Approximately 300 species of rhizomatous and bulbous perennials, mainly from the northern temperate zone. They are tufted or clump-forming, with mainly basal, linear to sword-shaped leaves in a two ranked (distichous) formations, the sword-shaped ones forming fans. In the rhizomatous species the rhizomes range from short to long and thick to thin, and in a few cases underground stolons also develop, e.g. I. japonica. Bulbous species also vary in size and bulb shape and in the group known as Juno (Scorpiris) each bulb bears several swollen, tuberlike roots. These are additional storage organs and should not be broken off when transplanting or repotting. I. decora and its two allies are unique in having a bunch of swollen roots of this kind, being neither bulb nor rhizome. Basically, the flowers have a tubular base and six tepals but the form and arrangement of the latter is highly distinctive. The three outer and generally larger tepals are composed of a flattened stalk (or haft) and a blade-like tip. They often arch outwards or hang down and are known as falls. The three minor tepals may be erect, arch inwards or outwards and, very rarely, droop and are called standards. Alternating with the standards are three winged and often tepal-like style arms, each one sheltering a stamen. The proportionate sizes and shapes of the falls, standards and style arms varies greatly. In addition, both falls and/or standards may be bearded, that is, bear a zone of more or less fleshy hairs, often in a contrasting colour. A few species have crests of petal tissue on the falls.
Among the small species there are subjects for the rock garden, raised bed, scree, sink and alpine house. All have either beautiful or fascinating flowers. A sunny site is needed and for most species any well drained soil is suitable. Some species however, have more specialised requirements and these will be noted at the end of the individual descriptions. Where 'alpine house' is added at the end of the description this means that a warm dry summer rest is required to ensure survival and flowering. (The species concerned are mainly native to western and central Asia where the summers are hot and dry followed by some autumn rains, a cold winter and wet spring). A gritty, loamy soil is required with annual repotting in late summer and a good soaking in autumn to encourage root growth. Thereafter, little water is given until top growth commences, and even then it is sparingly applied until the main burst of growth from late winter to early spring onwards. Water is then given freely, plus a few applications of a low nitrogen liquid feed at seven to ten day intervals. Watering must be reduced after flowering and stopped as the foliage yellows. Propagation of rhizomatous species by division after flowering or spring, and of bulbous species by separation of clumps or removing offsets when dormant. Both types can also easily be raised from seed sown when ripe or as soon afterwards as possible. In some cases packet-dried seed may take up to twelve months to germinate or may do so erratically over a period of several years. In such cases it is worthwhile making a note of the number of seeds at sowing time which can then be checked against subsequent germination.
Being such a large and varied genus and for ease of classification, botanists have split it up into six subgenera which, where necessary have been further divided into sections. The following classification is adapted from that used in the standard reference work on the genus: The Iris, by Brian Mathew published by Batsford Ltd, 1981.
Subgenus 1 Iris. Plants with a well developed rhizome and falls and/or standards having a prominent beard.
Section A. Iris. Stems often branched, with three or more flowers, hairs of beard multicellular and seeds without an aril. Section B. Psammiris. Flowers one to three per stem, more than 5cm in diameter, various colours sometimes veined or spotted but never freely blotched or mottled, the falls bearded, seeds with an aril. Section C. Oncocyclus. Like Psammiris, but flowers always one per stem, not less than 5cm across, usually much more, seeds with an aril. Section D. Regelia. Flowers usually two per stem, various colours, sometimes veined or spotted but never blotched or mottled, both falls and standards bearded, seeds with an aril. Section E. Hexapogon. Flowers three or more per stem, various colours not mottled or blotched, falls and standards bearded, seeds with an aril.
Section F. Pseudoregelia. As for Section Iris, but flowers only in shades of lilac, purple or blue, prominently blotched or mottled, seeds with an aril.
Subgenus 2 Limniris. Plants with stout to slender rhizomes, sometimes stolon-like, falls without an obvious beard but sometimes having a coxcomb-like crest.
Section 1. Lophiris (evansia irises). Plants rhizomatous and stoloniferous, the falls with a cockscomb-like crest derived from petal tissue and not formed of hairs. Section 2. Limniris. A diverse group of plants mainly classified on the shapes and sizes of stigmas and seeds. It is the largest grouping of species and has been divided into sixteen series, none of which have clear cut separating features. The following species are representative of the section (not all are smah enough to be described here): douglasiana, foetidissima, fulva, lactea, longipetala, masia, minutoaurea, prismatica, pseudacorus, ruthenica, setosa, sibirica, spuria, tenuifolia, unguicularis and verna.
Subgenus 3 Nepalensis. The main feature of the three known species is a lack of bulb or rhizome, their place being taken by a bunch of tuberous roots. Two species have falls with a linear crest (/I. decora and collettii). Unlike most irises the flowers last less than one day. Subgenus 4 Xiphium. Bulbous rooted species rarely less than 30cm in height. They have slender channelled leaves and except for one species (I. serotina) produce beardless flowers. Subgenus 5 Scorpiris (Juno). Bulbous rooted species with the addition of tuberous roots. The channelled leaves are much broader than those of Xiphium and the flowers have very small standards, in some species reduced to tiny bristles. Subgenus 6 Hermodactyloides (Reticulata). Bulbous rooted species less than 15cm tall when in bloom with only one or two leaves which are more or less square in cross section (except II. bakeriana and kolpakowskiana, q.v.). Flowers fairly typical of the genus as a whole, except in I. danfordiae, where the standards are reduced to bristles.
The numbers and letters which start each species description refer to those used in the above classification.