Autumn is the time to divide clumps of iris ensata (Japanese iris) that have become too large. The best way to know if your irises need dividing is by the bare spot in the middle with blades splaying outward in a circle. This usually needs to be done every 3-5 years. My irises were planted in fall 2006, but they were crowded in gallon pots. My iris are planted in the rain garden where the soil stays moist. I've not had to use supplemental irrigation this summer due to frequent rains. During the drought last year, I sparingly used the drip irrigation (we are on a well).I will dig out the clumps and separate into individuals. Before dividing, I will have the planting spot ready to keep the roots from drying out. Japanese iris like moist, acid soil, full sun. It is best to plant at least 24" apart to plan ahead for rapid expansion. Plant with the junction of the fans and roots about 1-2" below soil level. I will probably separate the individuals into about 3 fans each.
In my research, I've read that you shouldn't replant Japanese iris in the same spot. I did further research and found suggestions of digging out the soil, using new soil and/or flushing the soil to wash away the root secretions from the previous plants. Given the layout of my garden, I will have to replant some of the individuals in the same place.
When the foliage dies back (yellow) in the fall, I cut it and clear it away to keep thrip eggs from overwintering. Other problems are deer nipping blooms when the irises bloom in late May through mid-June. The damage hasn't been severe enough for me to stop growing Japanese iris.
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