It's time to beard the task of dividing iris
It's rare to meet someone who doesn't admire iris, especially tall bearded iris with their large, delicate blooms and array of sumptuous colors. Some sport ruffled petals, some are bicolored and almost all produce a lovely fragrance unlike that of any other flower.
Tall bearded are the most popular member of the iris family. Other relatives include crocus, gladiolus, freesia, blackberry lily, and of course, all sorts of irises, such as Siberian iris and the moisture-loving blue flag and yellow flag irises that grow wild in Minnesota marshes.
This year, bearded iris were truly spectacular. Like most early-flowering trees and perennials, their blooms, boosted by our cool spring weather, lasted longer than usual. But as temperatures escalated, they went into summer dormancy, a period marked by very slow growth. It's during this phase that you can safely lift and divide existing clumps of iris, as well as plant new ones.
Most bearded iris should be divided every three or four years. If you neglect this chore, the plants become crowded and fail to bloom well. Dividing and replanting will not only revitalize your iris and encourage better blooming, it also will give you a chance to inspect their roots and rhizomes for borers and rot. If your irises have multiplied, it's also an excellent time to share or swap with friends and neighbors.
Iris rhizomes sit just below the surface. To dig up a clump, slide your garden fork carefully beneath the clump and lift it out of the soil. Then spread the clumps on the lawn and spray them with the garden hose to wash all the soil off. Once you can see the rhizome, it's not difficult to cut each clump into smaller portions for replanting.
Labor of division
Each clump of iris will have two or more fans of leaves growing out of its rhizome. Use a sharp knife to divide the rhizome, separating it into individual fans, each with a thick chunk of the rhizome (with fat roots) attached to it. Discard the oldest woody part of the original rhizome, along with any parts that are soft or rotted. Next, trim any broken roots and cut the green foliage back to 4 to 6 inches. You can replant the rhizomes immediately or store them for several weeks in a cool, well-ventilated place.
Back to earth
Unless your soil is just about perfect, it pays to improve it before replanting the iris. Incorporate organic matter such as finished compost or peat moss, especially if your soil is fairly sandy. At the same time, work in a little garden fertilizer such as 5-10-10. A standard recommendation would be about 1/2 pound per 25 square feet of garden (an area roughly 8 feet long by 3 feet wide).
Plant groups of the same iris in drifts with their leaf fans facing outward from the center of the garden. Leave a minimum of 8 inches between each rhizome for expansion.
Rhizomes should be positioned horizontally, right below the surface. Dig a shallow hole for each rhizome, with a small ridge of soil for it to sit on. Spread the roots to both sides of the ridge, then firm more soil over the rhizome and roots and water it thoroughly.
When the soil begins to freeze in November, mulch these iris with 4 to 6 inches of straw or marsh hay, or 10 to 12 inches of dried leaves. (When used as mulch, leaves pack down and don't trap as much insulating air as straw or hay.) Remove the mulch gradually as it thaws in spring. And when your iris bloom beautifully next spring, you'll be glad you took the time to work on them this summer.
Deb Brown is a horticulturist with the University of Minnesota Extension Service Yard and Garden Line. For help with garden, plant and insect questions, call the Extension service at 612-624-4771.