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Finally found some of the details I was looking for. From the book "Magic of Irises" by Barbara Perry Lawton pp172-173
"Rhizomatous bearded irises should be divided every 3 to 5 years...(more often in some regions)...
Although you can dig and divide bearded irises anytime you can work the ground, the best time for transplanting is when the rhizomes have reached their maximum growth and are semi-dormant -- in July, August, and September in most parts of the US. The point is to divide the rhizomes and replant them in time for them to grow new roots during the fall months.
If you time things right, you may even have significant blooms from the new divisions next year. If you divide bearded irises in the spring, the plants will probably be fine, but you probably will sacrifice the current year's flowers.
If you divide and transplant bearded irises too late in the fall, the plants wont have time to develop new roots and become reanchored in the soil. The result may be that freeze-thaw cycles will heave them out of the ground. Knowing the growing cycles of bearded irises explains the timing of transplanting.
When the soil warms up in the spring, irises begin growing aboveground, developing this year's foliage, buds, and flowers. The roots on last year's old rhizomes decay and disappear during this period. At the same time, the new iris roots on this year's productive rhizomes are growing, supplying nutrients and water to the foliage. Once the plants have flowered, rhizome and root growth continue for about 2 months in this year's blooming rhizomes.
During this stage,nutrients are being stored in the rhizomes for the following season's growth. At this time, the rhizomes develop the new rhizomatous growing ends from which next year's foliage fans and flower buds will arise.
The growing productive parts of the rhizomes look like a Y. The leg of the Y is the older part of the rhizome and the arms are the newer parts where next year's growth will take place.
Once this growing period is over, most of the bearded irises rest until the late-summer rains arrive. The irises are semi-dormant during these weeks. The exceptions are the reblooming irises that grow continuously throughout the summer and so must constantly gather moisture and nutrients for repeated buds, stalks and flowers. The late-summer rains and slightly cooler weather trigger a new round of root growth. Extra water during hot, dry summer months will cause these irises to begin their late-season period of root growth sooner.
(To be continued...)
This message was edited Jun 26, 2006 8:49 PM
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