Louisiana Iris Growing Tips and Culture

The garden culture of Louisiana irises is fairly simple.  While these irises do have a few preferences, they are not difficult to satisfy.  

The growing culture described on this page is specific to the lower South, even though Louisiana irises are adaptable to most parts of the country.  If you live outside the South, start with these basic cultural recommendations for the Gulf Coast where Louisiana irises are most numerous in nature (but remember that one species is indigenous as far north as Indiana and Illinois), and then make modifications to more northerly or westerly conditions.  Less has been written in detail about growing these irises elsewhere, but growers in a number of diverse areas have written accounts of their experiences and have offered suggestions.  Some recommendations for other regions are available by clicking on the areas listed above.  Others accounts can be found on various web sites, and links to these can be found under Culture Resources, above.

General Location

The low and wetland origins of the wild species should not imply that Louisiana irises require aquatic culture.  They do love water and thrive in ponds and boggy settings, but Louisianas also grow and bloom exceedingly well in typical garden beds. These irises should not be allowed to dry out during periods of drought, however. They will stay green and grow through hot weather only with ample moisture.

Louisiana irises should be grown in half to full sun.  Less than a half day of sun will diminish bloom.  It is highly advisable to avoid close competition with large trees or plants with extensive root systems that would use most of the soil moisture.

Louisiana irises can be mixed with most smaller ornamental plants.  They also can be grown in beds consisting entirely of irises, although iris-only beds may not be as attractive in late summer as the foliage begins to die back in preparation for the new growth cycle that begins in the fall.

Soil Conditions and Preparation

An acid soil is often recommended for Louisiana irises, but this is not necessary.    Soils in much of the Gulf Coast are acid, but some areas, such as parts of New Orleans and the Mississippi River flood plain, may be neutral or a bit alkaline. Since Louisiana irises were found in the wild in these areas, they had to be happy with natural conditions.  Louisiana irises grown in very alkaline soils, such as in some Western states, will exhibit yellow leaves and stunted growth.  There has been no systematic test of the limits of tolerance in an acid or alkaline direction, but experience in the 6.5 to just over 7.2 range has produced excellent results.

Irises need a soil high in fertility and organic matter.  Finely ground pine bark, composted oak and other leaves or rotted manure, for example, are excellent additions that should be worked into the soil when beds are made.

If the soil has a clay texture, adding some sand may be helpful.  Although these irises in the wild may be found in clayey muck, a loose and friable soil is helpful in promoting growth and bloom in garden conditions.  If the irises are grown in water instead of a garden bed, there are other factors to consider.   With any container placed in a pond, a soil too light may tend to float away.  Store-bought soils and amendments, unless designed for aquatic culture, may not be well adapted for pots immersed in water, and a heavier soil is advised.  In water culture where the appearance of the water is not an issue, such as in natural ponds or large containers not intended to fit into a naturalistic pond setting, a wide variety of soil types seem to work well, from nearly soil-less mixes to typical garden soil.

If possible, prepare iris beds a few weeks before planting, but don't fret if you have to plant immediately.  The traditional recommendation is to add the organic material and a generous amount of commercial fertilizer (for example, 8-10 lbs. per 100 sq. feet of 8-8-8) and work in.  An azalea/camellia fertilizer is a good choice if there is a need to adjust for an excessively alkaline soil.  Organic fertilizers are also effective, although the amounts equivalent to a balanced chemical fertilizer are difficult to estimate, and not much guidance is available specifically applicable to Louisiana irises.  If you want to use organic fertilizers, however, click here for an article that may be useful.

Planting and Dividing

Unlike most plants, which go dormant or grow little in the winter, the growth cycle of Louisiana irises actually begins in the fall, continues through winter and culminates in a burst of rapid growth and bloom in the spring.  When the weather gets hot, growth may slow or even stop, especially if moisture and soil fertility are not optimal.

Timing. Consistent with this cycle, the best time to plant and divide Louisiana irises along the Gulf Coast is mid-to-late August, September, October, and early November, not long after the period of new growth has begun. Irises planted later than November are not likely to become well enough established to reach normal size and bloom in the spring.

Transplanting immediately after bloom is not recommended. The hot weather following closely upon the bloom season will stress the plants, and little growth or even an early dormant period may result.  The longer into the season foliage growth can be continued, the better the chances of good bloom the next year, because the plants have a greater opportunity to grow larger rhizomes to support bloom.  If it is necessary to transplant after bloom, be sure to supply extra water.   Alternatively, hold the irises over in water, or in pots set in shallow water and partial shade, until new growth has resumed or until the fall planting season.

Placement. Irises should be planted with about ½ to 3/4 inch of soil covering the rhizome and, ideally, 1-2 inches of mulch over the soil.  Planted about 12 inches apart, Louisiana irises can be left in place 3-4 years and will form clumps. Every several years, it is helpful to dig and divide the irises, thinning them out and replenishing the soil with organic matter as if preparing new beds.

Remember that the rhizomes grow longer as new leaves emerge, and varieties planted too close together will become mixed and difficult to identify.  The rate of "traveling" varies with the particular variety.  Two or more offsets generally form each year on either side of the rhizome, and their growth extends roughly perpendicular to the original rhizome, which results in the tendency for a clump to develop.  Each rhizome blooms only once, then the offsets bloom in subsequent years.  Keeping this grown pattern in mind is helpful in deciding where to plant Louisiana irises.  (Plants produced as offsets will be identical in every respect to the parent rhizome; plants grown from seed will vary, perhaps markedly, depending upon the identity of the particular parents).

Watering

Watering is often necessary to achieve a sufficient growing season for good bloom. Once new growth has begun in the fall, the beds should not be allowed to dry out. September and October are relatively dry along the Gulf Coast and it is important to give the iris beds a thorough soaking at least once weekly during such dry periods.  In summer, a dry spell without good watering will cause growth to halt or even cause the plant to go dormant.

Fertilizing

In truth, fertilizing practices vary a good bit among Louisiana iris growers.  The traditional advice has been:  For new or replanted beds (to which 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 one at the start of the growing season, and then the light dressing just before bloom.  Complete fertilizers such as 8-8-8 are said to be preferable to high nitrogen fertilizers.  High nitrogen can cause the plants to produce leafy growth while suppressing flowering; it also is thought to render the plants more susceptible to some diseases.  An acid-forming azalea/camellia fertilizer should be used if the soil tends to be too alkaline.

Many serious growers of Louisiana irises deviate from this recommendation in several respects.  Some recommend a third feeding after bloom. Some favor liquid fertilizers and some select fertilizers with additional nitrogen. Timed-release products are occasionally recommended, especially in water culture.  We do know that Louisiana irises are heavy feeders, and there is little doubt that the average gardener fertilizes them a good bit less than optimal.  Failure of foliage to remain pretty and green in the hot months after bloom probably is due to insufficient water and less than optimal nutrition.  Cases of problems from over-fertilizing are rare.

Mulching

Mulching is an important part of good culture.  Mulches serve many purposes, such as maintaining soil moisture, keeping weeds under control, increasing organic matter in the soil, and protecting against sun scald (see below).  About 2-3 inches of rotted or even fresh leaves, pine straw or pine bark are excellent mulches.  Materials available locally, such as bagasse (sugar cane fiber), should be considered.

Fortunately, Louisiana irises are not prone to many diseases and insect pests.  Most growers are able to easily endure any small nuisances without having to resort to insecticides or other treatments.

Rust appears as red to dark-brown powdery spots, often surrounded by a yellow margin, on leaves and stems.  Leaves may become severely discolored and die back. The rust does not appear to damage the plants severely, but it does make the beds look bad.  Avoiding too much fertilizer, especially high nitrogen, is said to potentially limit the spread of rust.  Some growers feel that cow manure, specifically, may promote rust and that sedge may be a carrier of rust (and a nuisance in general).

The best way to avoid rust is to remove and discard withered leaves.  Rust is a soil borne fungus, and if infected leaves are not permitted to decay in the garden, the life cycle of the fungus is disrupted.  Stands of Iris fulva in the wild are often infested with rust.  This is not due to gardeners who applied too much nitrogen, but rather by their unavailability to clean up and dispose of the infected foliage.

If an infestation of rust does begin, it is important to remove affected foliage to the extent possible (put it in the garbage, not the compost pile).  Some growers recommend spraying plants in the area of the rust with such products as Ortho's Garden Disease Control or Fertiloam Systemic Fungicide.  

Leaf Miner is indicated by white steaks along the leaves, particularly near the base, in hot weather.  The outermost leaves may collapse, and, if they become unsightly, can simply be removed.  Leaf miners rarely cause enough trouble to warrant treatment with an insecticide.  A freeze will kill them.

Iris borer may be a problem now and then, but it is more serious in areas where bearded irises are grown.  The worm enters the rhizome and may hollow it out and effectively kill it or destroy its ability to produce a bloom stalk.  The first sign of a borer may be a center leaf in a fan that turns yellow and dies.  This occurs because the borer, first deposited on the leaves, has worked its way down to the rhizome and devoured the growing tip.  If one watches for center leaf damage, the borer can sometimes be found and destroyed before the rhizome is seriously injured.  A rhizome damaged by a borer will often produce offsets unaffected by the problem.

Snails and slugs may climb up the scapes and disfigure a bloom, particularly under conditions of high humidity and rainfall.  Some growers use a commercial snail bait and others use saucers of beer.  Snails are not a problem of sufficient severity that most do anything about it at all.  Those who grow irises for entry in shows may be the exception.

Sun scald is not a disease, but a condition resulting from the sun scorching the rhizomes in midsummer, causing them to become mushy and to deteriorate. Mulching solves this problem.

Cutworms are sometimes a problem.  Cutworms are chewing insects that will cut base leaves and disrupt growth.  One recommendation is a fertilizer fortified with an insecticide, such as that sold for control of lawn chinch bugs.