A Basic Guid to Recommended Cultural Practices
The Louisiana iris is a relative newcomer to general garden culture. Fifty years ago only a handful of specialists were really aware of this native iris. One of the biggest hindrances for their widespread use in gardens has been a misunderstanding of their cultural requirements. No, you do not need a pond or bog to grow them. This is just another example of their adaptability, and that is what makes a great garden subject-adaptability.
The species of the Hexagonae series grow in Georgia and the Carolinas, from Florida to Texas along the Gulf of Mexico, and up the Mississippi valley into Arkansas, Missouri, Ohio, and Indiana. The geographic regions represented are quite varied in temperature, annual rainfall, and soil conditions. The range of the modern garden hybrids have encompassed the globe. Louisianas are being grown successfully from Canada to Australia, from]apan to Germany.
SITE & SOIL
The site chosen should be as sunny as possible, unless you live in an area where summers tend to be long, hot, and dry. In this case, some afternoon shade may be beneficial. Areas of deciduous shade are acceptable, but avoid areas of heavy shade. The irises will grow, but bloom will be reduced greatly.
The soil must be the grower's first consideration. A thorough soil test should be performed before you add amendments. Take the samples from several locations in the area chosen for planting to insure a true representation. Consult your Agricultural Extension Service for testing facilities, or buy a home test kit for the simpler tests. The things you'll want to test for are pH (acidity or alkalinity), soluble mineral salts, nutrient content (nitrogen, phosphorus and potassium), and organic matter content.
The pH scale ranges from 0 to 14 with 7.0 being neutral. Any value below pH 7.0 is acidic, and any value above pH 7.0 is alkaline. The numbers make up a proportional scale. In other words, the lower the number (below 7.0) the more acidic the soil; the higher the number (above 7.0) the more alkaline the soil. Louisiana irises prefer an acidic soil and are native to regions of the country where acidic soils predominate. But Louisianas do appear to tolerate slightly alkaline conditions, especially if kept constantly moist.
The pH affects the way in which plants grow because certain nutrients become unavailable chemically in overly acidic or alkaline soils. One exam ple of this is iron deficiency (chlorosis). In acidic soils iron is readily available, but in alkaline soils it becomes chemically "locked up" - iron is present in the soil, but only in a form which plants cannot absorb.
There are a number of ways in which pH can be lowered. The most economical probably is the addition of organic matter, but the addition of organic matter alone affects soil pH only very gradually. Chemical alternatives are available through the use of sulphur-based compounds or acidifiers such as aluminum nitrate. Raising the pH, on rare occasions when that step appears advisable, will require the incorporation of agricultural lime. A pH increase of 1.0 will require from 50 to 75 pounds per 1,000 square feet. Read and follow label instructions when applying commercial amendments.
Soil nutrients come in two basic categories, primary elements and trace elements. The primary elements are nitrogen (N), phosphorus (P), and potassium (K) Each plays a vital role in the overall health and performance of plants Nitrogen promotes leafy growth and overall vigor; phosphorus stimulates root growth, flowering, and fruiting; and potassium regulates a plant's response to heat and cold and promotes overall hardiness. These elements can be purchased in various strengths and mixtures and are represented by the numbers found on fertilizers, in the order of N, P, K. Trace elements most commonly concerning gardeners are iron (Fe), sulphur (S), and zinc (Zn).
Fertility and organic content are very important in growing Louisianas, for they are very heavy feeders. It is doubtful that there can be too much organic matter in the soil. Adding organic matter is always a good idea, but be careful what you use. The most desirable sources are finished compost, brown Canadian peat moss, leaves, or shredded bark. Grass clippings can be used, but have some drawbacks. Green clippings will rapidly deplete nitrogen from the soil due to its rapid decomposition. Adding Nitrogen at the time the green matter is incorporated helps offset the problem. Avoid fresh manures, fresh sawdust or wood chips, and domestic black peat moss from the Northern U. S.
The addition of organic matter will help to loosen heavy clay soils (although heavier soils, especially if kept moist and fertile, are not a problem for Louisianas), improve moisture and nutrient retention in loose soils, increase acidity, make the soil more friable, and allow the soil to warm faster in the Spring due to increased air movement through the soil. Till the soil to a depth of 12 inches before adding the organic matter. A four-inch layer of compost or peat moss is a good rule of thumb.
My favorite source of organic matter is leaves. They are easily obtained in the fall, since most people have no use for them and will rake and bag them for garbage collection. Shredding leaves will allow for faster decomposition. If you do not have a shredder, a lawn mower can be used. Whenever possible, I start preparing a bed a year in advance. This allows me to keep adding organic matter at monthly intervals.
The goal in preparing soil for Louisiana irises is to end up with a soil that is on the acidic side. This is an interesting point of study for growers. In water culture, pH does not seem as important. I know of one site where a pond was excavated in pure caliche rock and the surrounding soil was not amended, yet the plants show no ill effects from the high pH. (Admittedly, the irises have been growing in this environment only a couple of seasons, so some ill effects may yet be observed.)
In southern Cameron Parish, Louisiana, there are large colonies of I giganticaerulea growing in beds of Oyster shells and water, and once again the plants show no signs of a pH problem. Due to the alkaline nature of the water supply (8.5-10.) in the Dallas area, it is almost impossible to maintain an acid bed. My beds test somewhat alkaline, but growth, flowering and multiplication does not appear to be depressed as a result.
To combat this creeping alkalinity, it is recommended that soil acidifiers be added both Spring and Fall, and that your fertilization program make generous use of "acid-forming" fertilizers (often labeled "Azalia-Camellia-Gardenia Fertilizer"). Water-soluble fertilizers applied with a hose-end sprayer are the easiest types to apply to already-established beds. The acidifier will be absorbed through the leaves for quick results, and through the roots for longer-lasting effects.
Watering questions are the most frequently asked. The beds for Louisianas should not be allowed to go dry, but they do not need to be kept wet. The soil should be kept as evenly moist as is possible. Mulching is the best aid in this quest, and will be discussed in a moment. The prime growth time for irises are in the Fall and Spring, the normal wet times of the year. In my area I rarely have to water once the Fall rains set in. If there is an extended period of no rain, I water. Watering is just as critical in the Winter as it is in the Summer. Allowing the plants to endure a long, dry cold snap will set them back. When watering is required, at least 1 inch of water is recommended. This is the same amount as is recommended for watering the lawn. A good deep watering is always preferable to shallow watering. The only exception I make to the rule of keeping beds evenly moist is when plants are being put in. For roughly 1 month after planting, or until appreciable new growth has begun, I try to keep the beds pretty wet.
In my opinion, mulching is important when growing Louisiana irises in beds (as opposed to water culture). The easiest way to keep soil evenly moist all the way up to the surface is to mulch. Mulching also regulates the fluctuations in soil temperatures and will greatly enhance water conservation. The rhizomes of Louisiana irises are subject to sun scald, which leads to rot and, ultimately, to death of the plant. It is the tendency of the rhizomes to grow at ground level. The foliage will shade them to an extent, but mulching is the answer. Especially across the South, gardeners with beds located in full sun areas must be very careful to protect against scald. Phosphorus absorption is enhanced in alkaline soils as the mulch decomposes, but more about this later.
Mulching is also good for weed control. With properly mulched beds you will have very few weeds to pull, and within a year can break the weed seed cycle in the soil. In my opinion, pine needles make the best mulch. Pine needles, or Pine straw, will not blow away or mat down, lasts for several years, and is attractive as well as aromatic in the landscape. Pine needles can be purchased in bales, but finding a supplier may prove a challenge. Hay can be used, but often contains huge quantities of weed seeds. Thrashed wheat and rice stalks make an excellent mulch but are hard to locate, and are usually fairly expensive. Leaves should be shredded to keep them from rotting down too much. The only drawback with leaves-is the difficulty keeping them in place in a wind, Shredded Pine or Cypress bark can be used with good results and are readily available at most garden centers. Grass clippings should be considered as a mulch material only after a thorough drying out. Green clippings act like a nitrogen.vacuum and will literally rob it from the soil - although the nitrogen eventually will be returned as the clippings decay into humus.
PLANTING & TRANSPLANTING
Planting and transplanting the rhizomes are best accomplished in the Fall, at least 3 weeks prior to the average first freeze date for the area. Rhizomes should not be allowed to dry out, ever. As soon as they arrive, remove the wrappings and keep them in pans of water at least overnight, or until planting. They can be held in water for some time without fear. If it is necessary to hold them for planting for several weeks, embedding the rhizomes in wet vermiculite should be considered The addition of a root stimulator to the water has worked well for me. Since my water is so alkaline (pH 8.5 on the average), I add an acidifier when the plants will be held in water for any period of time before planting.
A regular fertilization schedule should be maintained for both Spring and Fall The hose end sprayer is a quick and easy way to accomplish the task. Digging in the fertilizer around plants is a laborious job and can damage roots and rhizomes. I take to heart that these irises are heavy feeders, and I tend to fertilize more frequently than most growers. Spring applications should commence 4 to 6 weeks prior to the anticipated emergence of the flowering stalks. Louisianas normally start blooming about 2 weeks after the Tall Bearded irises. The fertilizer should stipulate that it is for blooming plants, such as Super Bloom. Acidic fertilizers are available and are generally marked as Rose and Camellia fertilizers. Applications at 2 week intervals prior to bloom works well for me. Fall fertilization should be based on the average first freeze date for your area. As with planting, fertilization should cease 3 to 4 weeks prior to this date. Two or three applications by hose-end application, or good side dressing should yield the desired results. New plantings can be lightly fertilized once, after appreciable new growth has begun. A word of caution about the use of super-phosphates. These compounds are highly alkaline and can build up in the soil Moreover, if soil pH is too high or low the addition of phosphorus will not help plants, even if the plants are suffering from phosphorus deficiency. Phosphorus is particularly sensitive to both high and low pH readings. At a pH of 6.0 it begins to become less available to plants, and at a pH of 5.0 the phosphorus present in the soil cannot be used by plants. On the alkaline side of the scale the same things occurs between 6.9 and 7.8 pH. Applications of lime and phosphate should be separated by at least 2 weeks.
Pests and diseases
Louisiana irises are amazingly immune to most pest and disease problems. The biggest problems for me are slugs (controlled with baits) and grasshoppers (controlled with Sevin). Rust is not a problem in areas of low summer humidity, but responds to treatments with maneb or a similar fungicide. Leaf miners are not a problem except in areas with too heavy a shade. Iris borers and leaf spot are possible. These problems have been reported in Northern States and California.