The iris is a perennial plant available in several varieties ranging in size from 3 inches up to 4-feet. While many consider the iris a bulb plant, the correct terminology is a rhizome root system. The best time to transplant an iris is late summer through early fall since the plants begin blooming in early spring. Transplanting too early in the growing season may disrupt the plants natural growing cycle and prevent blooming the following spring. It may be necessary to divide and transplant an iris every four years since they grow in clumps that become crowded. An iris with a decrease in flower production is most likely compacted and in need of dividing.
The first step in dividing an iris plant is cutting the green foliage to a height of 2 inches early in the fall season. Remove the plant from the ground by digging around the entire rhizome root structure with a sharp spade. Make sure you dig deep enough to remove as much of the root system as possible. Lay the plant on a plastic tarp or grassy area and gently pry the clumps apart. Make sure there is a section of green foliage stem attached to each rhizome root clump. Inspect the rhizomes and discard any that appear shriveled, diseased or soft.
Transplant the iris rhizomes in a location that has a well-draining soil and receives full sunlight. Dig a hole that wide enough to accommodate the rhizome root section. Plant the rhizome so it is even with ground level in clay soils or buried 1 to 2 inches below ground level for loams and sandy soils. Set iris plants at a distance of 1 to 2 feet apart to prevent premature crowding of plants. Applying a thin 1/2-inch layer of organic compost around the iris plants works as a mulch layer and provides nutrients to the soil. Do not mulch the bearded iris variety or those planted in clay soils as this may hold too much moisture and cause the rhizomes to rot.
Water the soil around the iris plants immediately after planting to stimulate root growth before the winter season. Provide winter insulation to protect iris plants growing in cold climates that do not receive snow cover. Good choices for winter insulation are loose straw or evergreen branches. Do not use dirt or mulch that retains water as this promotes root rot in the plant. Make sure you remove the mulch in late winter to stimulate plant growth in early spring for blooming.