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Growing Aril Irises

Ten years after moving to the USA, I continue to be amazed at the number and variety of plants that thrive in the garden under conditions that conventional wisdom deems unsuitable.  Having gardened almost exclusively with bulbs and alpines in pots in the SE of England, it took a while to overcome the urge to ‘protect’ plants by potting them up.  The more special a plant was perceived to be, the stronger the desire to save it from the rigors of the open garden, even though in many cases the plants in question were eastern US natives that had spent thousands of years enjoying conditions similar to those offered by our new garden!  Now I am reborn I have no doubt that the best place for any plant is in the garden, as long as conditions can be provided that give it at least a reasonable chance of survival.  Finding such conditions is a matter of experimentation, they won’t be found reading a book.  Although my Oncocyclus and Regelia iris cultivation in pots was reasonably successful, there was always a feeling that they would do far better given a free root run and more consistent growing conditions.  This has indeed proven to be the case, and my methods for growing them are described in this article.

Many things changed as a result of our move, not the least being the amount of space we have to garden in.  That certainly provided a major stimulus to develop a garden, rather than a collection of greenhouses and cold frames, although the latter are still very valuable for bringing on seedlings and housing certain tender plants.  Here we have a little over 1.5 acres of land in Exton, SE Pennsylvania in the NE USA, about 35 miles west of Philadelphia.  Exton is in USDA zone 6b, winter minimum absolute temperatures can reach -5F (-20C), summer maximum is over 100F (38C).  During January and February several weeks usually go by without the temperature rising above freezing.  The ground freezes hard for a month or more in a normal winter, raised beds are blocks of ice, but frost is very rare because of the low humidity.  The latter is very low from late fall to late spring but is well over 90% in July and August, with accompanying temperatures in the 90-100F (32-38C) range.  This is an interesting experience!  Much of the winter can pass without snow cover, approximately 24" being the total depth of snow falling in an average winter.   However, we have had up to 66" and down to less than 3".  Ice storms can be as frequent as snow storms, but weather systems come and go very quickly.   Thus, the number of days with abundant sunshine is great, on average 5 or so days a week and, consequently, the conditions are excellent for growing 'in character' plants and bulbs, in particular.  Four well defined seasons is the norm, fall colors are spectacular and the growing season for hardy plants outdoors is very long, generally from late February through late October (and into December for some crocus species).  Writing this in mid-November we have at least 8 species of crocus in flower in the garden, with still more to come.  January is usually the only month between September and April that we don’t have a crocus in flower in the garden.  Winters with more prolonged and deeper cold mean we don’t see most snowdrops or the main flush of hellebores until late February, but we are spared the temperature fluctuations of an English winter that can fool plants into thinking spring has arrived in January. 

Our garden faces south and is situated just below the ridge-line on the northern side of the Great Valley that heads west from Philadelphia, and is thus sheltered from the worst of the prevailing winds.  It is around 700 feet above sea level, 300 feet above the valley floor, making air drainage excellent.  The soil is moderately acid and is superbly drained; in many places the ground is very rocky, with many exposed large rocks; a few pockets tend towards pure sand.

Whilst the climate and conditions are very definitely not favorable for the cultivation of high alpine cushions, which are rapidly reduced to fungus-ridden mush in the summer months, it became clear that they were perfect for growing a wide variety of hardy bulbs, including many irises.  The garden is home to a large collection of irises, including representatives of all the subgenera and sections of interest to rock gardeners.  Virtually all are grown outdoors, the exceptions being a few members of sections Hermodactyloides, Oncocyclus and Scorpiris that are likely to be tender or not of sufficient numbers to risk testing them yet.  The Oncocyclus irises include I. atropurpurea, cedretii, sofarana var. kasruensis and jordana, which are grown in pots in a frost-free greenhouse with forced ventilation.   The compost they are grown in is identical to that used for seed germination and is discussed later.  These species are kept well watered when in growth but dried off during summer dormancy, from June to mid-September.  They do not maintain quite the short sickle-shaped fans of foliage of those grown outside, but they make a passable attempt when grown without any shading from fall until early-spring, after they have flowered.  Even under frost-free conditions, the flower stems of Iris atropurpurea are susceptible to sudden low temperature shifts if grown too close to the greenhouse glass.  Four flower stalks collapsed suddenly in February last year after a warm day was followed by a very cold night.  This phenomenon has also occurred occasionally on Regelia species grown outdoors.

Aril irises that are grown outside include I. acutiloba, barnumae, darwasica, ewbankiana, gatesii, hoogiana, iberica ssp. iberica, iberica ssp. elegantissima, iberica ssp. lycotis, iberica x paradoxa, kirkwoodii, korolkowii, meda, medwedewii, paradoxa ssp. paradoxa, paradoxa var. Choschab, sari, sprengeri, stolonifera and urmiensis.   A number of others are coming along from seed.  These are grown in raised beds in full sun.  The raised beds are made from 8’ x 3” x 5” treated landscape timbers fastened together to fashion beds approximately 24’ x 4’ and around 12” high.  The price of treated lumber in the USA was also a pleasant surprise!  These are a convenient size to work with, and also ideal when it comes to making covers.  The beds are filled with pure coarse sand over native soil.  The source of the sand seems to make little if any difference to the plants, as long as it is coarse and provides adequate drainage and air-filled porosity.  Although numerous publications suggest incorporation of a wide range of materials, from gypsum to manure, I find the sand alone works exceptionally well.  It is also unnecessary to compact the sand, as has also been suggested.  As can be seen in the accompanying photographs, rhizomes are planted horizontally such that the lower portion is in the sand whereas the upper surface is exposed.  Friends gardening in drier climates may need to plant the rhizomes a little more deeply.  These are then covered with approximately 1” of 3/8” gravel as a top dressing.  I like to place the rhizomes so that the new growing points sit just clear of the sand surface, roots easily find their way down into the compost, in many cases way down, even into the sub-soil.   Oncocyclus irises behave much like Cyclamen graecum during ‘dormancy’, although the tops can go quite dormant in the height of the summer, the roots are always active.  Established clumps of Oncos have perennial roots that penetrate over 15” into the soil, still seeking some moisture, even during the hottest months.  It is no wonder that lifting and dividing clumps generally sets the plants back such that it can take a couple of years for them to fully reestablish. 

The summer months of high temperatures and humidity pose the greatest risk to the plants, especially during those summers with frequent and heavy rainstorms.  Thus, raised beds containing arils are protected from mid-June through mid-September.  Although I suspect protection is not necessary for many Regelias and some established Oncos it is necessary for to provide a dry dormancy for some of the choice Juno irises with which they share the beds.  The covers are 8’ x 4’ sheets of twin-wall polycarbonate framed out with timber to make them sturdier.  They are simply propped up about 12” above the beds, overlapped and sloped to ensure heavy rains from summer storms run harmlessly away – their sides are completely open to ensure excellent air circulation (and free access to the cats who love to bask underneath them).  Once on, the temperatures at soil level get well above 100F and the top few inches of the beds are very dry indeed.  Following removal of the covers in late September the plants receive water whenever it rains.  At this time I also provide a dusting of bone meal and in-line watering with a weak soluble low nitrogen fertilizer, which stimulates root growth but doesn’t encourage excessive top growth that would be more prone to winter damage.  Leaf growth at this time is nothing like that of the main spring flush.  Weak in-line fertilization is repeated a couple of times in early and mid-spring.  Plants grown hard are generally much less susceptible to a number of pests and diseases, and are more likely to be ‘in character’.  No protection from the elements is needed during the winter; the plants are left to take whatever weather the winter throws at them, and seem to come to no harm other than some cosmetic ice burn to the leaves.  It is interesting to note that the foliage of many winter-green irises, cyclamen and crocuses is impervious to damage that one might expect from extreme cold, but it is burnt by repeated exposure to snow and ice.  However, in the case of the irises, the damage soon becomes unnoticeable once they start back into growth in the spring, usually in March.  I have lost one or two premature and advanced flower buds to a hard freeze in early spring, but this is generally not a problem.  The main flowering period covers April and May.   Iris gatesii is always the last to flower and the time I collect its seed usually determines the time the covers go back on.

The Oncos and Regelias can be propagated by either seed or division.  Plants are easily divided and, although the process is simple, the timing is critical.  Mature clumps are generally divided every three years.  Freshly divided Oncos are exceptionally prone to bacterial rot for several weeks after division and replanting, infection appearing if they even get a hint of moisture, such as a period of high humidity.  Thus, freshly divided rhizomes are best planted in late September, when temperatures are starting to moderate and humidity is falling.  This time also coincides with the beginning of a period of active root and shoot growth, and the plants get their first watering following emergence from dormancy.

Although I have multiple clones of a number of species seed set is variable and usually poor; probably due to the lack of a suitable pollinator.  Seed has been set without artificial aid on Iris barnumae, gatesii, kirkwoodii and sari.  Many Regelias appear to set seed but provide me only with empty pods.  Seed is collected and surface sown as soon as ripe onto a compost of 50% BioComp BC5 compost and 50% super-coarse perlite.  The BC5 is a composted mixture of coarse peanut hulls and bark which is exceptionally well drained yet holds plentiful moisture and air without ever being waterlogged.  As a devout follower of the JI plus grit school in England, I had serious problems finding suitable composts in the US.  Had anyone suggested I’d be using such a bizarre mix in future I’d have considered them fit to be certified – but this is the only compost I use for all the bulbs and plants I grow, and it works wonderfully well.  Seed pots are covered with ½” of granite grit, watered, and placed on the floor of a greenhouse which is allowed to go no lower than a temperature of around 28F.  Germination of aril irises usually occurs during the fall of the year after sowing but can be immediate or delayed for at least 13 years.  Seeds collected in Israel in 1992 continue to throw occasional seedlings every fall!  Seedlings are left undisturbed in the seed pots for one year, fed with dilute liquid fertilizer with each watering and dried off to a degree during the summer.  I do find, however, that many seedlings prefer to keep ticking over during the summer rather than entering a true dormancy.  As they grow away in their second fall they are potted up singly, taking care to rescue and sow any ungerminated seeds, and after two years they are usually robust enough to be bare-rooted and planted out into the sand beds.

Without doubt viruses are the single biggest threat to the health of any iris collection and it is vitally important to eliminate aphid vectors (not just maintain them at a population level sufficient to feed predators – a currently recommended method guaranteed to ensure your entire collection is riddled with virus).  I find Marathon (active ingredient imidacloprid) is extremely effective and convenient to apply, in either wettable powder or granular forms, and has the bonus of remaining active for 3-6 months after a single treatment.  Using applications in both September and February I have thus far managed to keep virus and aphid-free stocks.

As is often the case with any specialist area of horticulture, complacency is the biggest killer and there is no substitute for constant observation and propagation.  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 the clump of I. gatesii bearing over thirty flowers that greeted me last spring made for one of life’s true highlight moments.  In addition to the pictures included with this article, many others can be found on my web site at