Iris come in different types, but all can be divided into two major types: bulbous and rhizomatous types.
A rhizome is a thickened stem that grows horizontally underground, producing roots along its lower surface.
The Bearded Iris is characterized by broad leaves, fleshy rhizomes and a beard or pattern of hairs in the falls (or down turned petals.) This group includes Bearded Iris, dwarf, border, flag and Louisiana Iris.
The Beardless Iris has narrower leaves, thick rhizomes and smooth falls. Included are Siberian Iris, Japanese Iris and many species Iris.
The Crested Iris has slender rhizomes and cockscomb-like crests on the falls, Included are iris Cristat and Iris Tectroum.
A bulb of the Iris is a modified bud surround by thick fleshy bulb scales. The purpose of the bulb structure is to act as a storage organ. Roots emerge from the bottom of the bulb when it is actively growing. Iris bulbs resemble an onion (Allium) bulb, having fleshy concentric inner layers and a thin outer layer. Commonly grown bulbous irises include Dutch iris, and the tiny Iris reticulata and Iris danfordiae.