By Dr. William C. Welch, Landscape Horticulturist
Louisiana irises are perennials that can be grown successfully in every area of Texas and the Gulf Coast, but thrive best in the eastern third of the state where their ancestors are native. They also occur naturally in Arkansas, Louisiana, and Mississippi. Mature plant size varies from 1 to 6 feet and flower sizes from 3 to 7 inches across. Flowers occur in March and April. Because all the primary colors are inherent in the various species that contributed to this group, there is no limit to the color range. The Louisianas, for example, include the purest form of red of any iris.
Louisiana irises prefer an acid soil in the range of 6.5 or lower. They like large quantities of fertilizer and water, but their greatest need for both of these comes during the naturally cool and moist fall and winter seasons. They are among the few irises that will thrive in poorly drained soils, and may be effectively used along streams and lakes where they may be inundated periodically during changing water levels. Foliage is lush and requires heavy fertilization to remain healthy and productive. Some varieties go dormant during the heat of summer, leaving dead foliage that should be cut back or removed. New foliage will appear again in the fall.
Fall is the best season for transplanting. Beds should be well tilled and amended with large amounts of compost, peat, or pine bark. Rhizomes should be planted just below ground level and kept moist until well established. Clumps spread quickly, and individual rhizomes should be spaced several feet apart to avoid need for annual division. Mulching in the summer protects rhizomes against sunscald. Winter protection is not necessary, but could help prevent the evaporation of essential moisture in northern and dry areas of the region. Azalea-camellia fertilizers are recommended, along with water soluble fertilizers designed to lower the soil pH.
After bloom is completed in the spring, the stalks should be cut back to the rhizome. Old rhizomes do not bloom again, but increase to produce the following yearís crop. These flamboyant flowers are attractive to bees, and the visits of these insects often result in pollination and the production of fertile seed in the irises' large seed pods. Ripening seeds sap the plantís strength, so they should be removed unless, of course, the grower has decided to raise new plants from seed. If so, leave the pods in place until they turn yellow-green in July or August, shell out the seeds before they dry, and plant at once into pots of well-prepared soil. Provide adequate protection over the winter, and plant the young seedlings into permanent locations in March.
Although not always available in a great variety of colors, Louisiana irises are sold by some garden centers in Texas. Mail order sources are another possibility. Special plant sales, such as the Bulb Mart in Houston each fall and March Mart at the Mercer Arboretum, usually offer a wide variety of Louisiana irises.
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