The culture of Louisiana irises in an upland garden is not too difficult if the grower will attempt to supply the plants with the conditions which make for survival and growth in the swamps and bogs, where they are found as native plants.
The soils in which these plants are found are among the richest in the world, from the standpoint of plant fertility, being mostly composed of alluvial deposits in the Lower Mississippi Valley. In addition to being high in the elements of plant food and in organic matter the native beds are usually poorly drained and are covered with shallow water during much of the growing season. In the summer after the water drains off or is evaporated, the rhizomes are protected from the heat of the sun's rays by shade or by leaves and other deposits of vegetation.
In translating these conditions to the upland garden it is desirable to first work into the soil an abundance of vegetable matter. Suitable sources are manure, compost from leaves, and peat moss. A complete, well balanced commercial fertilizer with a high percentage of N-P-K is needed for most soils. For the Coastal Plains of the Gulf Area and for the Mississippi River terraces the use of 8 to 10 pounds of 8-8-8 per 100 square feet is very beneficial. This should be well worked into the bed before planting the rhizomes in late summer or early fall. The newly set beds should be heavily mulched as soon after planting as possible. The mulch is desirable in the fall and spring as a means of conserving moisture and smothering down the growth of weeds and grass, and it is essential after the plants become dormant in the summer. If the rhizomes are continually exposed to the sun during the summer months they will be killed and will rot away. This of course will result in a poorer crop of flowers the following season, as the small rhizomes which survive will not produce many flowers.
Suitable mulching materials are sugarcane bagasse, rice hulls, cotton-seed hulls, .oak leaves, rotten sawdust and even pine straw. Leaves of the various oaks make excellent mulch when applied in the fall, but as they rot down it is necessary to renew the mulch the following June or July. When nothing better is available, the iris rhizomes may be saved by covering about one-half inch deep with a fine sandy loam soil when the plants commence to become dormant in early July.
The best time for transplanting and re-working the beds is in late summer and early fall, with September being the optimum period in the Gulf Coast Region. If planting is delayed into late November the crop of blooms will be greatly reduced the following spring.
Enthusiasm for this fine native plant should not blind us to its limitations. They may be and are grown in the arid regions of the South-western United States, but in order to succeed the beds must be heavily irrigated throughout the year. In the northern her of states where the winters are long and severe these irises may be grown, but they are not as satisfactory as some other plants which may be better suited to the region. The reason is that in their native habitat Louisiana irises make their vegetative growth in the fall, winter and spring. If attempted in areas where the temperature reaches zero or below, the plants are forced to lie dormant during their natural growing period and the following summers are usually too hot and dry for best results. Another factor which should be considered is the soil. From the standpoint of latitude and climate these natives are well suited to the peninsula of Florida, but much of that state is composed of deep, coarse sand which does not possess the necessary moisture holding capacity for good growth.
Diseases are of negligible importance with the exception of rust, which can seriously damage the foliage of some species and their hybrids under favorable weather conditions in the spring. Rust is a fungus disease which can be controlled by systematic spraying of the foliage with a suitable fungicide at regular intervals during the late winter and spring, but it is doubtful that "the game is worth the candle".Iris.Fulvia and Iris.foliosa are the most susceptible of the species, but Iris.giganticaerulea carries a high degree of natural resistance.
In humid regions a heavy infestation of leaf-miner can cause foliage loss and partially check growth of the plant. Since this insect is inside the tissue near the base of the leaf it cannot be controlled by contact insecticides or stomach poisons, but it is easily controlled with one of the systemic materials which is absorbed by the plant. A thorough treatment of the iris bed with Di-Syston, Systox or Cygon in the fall, winter or early spring will give effective control of the insects for an entire season.
Propagation by seed is quite easy and is also the basis for the production of new varieties. Most of the popular commercial varieties are fertile and produce viable seed whether selfed, crossed with other varieties or permitted to be open-pollinated. In the Lower Gulf Region the seeds mature in early July. The pods should be harvested before they get dry and hard and the seed planted immediately. If the seed-bed is kept moist and covered with boards or plastic to help retain surface moisture, germination takes place in October and November. The little seedlings may be transplanted to the permanent bed in the late winter when they are four to six inches high. Under good cultural conditions most of the seedlings should bloom the second year after the seed are formed. Germination of the seed is improved if the dry, outer husk is broken so as to permit moisture to more quickly reach the embryo. This is easily and safely done by breaking off the "horn" of the outer seed coat with the thumb nail.