URBANA, Ill. - Late summer through early fall is the best time to divide and plant bearded iris, according to Elizabeth Wahle, University of Illinois Extension horticulture educator.
“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.”
A question arises about potted irises available throughout the season in many garden centers—should gardeners wait until late summer to early fall to plant them?
“It’s not necessary to wait,” Wahle explained. “Containerized iris can be planted immediately. Usually containerized plants are divisions from clumps dug and divided the previous fall, then allowed to overwinter prior to shipment to nurseries this spring.”
Whether iris was planted as a bare-root rhizome or as a potted plant, 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.
Plant spacing can vary. “It really depends on how often you want to dig and divide. The closer you plant, the sooner plants grow together and need to be divided. Plants spaced 12 to 18 inches apart allow a gardener to put off division for three to five years. Planting less than 12 inches apart will accelerate the process to every two to three years,” Wahle explained.
Like most garden plants, bearded iris benefits from fertilization. Avoid using fertilizers high in nitrogen, which results in succulent leaf growth and fewer flowers. Wahle said products such as bone meal or a 6-10-10 fertilizer are effective. “Fertilizer can burn the rhizomes so it should be applied around but not directly on the plants,” she cautioned.
There are two times during the active growing season when bearded iris should be fertilized. For existing clumps, the first application is about six to eight weeks prior to flowering, which for much of Illinois is between mid-March and mid-April. The second application should be just after transplanting or, for any existing clumps not being dug, in mid-summer.
Source: ACES