Cultural Information: Planting and Growing Iris

by Betty Wood

Irises are among the easiest of perennials to grow, and they give an abundance of beauty with minimum care. The iris has a thick fleshy root called a "rhizome" (pronounced rye-zome) about like a tough potato in texture. When you buy a new iris, you will probably receive a rhizome with clipped roots and leaves. It can remain out of the ground for a week or two without serious harm, but the sooner it is planted, the better.

To plant your irises, choose a sunny spot in well drained soil. Prepare the soil well, by spading or turning over the soil with a garden fork to a depth of at least 10 inches. Spread fertilizer and work it into the top of the soil. If possible, this should be done 2 to 3 weeks before you are ready to plant. A well prepared bed will result in better growth and more bloom. Don't starve your irises or make them compete with nearby grass or weeds for food and water. Many gardeners, iris and otherwise, have soil analyses made of their garden soil, then add the fertilizer of the kind and quantity the tests show the soil needs.

The soil should be light. If it is clay soil, add very coarse sand and humus. Bone meal and a good garden fertilizer, low in nitrogen, are good for irises, but manure should be used only after it has aged for about a year. Otherwise, it may cause rot. The roots must be buried firmly to hold the plant in place, but the rhizome should be near the surface. An easy way to achieve this is to dig two trenches with a ridge between them, place the rhizome on the ridge and spread the roots carefully in the trenches. Be sure to firm the soil tightly and allow enough for settling to keep the rhizome above any possible standing water. Then fill the trenches with soil, letting the top surface of the rhizome be just barely beneath the surface of the soil.

If you have several plants, plant them at least a foot and a half apart, "facing" the same way. The rhizomes will then increase in the same direction, without crowding each other too soon.

From the new parts of the rhizome, new bloom stalks will come up in later years and the flowers will be exactly as the original flower. This is called "vegetative propagation".

In about 2 or 3 years, the new rhizomes will begin to crowd each other and you will want to divide the plant, cutting the newer parts of the rhizome free from the old, which may then be discarded. Unlike the other bearded irises, arils need to be transplanted annually.

You will have so many new rhizomes that you will share them with your friends. Perhaps you received your first rhizomes from a friend. When digging, keep all plants carefully labeled with their names, for sure identification. It is wise to keep diagrams of your planting area to double check individual labels on the plants.

This digging and separating is best done between one and two months after bloom season, usually in July or August. Soon after this the irises grow roots which help to hold the plant firmly during the winter in areas where freezing and thawing can result in heaving the rhizome out of the ground. If you live in this type of climate, a mulch of salt hay can be very beneficial.

Culture of the beardless irises differs somewhat from culture of the bearded irises. They should be transplanted in the fall or in early spring. The roots should never be allowed to dry out while they are out of the ground and they should be watered heavily after transplanting. They should be set slightly deeper than the tall bearded. Japanese iris should be planted in a distinct "depression" in heavy soil to assist in supplying moisture to the plant. Siberians and the Pacific Coast Natives can tolerate light shade but the Spurias, Japanese and Louisianas demand full sun. Louisianas and Japanese require moist conditions during the summer months while the Pacific Coast Natives enjoy a very low humidity and dry soil no matter how hot it may get. All, except Louisianas, should be planted in a permanent spot where they can remain for many years as they resent being disturbed. Louisianas tend to "creep" and therefore, should be tended to every few years. All are heavy feeders and need to be fertilized regularly.