Talking About Arilbred Irises

By Maggie Asplet

Aril and Arilbred Iris are not something that you would expect to see a lot of in New Zealand.  We are very fortunate to have Bill Dijk and his wife Willy, living in Tauranga, North Island of New Zealand.

Bill is one of those very lucky people with green fingers.  Whatever he does, it just works and is very able to get the impossible to be possible in our climate, with a lot of hard work I must say.

This article was been written by Bill and he has given me permission to publish his take on Aril and Arilbred Irises.

I. acutiloba susp. lineolate growing in coarse material.

When we talk about the aril irises, two very different types of irises are grouped together under the term "aril".   These are the oncocyclus and regelia irises of the Near East.  Although they have beards, they are not classified with the other bearded irises because they are so different in their makeup. Aril irises have derived their name from a little cream aril or a collar-like fleshy appendage of their seed.

Aril seed cut - showing the embryo

The arils show dark signal spots below the beards with much veining and speckling, in an unbelievable range of colours. Unfortunately, the arils are difficult to grow in all but the warmest and driest regions of New Zealand. I will start off by showing a few true aril oncocyclus/regalia species.  

Close-up of spcekling on I.samariae x I. hermona) X I. kirkwoodiae

In this century, hybrids were produced from crossing the arils with the more common bearded irises. These are called "arilbreds" (ABs), and are usually very easy to grow and still display the spectacular features of the arils. The arilbreds are as diverse in colour and form as they are in their genetic makeup and the combinations of these features make this an exciting and challenging group of irises. Unlike their aril ancestors, arilbreds can be grown successfully in a wide range of climates. They give gardeners the opportunity to enjoy the beauty of aril-type flowers without having to provide the special environment the pure arils require. They usually bloom earlier than the TBs, with the SDBs and the IBs.

I. Sheba's Jewel

Culture of Aril and Arilbred Irises

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.  These two types, however, are very different in their cultural requirements and their capacity to grow and thrive without special attention.

Growing the pure arils like the oncocyclus and regelia species successfully is a real challenge, and it’s often a question of understanding their cultural requirements and adjusts them accordingly. Not always easy with our sometimes excessive wet, and humid climate in the Bay of Plenty in New Zealand. A warm and dry climate like central Otago would be more suitable, somewhat similar to their native habitat.

I. paradoxa atrata. Note its small, dark purple falls.

Today's AB’s (arilbreds) are not hard to grow in most climates. A selection of arilbreds interspersed among a bearded iris planting will find that most of them will grow and flower without any special attention; however, some understanding of  their cultural preferences increases the odds, ensuring 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 descendants.

The aril irises are the oncocyclus and regelia species 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 don’t apologise for being temperamental, sometimes thriving for four or five seasons and then simply dying for no obvious reason. Regelia’s are much more adaptable, but still prefer dry hot summers.

Many different methods have been used for growing/protecting oncocyclus irises, especially during their summer dormancy when they must be kept dry. In cool, wet climates, most growers make use of a shelter/cover/frame, greenhouse or any other form of protection. I build this structure (picture) which is open on both sides for extra ventilation, and elevated bed for extra drainage, that is covered with polycarbonate plastic cover to keep the rain out.  I prefer to leave it on all season in our New Zealand climate to control the often excessive rainfall, warm temps and high humidity at the wrong time during the summer, which could results in rotting of the rhizomes.

Raised protected bed for Arils

This way I do control the cultural requirements like watering when needed, air circulation, feeding, and spraying for any fungal or insect problems.

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 as 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 benefits 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, preferably slightly raised location your garden can provide.

Preparing the site

For all arils: first and foremost, sharp drainage is important and the prime requisite for successful culture. They are desert plants, so they need full sun for at least two-thirds of the day. If possible, some protection, or shelter, from rain and cold is helpful.

Washed brick sand, granite, course pumice, or other coarse material, can be worked into the soil to improve drainage. There should be a good supply of calcium. (Gypsum can be used to provide calcium and loosen heavy soil.) If the soil is acid, lime should be added. Planting the irises on hills or ridges can help the drainage in marginal soils. Many people plant arils in raised beds where sharp drainage can be "built in."

To summarise:

•   full sun
•   sharp drainage
•   no water in summer for the pure aril irises while dormant.

Normally, the colours of aril blooms are extremely pure as well as clear. Alternatively, their blooms may even have wonderful blotches that contrast the colour of the flowers. When arils are hybridized with the standard bearded irises, the progenies retain a number of these attributes, while some other progenies may have new, but mesmerizing hues, patterns and streaks.

The falls of aril flowers have another typical characteristic. They may have veins and dotting or stippling in subdued or strong shades. These features may also appear in the standards of aril flowers. The dark, circular spots, also known as "signal", which appear at the end of the falls, are another typical trait of the flowers of oncocyclus irises, which distinguishes them from other iris flowers.

Iris mariae

Ideally, arilbred irises should be planted when they are just getting out of their hibernation or dormant period. You should avoid planting irises during the summer heat, as it is very stressful at this time of the year. Similarly, irises should not be planted during the late autumn just before the harsh winter months. In fact, the best time for planting irises is actually subject to the climatic conditions in your region.

Classification:  Nine Types of Arilbreds? Yes, Really

Although for awards purposes, the American Iris Society sorts all arilbreds into only two classes (less than 1/2 aril content and 1/2 or more aril content), the Aril Society International uses a more detailed system of categories that tracks not only the amount of aril content, but also the type of aril content (oncocyclus, regelia, or both).  

Close up of Iris paradoxa

An arilbred with only oncocyclus and bearded ancestry is an oncobred (OB). One with only regelia and bearded ancestry is a regeliabred (RB). If both oncocyclus and regelia ancestry are present, it is an oncogeliabred (OGB). This is by far the largest category.

If the arilbred has less than 1/2 aril content, it is marked with a "-" sign. If more than 1/2, with a "+" sign. If it has 1/2 aril content exactly, neither a "-" or "+" is used.

Aril seed as a rule are not easy to germinate, and there are several methods of germination:

Stratifying,  cutting and forced germination.  Aril iris seeds can be germinated with the following technique.

Forced germination" this is a technique that is often used for pure aril seeds to hasten germination. This method bypasses the need for any cold treatment.  The forced germination procedure involves cutting  with a scalpel or razor blade across the micropyle, across the end of the endosperm and embryo, in order to create an artificial rupturing of the micropylar barrier, which in natural situations germination could take a long time sometimes years to archive.  I use a special sharp grafting knife which I find personally more reliable and safer.

After soaking the seed for a few days in water (with some fungicide) to soften the seed, the aril and half the seed coat is removed, followed by cutting or slicing enough of the endosperm to expose the end of the embryo. I also borrow my wife’s art-craft 5X magnifying desk lamp with build-in lights for more close-up, hands free detail when slicing or cutting the seed.  

Most people do not do this with arilbreds, which germinate more easily.

Some people would try to stratify them and see what germinates first.  Sometimes temperature cycling is used as well. After all of that you could then try cutting or slicing them for faster germination. Be sure to sterilize the seeds before cutting them, especially for fungal protection when germinating seed in plastic bags or damp sterilized paper towels or whatever method you decide to use.

Points to recommend and remember:

1.   Hydration: Soak the seeds for up to a week in water with systemic fungicide.

2.   Remove the aril carefully (if it's an arilated species), cut the skin and slice a little layer to expose the embryo, which will be visible in the little hole of the endosperm.

Slicing aril seed

You need to be careful not to slice off too much of the embryo or you will negatively affect root formation and also risk damage to the embryo.

3. After cutting, put the seeds in damp perlite or vermiculate in little plastic bags.

I prefer damp sterilised kitchen paper towels for germination.

4.   When germination takes place in 2 or 3 weeks -

I prefer to very carefully transplant the little delicate seedlings directly into a 7cm X 9 cm peat pots, with a spray of systemic fungicide, outdoors in a cool, frost free place. Peat pots have the added advantage of no root disturbance when planted on into its permanent place or suitable container.

Having initial success with the germination, either forced or the traditional method is just the start of further necessary and ongoing special cultural treatment of the beautiful oncocyclus/regalia group.

After cutting/slicing I prefer germinating the seed in damp folded sterile kitchen towels, the moisture content when damp imho is just right for steady germination.  I then place the folded kitchen towels in an ice-cream container with the lid securely in place to prevent moisture loss, in a cool part of the nursery ( 10-15 C ). I inspect the seed at regular intervals for any sign of germination, with many seeds showing a radicle after 2-3 weeks in the damp kitchen towels.

I then proceed to very carefully prick out the sprouted seed, one at a time, in a 7cm X 9cm cm peat pot with 50/50 mixture of compost/fine pumice, water carefully to settle in the little seedling properly. Usually the seedlings ( 5-10 cm) will be ready for transplanting in its permanent place after 4-6 weeks. The very important advantages of the peat pots is no transplanting shock to the delicate seedlings, roots will easily penetrate the peat wall with no loss of growth. Don’t forget to spray the young seedlings with an appropriate fungicide at regular intervals for any possible fungal problems.

As is often the case with any specialist area of horticulture, complacency is the biggest killer and there is no substitute for constant observation, care and proper treatment.  The Oncocyclus and Regelia irises constitute an incredible group of plants that deserve nothing but the best.  The sight of just a single flower takes your breath away and a sight to behold.

To quickly summarise again :

    full sun.

    sharp drainage.

    dry for the pure aril irises, no water in summer while dormant.

These are some cultivation notes on how to grow the beautiful aril and arilbreds irises.

Editor's Note: For more information about aril and arilbred iris, contact The Aril Society International.  For more information about irises in general, contact The American Iris Society.