My iris were especially pretty this year, and I think I need more! My colleague Elizabeth Wahle, University of Illinois Extension horticulture educator, explains below why late summer through early fall is the best time to divide and plant bearded iris.
"Growers need to allow four to six weeks following flowering for rhizomes to fully develop before digging and dividing," Wahle said. "That's why most iris plant sales are held later in the growing year and why nurseries don't ship bare-root plants until mid- to late-summer."
Iris grows well and eventually the clump will need to be dug and divided, according to Wahle. "Cultivars vary in growth rate, but on average, a gardener can expect three or more fan increases each year from each 'mother' rhizome," she said. "Some cultivars are slow to increase with less than three fan increases whereas others rapidly multiply the mother rhizome by a factor of eight or more.
"Most gardeners find it necessary to dig, divide, and replant new divisions every three to four years. A gardener should definitely have a plan in place for where new iris rhizome divisions will go before digging, whether that be a new patch of ground or maybe a gift to a friend," she added.
Wahle suggests digging one clump at a time to avoid mixing up cultivars. Once the clump is dug, cut away individual new rhizomes from the mother rhizome with pruning shears or a knife. Discard the old mother rhizome. "Each new division will look like the original rhizome you planted," Wahle said. "Cut the leaves back in an arrow or inverted 'v' shape, with the point centered about four to five inches above the rhizome."
The rhizome divisions are then ready to plant. Irises grow best where they will receive six to eight hours of direct sunlight, Wahle said. Soil should retain uniform moisture but still be well drained.
"Probably the most common mistake gardeners make with planting iris is planting the rhizome too deep. Despite its bulb-like appearance, a rhizome is actually modified stem tissue that grows best at or just below the soil surface," she added.
Want to learn more? University of Illinois Master Gardener and Illinois Iris Society Vice President Margaret Kelly will present "The World of Iris" at the next ICC Summer Gardening Educational Series session on August 22 at 10:00 a.m. All seminars are held at the ICC Horticulture Building on the ICC campus, East Peoria. Reservations are not required and seminars are open to the general public. More details are available online at http://web.extension.illinois.edu/fmpt/downloads/59419.pdf.