves. Japanese iris leaves have a distinct rib that runs lengthwise down the leaves. This rib is a good way to distinguish japanese iris from other types of iris. The iris to the left is Exuberant Chanty, a 3 petaled japanese iris. In the background is a good example of japanese iris foliage. To the right is Double First, a 6 petaled iris. Double First, a 6 petaled japanese iris. width=

Water ---No other iris is influenced to as great a degree by culture as are Japanese irises. They require ample moisture, especially up to bloom time. You will be rewarded with much healthier plants and bloom if the plants are kept watered all summer long. Lack of moisture will stunt the plants and produce miniature blooms. A good place to plant them is near a pond or stream, though not required.

Soil ---Japanese Irises prefer a rich soil with ample organic matter, which helps in water retention as well as adding nutrients. The soil pH should be slightly acidic, ideally between 5.0 and 6.5. NEVER ADD LIME, as this will raise the pH and eventually kill the plants.

Fertilizer ---Japanese Irises are heavy feeders. A liberal application of a balanced fertilizer (10/10/10) in the spring and just before bloom is beneficial. Weak plants will be invigorated with a biweekly foliar and drench application of a water soluble acid fertilizer such as Miracid. The preparation of your iris bed with composted manure and peat will be a good start for your JI, but once planted do not fertilize transplants until they have become established. (The same is true for Siberian Irises)

Planting --- The rhizomes should be planted 2 to 3 inches deep. Plant them in a depression which will help catch water, fill the depression with organic mulch. Plants under good culture require division every 3 to 4 years. After planting or division it is essential to not let the JI dry out. Planting the iris in a depression is important because new JI roots grow in above the old roots. After 3 to 4 growing seasons, the new roots will be coming out right at the soil level and it will be time to divide the clump again. Best bloom is usually on 2 to 3 year old clumps.

Mulch and Pests--- We recommend heavy mulching year round for Japanese Irises, 2 - 3 inches. The mulch helps to conserve moisture, discourage weeds, and prevents heaving of fall transplants. Remove old foliage after the first frosts with a serrated knife. Destroy the old foliage, which may contain borer eggs or thrips. These two main pests of Japanese Irises can be controlled with systemic insecticides, such as Cygon-2E or Isotox./Orthene. Cygon is no longer sold, so when looking for a replacement, ask for a systemic poison. UNDER NO CIRCUMSTANCES do you exceed the manufacturer’s directions. These are strong chemicals and sometimes may be used at half strength.

Division-- Japanese iris need to be divided every three to four years. A clump that has formed a solid ring with a bare center needs to be divided. After the plants have been divided, trim the leaves to a height of four to six inches. Replant the divisions in a depression and it is essential to keep the new divisions wet. JI can be divided in most any time during the growing season and right after bloom is a popular time. Spring is a good time in New England since it is easier to keep the divisions moist in the spring and the plants to be well rooted by the time winter sets in.

At the time of division, the iris should be moved to a new location where japanese iris have not grown before. It appears that JI excrete something into the soil that causes the iris to lose their vigor over time and decline. It is recommended that something other than iris be planted in the soil since other types of iris will grow there but not vigorously. The one exception to this is soil that has a lot of water leaching through it such as soil under a downspout or by the side of a stream. Whatever is causing the iris to lose vigor appears to be removed by water leaching. Research is ongoing in Japan to determine what is causing this effect. Japanese iris in Japan are commonly grown in pots and it is easier to replace the soil the iris grow in.

Siberian Iris Culture

Hubbard, a siberian iris. 
Note the grass-like foliage. Siberian iris have no beards and typical bloom in New England begins in late May or June. They have smooth, thin, grass-like leaves without the distinct rib that runs lengthwise down the middle of Japanese iris leaves. Siberian iris shine in the perennial border and they are a low maintenance iris. The iris to the left is Hubbard, to the right is Yankee Counsel. Yankee Counsel, a siberian iris. 
Note the grass-like foliage.

Siberian iris culture is similar to that of japanese Irises. They are among the easiest perennial plants to grow, but until they are well established they must be kept moist. If the roots are dry on arrival, soak them in water for several hours or overnight before planting. Plant where they will receive full sun light for at least half a day. In Maine full sun is best. The most spectacular bloom is produced in full sun. Plant with the top of the rhizome 1 to 2 inches below the surface. It is a good idea to place the rhizome on a mound of dirt in the hole dug for it to insure that there is not an air pocket beneath it. The roots should be spread downward over mound and then replace the soil, and water well. Siberian iris will often not bloom the first year after replanting, fortunately they do not have to be divided regularly like japanese or bearded iris. A clump that has formed a solid ring with a bare center is a candidate for division as are clumps that appear to be "crowded".

Each spring a small handful per plant of ordinary (10/10/10) commercial or organic fertilizer may be used. An acidic fertilizer such as Miracid is recommended as a foliar drench for weak plants. DO NOT use lime around siberian irises. The species prefer an acid soil. Use of lime may cause your siberian irises to decline rather rapidly.

Photo of Exuberant Chanty courtesy of Ensata Gardens.

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