Get It Growing: Its Time To Divide Louisiana Irises

Daniel Gill, Merrill, Thomas A.  |  8/25/2006 11:57:58 PM

GIG

Get It Growing News For 09/08/06

By Dan Gill
LSU AgCenter Horticulturist

Some of the most beautiful irises we can grow in our gardens are the hybrids of several species that grow right here in Louisiana. Called Louisiana irises, they are becoming increasingly popular in gardens all over the world.

I hope you have some of these wonderful plants growing in your garden, and if you do, now is a good time to divide them and transplant them. If you don’t have any Louisiana iris, it’s also a good time to purchase some and plant them.

The species called Louisiana irises are Iris giganticaerulea, I. fulva, I. brevicaulis, I. nelsonii and I. hexagona. Only in Louisiana do all five species occur. Since they are closely related, they will cross with one another, and crosses between the species have produced an amazing array of outstandingly beautiful hybrids that are also called Louisiana irises. Most of the Louisiana irises you find for sale will be hybrids of these species, although the species also are beautiful and worthy of use in the garden.

Louisiana irises can be divided and transplanted anytime from August through early October. Unlike most plants, Louisiana irises are at their most dormant stage in the late summer – making now the ideal time to divide them.

You might have noticed how many brown or yellow leaves are on your plants now. Even if you decide you don’t need to divide them this year, it’s a good idea to get in and clean out the unattractive foliage before the new growth starts in earnest. This will make the planting more attractive.

Each year Louisiana irises grow and spread – creating more rhizomes and shoots. Eventually, the plants may become crowded, which can lead to lower vigor and poor flowering. This generally occurs two to three years after the bed is planted, depending on how close they were planted to begin with and how much room they have to spread.

Clumps also may grow beyond their allotted space, and dividing will help keep the clump the size you want and prevent the irises from taking over areas where they were not intended to grow.

The first step in dividing these plants is to use a shovel or garden fork to lift the Louisiana iris plants from the bed. Try to get as much of their root systems as possible, and do not damage the fans of new growth at the ends of the rhizomes. Put them aside in a shady area and hose them down to keep them from drying out.

Once the bed is empty, take the opportunity to improve the soil in the bed before you replant the irises. Spread a 2-inch layer of compost or other organic matter over the soil, sprinkle a light application of a general-purpose fertilizer over the area and thoroughly incorporate everything into the bed.

To divide your irises, look over the clumps carefully. You will see that young rhizomes branch off from the older rhizomes. The younger rhizomes have a fan of green leaves at their tips with roots growing out from the rhizome at the base of the leaves. Break or cut off the young rhizomes at the point where they branch off from the old rhizome. Discard the old rhizome and replant the young rhizomes.

Plant the rhizomes horizontally with the fan of foliage facing the direction you want the plant to grow. Then carefully cover all of the roots. Space the rhizomes about a foot apart. The top of the rhizome should be just below or barely show above the soil surface.

Mulch the bed about 2-inches deep and water thoroughly, and if you have any rhizomes left over, pot them to share with friends.

Should the weather become dry this fall, winter or spring, water once or twice a week to keep the plants well supplied with water. An application of fertilizer in February will keep the plants growing vigorously into the blooming season.

Louisiana irises growing in containers in aquatic gardens may also need to be divided and repotted, and this is a good time to do that as well. Remove the plants from the pot and follow the same procedure I described for dividing those that are planted in the ground. Use pond fertilizer tablets according to label directions to fertilize your containerized Louisiana irises in aquatic gardens.

This also is a good time to purchase and plant new Louisiana irises into sunny beds or aquatic gardens. Some local nurseries may have them for sale, so it’s worth checking.

If you don’t see them available, don’t despair. Blooming plants generally are available in the spring, and they also do just fine when planted then. Or check out possible mail-order sources online by typing "Louisiana iris" into your favorite Internet search engine or by visiting your favorite gardening sites.

In addition to Louisiana irises, there are a few other perennial plants grown in Louisiana gardens that should be transplanted and/or divided over the next few weeks. Like the Louisiana irises, they tend to be dormant now and will begin to grow actively sometime in October as the weather cools down.

Feel free to divide and transplant Easter lilies, acanthus and calla lilies. If you’d like to divide or transplant spider lilies (Lycoris radiata, also called hurricane lilies or naked ladies), you may do so as soon as they finish blooming.

Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com. A wide range of publications and a variety of other resources are available.

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Contact: Dan Gill at (225) 578-2222 or dgill@agcenter.lsu.edu
Editor: Tom Merrill at (225) 578-2263 or tmerrill@agcenter.lsu.edu

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