Time to divide Louisiana irises

Daniel Gill, Young, John, Owings, Allen D.  |  9/11/2009 7:37:02 PM

Sustainable Landscape News From LaHouse

By LSU AgCenter Horticulturists Dan Gill, Allen Owings and John Young

Some of the most beautiful irises for our gardens are the hybrids of several native species. Called Louisiana irises, these plants are becoming increasingly popular in gardens all over the world.

Sustainable landscaping encourages the use of native plants. Hopefully, you have some of these wonderful irises growing in your garden.

Louisiana irises include Iris giganticaerulea, I. fulva, I. brevicaulis, I. nelsonii and I. hexagona. Only in Louisiana are all five species found. They are closely related and will interbreed with one another.

Crosses between the species have produced an amazing array of outstandingly beautiful hybrids that also are called Louisiana irises. Most of the Louisiana irises you find for sale will be hybrids of these species, although the species themselves are also beautiful and worthy of use in the garden.

Louisiana irises can be divided and transplanted anytime from August through mid-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. 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 three or more years after the bed is planted, depending on how close together they were planted to begin with and how much room they have to spread. Clumps also may grow beyond their allotted space. 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 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 decide where 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, and carefully cover all of the roots. Space the rhizomes about 1 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. If you have any rhizomes left over, pot them to share with friends.

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

Container-grown Louisiana irises in aquatic gardens also may need to be divided and repotted, and this is a good time to do that. Remove the plants from the pot, and follow the procedure just described to divide them. Use pond fertilizer tablets according to label directions to fertilize the plants.

This is also 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 (the semi-dormant plants will not look that great, but that is normal for this time of year). If you don’t see them available, don’t despair. Blooming plants are generally available in the spring and do just fine planted then. Or, check out possible mail-order sources online by typing “Louisiania iris” into your favorite Internet search engine.

A few other perennial plants grown in Louisiana gardens 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. 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.

Come to LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is located near the intersection of Burbank Drive and Nicholson Drive (Louisiana Highway 30) in Baton Rouge across the street from the LSU baseball stadium. Go online to Louisiana Yards and Neighborhoods for additional information.

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Editor: Mark Claesgens

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