THE LOUISIANA iris continues to be a popular plant in local landscapes. Despite their common name, Louisiana irises are grown in much of the United States and even in Canada and other parts of the world.
Gardeners tend to be surprised to learn that they originated in America and are well adapted to the soils and climate along the Gulf Coast. Those commonly found in local landscapes are derived from five iris species, most of which are native to a limited area of south Louisiana and the Gulf Coast marsh areas between Texas and Florida.
These five species are closely related and have been crossed with one another to produce the 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 five species, although the basic species is also beautiful and worthy of use in the garden.
Because all the primary colors are inherent in the various species that contributed to this group, there is no limit to the color range.
Louisiana irises can be divided and transplanted at any time from August until early October. Some varieties go dormant during the heat of summer, so now is the ideal time to divide them.
If you have Louisiana irises in your landscape that are well established, you might notice some brown or yellow leaves 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 them the size you want and prevent the irises from taking over areas where they were not intended to grow.
The first step in dividing them is to use a shovel or garden fork to lift the Louisiana iris plants from the bed. Try to get as much of the root systems as possible and do not damage the fans – leaves – 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 two-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 into the soil.
To divide your irises, look over the clumps carefully. You will see that young rhizomes branch off from the older ones. The younger ones 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 ones.
Plant the rhizomes horizontally with the fan of foliage facing the direction in which you want the plant to grow and carefully cover all of the roots. Space the rhizomes about a foot apart. The top of each one should be just below or barely show above the soil surface. Apply two to three inches of mulch to the top of the soil and water thoroughly. If you have any rhizomes left over, pot them to share with friends.
Should extended periods of dry weather occur over the fall, winter or spring, water your irises once or twice a week to keep the plants well supplied with moisture. In addition, an application of fertilizer such as ammonium sulfate 21-0-0 in February will keep them growing vigorously into the blooming season.
You might also need to divide and repot Louisiana irises growing in containers in aquatic gardens and this is a good time to do that as well. Remove the plants from the pot and divide them by following the same procedures as those for irises growing in the ground. Fertilize them by using pond fertilizer tablets according to label directions.
One other note – there are a few other perennial plants grown in local landscapes that should be transplanted or divided this month. Like Louisiana irises, they tend go into a period of summer dormancy when temperatures are hot and will begin to grow actively some time in October as the weather cools.
Agapanthus, or lily of the Nile, Easter lilies and calla lilies can also be divided and transplanted. 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.
William Johnson is a horticulturist with the Galveston County office of Texas A&M AgriLife Extension Service. Visit his website at aggie-horticulture.tamu.edu/galveston/index.htm
The extraordinary beauty and reliability of Louisiana irises grown in the landscape have made them increasingly popular. They can be divided and transplanted at any time from August until early October.