I am honored that this article, which I wrote many years ago, is still featured in the AIS booklet Basic Iris Culture and was also selected for the online Iris Encyclopedia.
Arils and arilbreds have a reputation for being difficult to grow. This is partly deserved, but also partly the result of misunderstanding. Unfortunately, the word aril is often used rather carelessly to refer to both arils and arilbreds. (Arilbreds are hybrids between aril irises and bearded irises.) These two types, however, are very different in their cultural requirements and their capacity to grow and thrive without special attention.
Today's arilbreds are not hard to grow in most climates. One who plants a selection of arilbreds interspersed among a bearded iris planting will find that most of them will grow and bloom without any special attention, even in climates outside the sun belt of the West and Southwest, where arils are most commonly grown. However, some understanding of their cultural preferences helps up the odds and ensure a greater rate of success. Although pure arils are not widely grown, a quick review of their cultural requirements is valuable, because it casts some light on the needs of their arilbred descendents.
The aril irises are the oncocyclus and Regelia from the Middle East and hybrids having only aril irises in their ancestry. The oncocyclus in particular have always posed a challenge to gardeners living outside their native region. They go completely dormant in the summer, which leaves them susceptible to rot in rainy climates. Furthermore, they are unapologetically temperamental, sometimes thriving for four or five seasons and the simply dying for no obvious reason. Regelias are much more adaptable, but still prefer dry hot summers.
Some people, hearing that arils are desert plants, assume that they are particularly vulnerable to winter cold. This is generally not the case. Many of the aril species come from exposed, mountainous environments where winters can be severe. Some lowland oncocyclus species are somewhat less hardy, but this is a trait they share with many of the ancestors of the tetraploid tall bearded irises, which come from the same region. So winter care precautions suitable for tall bearded irises in your area should also work for irises of aril ancestry.
The greatest challenge in growing pure arils is to arrange proper conditions for their summer dormancy. After they lose their leaves, the rhizomes need to be in warm, very dry, well-drained soil until new growth begins in the fall. In desert regions, these conditions can be obtained without much trouble: raising the beds a bit and mixing in coarse sand may be all that is needed. In climates with a bit cooler, wetter summers, aril growers may need to erect translucent coverings to deflect rain, or even dig the rhizomes and store them indoors over the summer.
If they are not infected by rot during their summer dormancy, pure arils may grow rampantly in the fall and then again in the spring. It is not unusual for a single rhizome to present a dozen fans and perhaps even three or four bloom stalks by bloom time. The gardener, however, should not be lured into overconfidence. The initial burst of growth may leave only a few viable rhizomes after bloom is through, or may produce a clump so dense and lush that air circulation and drainage are impaired, inviting rot the following summer. The careful aril grower examines each clump individually to assess whether division would be beneficial. Much remains mysterious about the cultural preferences of the different aril species. In their native habitat, many of them are found in the vicinity of desert shrubs. Does this provide protection from foraging animals, or perhaps ameliorate the summer heat a bit? The "bursty" growth of arils is perhaps an insurance against harsh and unpredictable turns of weather, and may not serve them so well in the more controlled environment of a garden. It is also unclear how important seed propagation is for arils in the wild.
In the early decades of the twentieth century, many arilbreds were almost as difficult to grow as their aril parents. The diligence of breeders, coupled with natural selection, has changed this. Today, even irises of half aril content grow quite easily throughout the US and in other countries with similar climates. There are, of course, exceptions; a grower may discover a mismatch between a particular cultivar and the local climate. There is no magical way to know in advance how a particular cultivar will perform in your area: some experimentation is indispensable in finding the best growers for your own garden.
Knowing the cultural requirements of the pure arils, one can take a few basic steps to improve the rate of success with arilbreds. If you have a choice of planting locations, arilbreds should be placed where light and air circulation are best and where drainage is particularly good. Take steps to avoid or reduce excessive soil acidity. Don't make the mistake of coddling them in a sheltered corner for protection from winter cold; such locations may be shadier and damper during the summer months, and lead to more harm than good. It will not be necessary to dig them or protect them totally from rain during the summer, as most arilbreds do not go completely dormant and are not so vulnerable as the pure arils. However, it is still wise to practice very clean culture and keep an eye out for densely overgrown clumps that could benefit from division. Plan on dividing arilbreds every other year; you may even find a few benefit from annual division!
In general, arilbreds of less than half aril content (this includes most arilbred medians) are to be grown exactly like the bearded irises. Giving them special treatment is unnecessary and may even be harmful, if it causes you to depart from tried and true practices that your bearded irises thrive on.
Those of more than half aril content should receive some preferential treatment. They should not require the full-blown summer protection preparations demanded by the pure arils but will appreciate the best drained, most open location your garden can provide.
Alas, the modern arilbreds, which are not much different from bearded irises in their cultural requirements, suffer under an unfair reputation. Arils, indeed, are frustrating to grow in most climates, but arilbreds are not. A willingness to experiment, and a little common sense in addressing their preferences, particularly in planting location, is all that is required to enjoy the magnificent and distinctive beauty of the arilbred irises.
Unless otherwise noted, all text and illustrations copyright Tom Waters and all photographs copyright Tom or Karen Waters. Please do not reproduce without permission.