The genus is widely distributed throughout the north temperate zone. Their habitats are considerably varied, ranging from cold regions into the grassy slopes, meadowlands, stream banks of Europe, the Middle East and northern Africa, Asia and across North America.
They are perennial herbs, growing from creeping rhizomes (rhizomatous irises), or, in drier climates, from bulbs (bulbous irises). They have long, erect, flowering stems, which may be simple or branched, solid or hollow, and flattened or have a circular cross-section. The rhizomatous species usually have 3-10 basal, sword-shaped leaves growing in dense clumps. The bulbous species have cylindrical basal leaves.
The inflorescences are fan-shaped and contain one or more symmetrical, six-lobed flowers. These grow on a pedicel or lack a footstalk. The three sepals, which are spreading or droop downwards, are referred to as falls. They expand from their narrow base into a broader limb (= expanded portion), often adorned with veining, lines or dots. The three, sometimes reduced, petals stand upright, partly behind the sepal bases. They are called standards. Some smaller iris species have all six lobes pointing straight outwards. The sepals and the petals differ from each other. They are united at their base into a floral tube that lies above the ovary. The styles divide towards the apex into petaloid branches (see pollination, below).
The iris flower is of special interest as an example of the relation between flowering plants and pollinating insects. The shape of the flower and the position of the pollen-receiving and stigmatic surfaces on the outer petals form a landing-stage for a flying insect, which in probing the perianth for nectar, will first come in contact of perianth, three with the stigmatic stamens in one whorl surface which is borne and an ovary formed of three carpels. The shelf-like transverse projection on the inner whorl under side of the stamens, which is beneath the over-arching style arm below the stigma, so that the insect comes in contact with its pollen-covered surface only after passing the stigma, while in backing out of the flower it will come in contact only with the non-receptive lower face of the stigma. Thus, an insect bearing pollen from one flower, will in entering a second, deposit the pollen on the stigma, while in backing out of a flower, the pollen which it bears will not be rubbed off on the stigma of the same flower!
Up to 300 species have been placed in the genus Iris. Modern classifications, starting with W. R. Dykes' 1913 book, have subdivided them. Dykes referred to the major subgroupings as sections, but later authors have generally called them subgenera, while essentially retaining his groupings. Like some older sources, the influential classification by G. I. Rodionenko removed some groups (particularly the bulbous irises) to separate genera, but even if this is done the genus remains large and several subgenera, sections and/or subsections are recognised within it.
There are six subgenera, of which five are restricted to the Old World, and the sixth (subgenus Limniris) with a Holarctic distribution; the two largest subgenera are further divided into sections.Iris subgenus Iris: bearded irises, growing from rhizomes.
Some authors regard the Snake's Head Iris as lying outside genus Iris, and classify it as Hermodactylus tuberosus.
Irises are extensively grown as ornamental plants in gardens. The most commonly found garden iris is the bearded German Iris and its numerous cultivars. Various wild forms and naturally occurring hybrids of Iris pallida and I. variegata form the basis of most all modern hybrid bearded iris. Median forms of bearded iris [intermediate bearded (IB), miniature tall bearded (MTB), etc] are derived from crosses between tall and dwarf varieties. Other iris types commonly found in garden are I. siberica and its hybrids (Siberian irises) and I. ensata and its hybrids (Japanese irises).
The bearded irises are easy to cultivate and propagate, and have become very popular in gardens. They grow in any good free garden soil, the smaller and more delicate species needing only the aid of turf ingredients, either peat or loam, to keep it light and open in texture. The earliest to bloom are the dwarf forms of Iris pumila, which blossom during March, April and May; and during the latter month and the following one most of the larger growing 'tall bearded' varieties, such as I. germanica, florentina, pallida, variegata, amoena, flavescens, sambucina, neglecta, ruthenica and their modern hybrids.
A true red standard, tall bearded Iris remains an unattained goal of frequent hybridizing and selection. There are species and selections thereof, most notably Iris fulva, which has a relatively pure red color. However, getting this color into a modern Iris breed has proven very difficult, and thus, the vast majority of Irises are in the purple and blue range of the color spectrum.
The section Iris subgen. Iris sect. Oncocyclus contains the cushion or royal irises, a group of plants noted for their large, strongly marked flowers. Between 30 and 60 species are classified in this section, depending on the authority. Compared with other irises the cushion varieties are scantily furnished with narrow sickle shaped leaves and the flowers are usually borne singly on the stalks. The closely allied Iris subgenus Iris sect. Regelia, includes several garden hybrids with species in sect. Oncocyclus, known as "Regelio-cyclus" irises. They are best planted in September or October in warm sunny positions, the rhizomes being lifted the following July after the leaves have withered.
Iris unguicularis (syn. I. stylosa) is a late-winter-flowering species from Algeria, with sky-blue flowers blotched with yellow, produced (in the Northern Hemisphere) from November to March or April. Japanese iris (Hanashobu, Iris ensata var. ensata, syn. I. ensata var. hortensis、I. kaempferi) is extensively cultivated in Japanese gardens.
Many other smaller species of bulbous iris, being liable to perish from excess of moisture, should have a well-drained bed of good but porous soil made up for them, in some sunny spot, and in winter should be protected by a covering of half-decayed leaves or fresh coco-fibre refuse. To this set belong I. milifolia, I. junonia, I. danfordiae, I. reichenbachii and others which flower as early as February and March.
The cushion irises are somewhat fastidious growers, and to be successful with them they must be planted rather shallow in very gritty well-drained soil. They should not be disturbed in the autumn, and after the leaves have withered the roots should be protected from heavy rains until growth starts again naturally.
The artist Vincent van Gogh painted several famous pictures of irises.
The artist Philip Hermogenes Calderon painted an iris in Broken Vows (1856). He followed the principles of the Pre-Raphaelite Brotherhood. An ancient belief is that the iris serves as a warning to be heeded, as it was named for the messenger of Olympus. It also conveys images of lost love and silent grief, for young girls were led into the afterlife by Iris. Broken Vows was accompanied with poetry by Henry Wadsworth Longfellow when it was first exhibited.
In water purification, Iris pseudoacorus is used. The roots are usually planted in a substrate (eg lava-stone) in a reedbed-setup). The roots then filter the water by consuming nutrients within the water.
The iris is the symbol of Brussels, since historically, the important Saint Gaugericus Island was carpeted in them. The iris is now the sole feature on the flag of the Brussels-Capital Region. The Iris is the state flower of Tennessee.