Louisiana irises need not be grown in water or under bog conditions, however, don't overlook this important use of the native iris. Water gardening has become very popular in the last decade and Louisiana Iris are a natural in this setting; however, for decades they have been grown under ordinary garden culture with excellent results. The native iris through the work of backyard hybridizers and a few nurserymen, have transformed this simple wild flower into Belles of a Mardi Gras Ball. The culture is very easy, provided a few simple rules are followed. The conditions described are for Louisiana.
Louisiana irises should be grown in as full sun as possible, but under no circumstances should they get less than a half day of sunlight. The irises will not bloom properly in dense of full shade. Also, the irises should not be grown in competition with large trees or plants with extensive root systems that would use most of the soil moisture. Two types of garden locations can be considered: beds containing only Louisiana irises, or beds containing Louisiana irises mixed with other plants. Louisiana irises can be mixed with most of the smaller ornamental plants, subject only to the limitations of acid soil and extensive watering needed when drought occurs during the growing season.
Soil Conditions and Preparation
An acid soil is a must for Louisiana irises; soil pH of about 6.5 or lower is best. Any soil suitable for azaleas and camellias is ideal. Irises also need a soil high in fertility and organic matter. Add sphagnum peat, compost leaves or rotted manure if necessary. If the soil has a clay texture, adding some sand may help. Prepare the beds a few weeks before planting the irises if they are to be planted in new beds. Work the soil, preferably with a power tiller. Add the organic material and a generous amount of commercial fertilizer (8-10 lbs. per 100 sq. feet of a complete fertilizer such as 8-8-8 or 8-24-24) and work in.
Planting and Dividing
The best time to plant and divide Louisiana irises is mid-to-late August and September. Transplanting immediately after blooming is not recommended. If existing beds are to be divided and replanted, dig out all the irises and rework the beds, adding organic materials and a commercial fertilizer as described for new beds. Replant as soon as possible, preferably the same day. Do not allow the newly planted beds to dry out; water every few days until the plants are established. Planted in a triangular format (about 12 inches apart), they can be left 3 to 4 years and will form nice clumps. Irises grow in the direction they face; at least two offsets generally form each year, one on each side of the rhizome. Each rhizome blooms only once, then the offsets bloom the next year. Good growth and offset formation are, therefore, necessary each season for consistent bloom. The offsets represent vegetative reproduction and are identical in every respect to the parent rhizome.
Watering is often necessary to achieve a sufficient growing season for good bloom. Our growing season is about eight to ten months from north to south in the state. Once new growth has begun, the beds should not be allowed to dry out. September and October are generally quite dry in Louisiana, and it is important to give the iris beds a thorough soaking at least once weekly during such dry periods. Modify the watering schedule according to growth.
For new or replanted beds (to which commercial fertilizer and organic material have been added at planting time), a light dressing of a complete fertilizer (2-4 lbs. Per 100 square feet) is sufficient about two months prior to bloom (late January in Louisiana). Beds which have not been replanted in late summer are generally given two applications, a fairly heavy application at the start of the growing season (with good watering in), and then the light dressing just before bloom. Complete fertilizers such as 8-8-8 are preferable to high nitrogen fertilizers. High nitrogen can cause the plants to produce leafy growth while suppressing flowering; it can also make the plants more susceptible to disease.
Mulching is an essential part of good culture. Mulches serve many purposes, such as maintaining soil moisture and keeping weeds under control. But mulching during the summer dormant period serves one essential function which cannot be met by any other means, except planting the irises in water.
The rhizomes lie on or very near the surface; the hot summer sun on the rhizomes (after the foliage has died back) will cause a condition known as sunscald. This causes the rhizome to deteriorate and rot. The tip of the rhizome will continue to grow, but it is almost a certainty that no blooms will be obtained the next season. This is one of the main reasons for failure of Louisiana irises to bloom consistently.
The problem is easily solved by applying a mulch. About 2-3 inches of rotted or even semi-rotted leaves, bagasse (sugar cane fiber) or pine straw are excellent mulches. As little as one-half to one inch of soil can also be used if nothing else is available. But by all means protect the rhizomes from the sun during the dormant period. Keeping a mulch on the irises the entire year is best, and it is a necessity in colder areas.
Editor's Note: Author and source unknown