Answer: 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. Japanese Iris 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 your area since it is easier to keep the divisions moist in the spring and the plants will 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 Japanese Iris 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. 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.
Best wishes with your garden!
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