It's time to dig, divide and plant iris
In today's "Growing Together" column, Don Kinzler explains why it's important to divide the perennial flower named after the goddess of the rainbow and how to do it in August.
Which perennial flower has the greatest range of colors? It’s iris, appropriately named after the goddess of the rainbow, available in white, yellow, blue, purple, lavender, orange, rust, brown, black and near-red. With a flower shape so elegant, it’s no wonder it’s called the poor man’s orchid.
Iris, blooming mostly in late May through June, are among the winter-hardy perennials that form the backbone combination of a good perennial garden. Even the sword-shaped blue-green leaves give foliage contrast when flowers fade.
Iris purchased in pots can be planted throughout the season, but August is the traditional time to dig and divide established plantings. Bareroot divisions are often available from garden centers now. Late August and early September are ideal for planting so roots become established before winter.
There are different species of iris, but the most common is called bearded or German iris (from the botanical name Iris germanica.) Bearded refers to the bushy “beards” on each of three drooping petal-like sepals, called falls. The true petals are called standards and are the upright part of the iris blossom.
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Iris grow from thick underground stems called rhizomes that are sometimes mistakenly called iris “bulbs.” Leaves grow in a fan shape arising from the top of the rhizome, and the actual fibrous roots grow below.
As years pass and the iris grow, the center of the clump often becomes choked with old leafless rhizomes. This causes a reduction in flowering and makes iris more susceptible to insects and disease. It’s best to divide clumps every three to five years while they’re still healthy. Dig the entire clump and reset healthy divisions in its place.
To divide and transplant iris, lift the whole clump by gently prying them out of the soil with a spading fork, which works better than a shovel because it’s less likely to injure roots and rhizomes. With a sharp knife, separate the clumps into divisions, each having a single rhizome with a fan of leaves attached. Cut leaves back to about 4 or 5 inches, leaving a neatly trimmed fan shape. Discard leafless old rhizomes from the clump’s center.
Iris bloom best in full sun, with six hours a minimum. They also don’t tolerate “wet feet.” An easy way to ensure good drainage is to rake soil into a slight mound for an iris grouping. Heavy soil can be improved by adding peat moss or compost.
Dig a shallow hole for each division, leaving a ridge in the center. Place the rhizome over this ridge with the roots spread out on either side. Add soil over the roots and gently firm. Just barely cover the rhizome with soil.
Planting depth is very important. Whether using potted iris or bareroot divisions, the rhizome should be just barely covered. Remember, the actual roots that require soil contact are below the rhizome. It’s preferred if the rhizome is slightly visible at soil level after planting, especially in heavy clay. If planted too deeply, flowering is hindered and rhizomes can rot.
Space rhizomes 12 to 15 inches apart when planting groups. Arrange several iris of the same variety in drifts with the fans pointing in the same direction, or in a triangle with the “toes” pointing inward. Water well after planting. If iris rhizomes are plump and healthy with a nice fan of leaves, they will usually bloom the first year after replanting. Small, thin rhizomes might take until the second year.
Iris should be mulched for winter protection, especially if their location doesn’t receive snow cover. Apply 6 to 12 inches of leaves or straw in early November after cutting back leaves to about 4 inches.
Besides bearded iris, several other types merit greater use. Siberian iris are extremely hardy and produce a circular clump of deep green foliage up to 3 feet tall that’s attractive even when they’re not flowering. They can remain eight to 10 years without division.
Similar to Siberian iris, and also becoming increasingly popular, are Spuria iris. Forming an attractive circle of 4-foot-tall foliage, they create a nice accent at the rear of perennial beds.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at firstname.lastname@example.org or call 701-241-5707.