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Growing Irises

by Leslie Dybiec    

Bearded Irises

Irises are among the earliest blooming and easiest perennials to grow. Probably the most popular and widely grown iris is the tall bearded type. "Bearded" refers to the presence of a furry strip on each of three drooping, petal-like sepals, called falls. The true petals are called standards and are upright. Flowers appear in late spring to early summer on stalks above the foliage. Flower colors are many (except true reds) and also come in two- color combinations. The leaves are stiff and strap-like and grow in fan-shaped clumps.

Bearded irises are available in a wide range of heights; miniature dwarf (up to 10"), standard dwarf (10"-15"), miniature tall (15"-25"), intermediate (15"-27"), and border (28"). Despite size differences, all bearded irises thrive in full sun and well-drained soil.

Culture

Bearded irises grow from an underground stem known as a rhizome (RYE-zome). Healthy rhizomes increase in size and develop branches or "off-shoots" which can be broken off (do not remove attached roots) and replanted. This method of propagation (division) should be done one to two months after bloom (usually July or August) every two to three years. When dividing, cut back leaves to one-third their length to keep newly planted divisions from being whipped by the wind and pulled from the soil. Otherwise, cut back foliage in fall as it begins to brown.

The planting procedure is the same whether planting a new plant or division. Cultivate the soil 12 to 18 inches deep and mix in composted organic material. Dig a hole about six inches deep and wide enough to accommodate the rhizome and its roots. Build a small mound of soil in the bottom and place the rhizome on it, spreading the roots carefully. Adjust the height of the mound so that the rhizome is just beneath the soil's surface. Fill in the hole and water. Bearded irises should be spaced 12 to 18 inches apart.

Bearded irises require a well-balanced fertilizer such as 5-10-10 or 6-10-6, applied once in early spring and again in early summer following bloom. Ample moisture is required in spring when flower buds are developing and during flowering.

The Iris Borer

The most serious bearded iris pest is the iris borer. It overwinters in the egg stage attached to leaves. The eggs begin to hatch in late April. The tiny caterpillars crawl up iris leaves and begin chewing and mining their way down within the leaf fold, eventually reaching the rhizome. Once in the rhizome, they continue to feed, sometimes reducing the rhizome to a hollow shell. Leaves which yellow and are easily detached indicate damage to the rhizome. About mid-August the fully-grown borer crawls out of the rhizome and pupates in the soil. Pupation is completed in about one month when the drab, gray-brown moth emerges (mid-September to October). The moth lays eggs on dry plant material and other garden debris, then dies. Eggs overwinter until the following spring, when they hatch and the cycle repeats.

Ideas for borer control include:

• eliminating the eggs by removing and destroying debris in and around the iris planting both spring and late fall.
• digging rhizomes, and physically removing the borer, then replanting (usually done in July or August).
• treating leaves in spring, with a labeled insecticide. Dimethoate (Cygon 2E) and lindane are registered for this purpose. Treat when new growth is 6 to 9 inches in height, then as needed to kill borers within them.

Soft Rot

Another problem which may plague damaged rhizomes is bacterial soft rot. This may enter the rhizome through any wound, including feeding damage from iris borers. To get rid of soft rot, dig the rhizome, scrape out the affected tissue, allow it to dry in the sun, then dip in a 10-12% solution of household bleach for a few minutes. Rinse with water and allow to dry before replanting.

Other Irises

The beardless iris group includes those with smooth falls and leaves which are long (2'-4') and relatively narrow. Of this group, Japanese (Iris ensata), Siberian (Iris siberica and Iris sanguinea), and Louisiana (hybrids of Iris fulva, I. giganticaerulea and I. brevicaulis) are the easiest to grow in Ohio.

Japanese Iris

Japanese and Louisiana irises thrive in constantly moist soil or even standing water, while Siberian irises can tolerate moist conditions as well as those of a typical perennial border. They all prefer acidic soil and require regular fertilizations through the growing season. Unlike bearded irises, these should be transplanted in fall or early spring (divide in early spring) and require heavy watering during establishment.

All but Louisiana irises should be planted in a sunny location where they can remain for several years. They do not thrive when disturbed too often. Borers do not as readily attack these irises.

The crested or evansia irises have a cockscomb-like crest along the lower half of the falls, instead of a beard. Iris tectorum, I. cristata and I. tenuis range in height from 6-12", and bloom in May if provided with a moist, shaded location and soil rich in organic matter.

The bulbous irises include the so-called English, Dutch and Spanish irises, which have erect standards. Some have smooth bulbs while others have a netted or fibrous covering on the bulb. Bulbs are planted in fall. Iris reticulata and I. danfordiae bloom in late February or March.

Still other lesser known irises, Iris pseudacorus, Iris laevigata, Iris virginica, and Iris versicolor, do quite well submerged in a pond, at water's edge, or in a moist part of the garden. Iris versicolor also does well in ordinary garden conditions.


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