The following is derived from information on The American Iris Society (AIS) website at www.irises.org. The AIS divides irises most often used as garden plants into three main groups: Bearded Irises, Aril Irises and Beardless Irises.
Beardless Irises normally bloom after Tall Bearded Iris (except for the Pacific Coast Native which blooms before the TBs). There are 6 types of irises classified as Beardless Iris: Spurias, Siberians, Japanese, Louisianas, Pacific Coast Natives and Species. Because the majority of these bloom after the TBs, they often extend the iris season in the garden.
1. Spurias (SPU) are between 2 and 5 feet tall and very elegant-looking. The bloom shape is often like an orchid. Spurias come in many colors and often have a bright yellow signal.
2. Siberians (SIB) come in multiple colors: blue, purple, red-violet, yellow, brown and orange. They prefer cooler temperatures, moisture and like a slightly acidic soil. Their blooms can be upright or round and flat. They look best when grown in clumps at a height of 2 to 4 feet. They usually bloom right after the TBs and their grass-like foliage is an attractive garden feature.
3. Japanese (JI) iris also prefer slightly acidic soil. Their blooms are gorgeous – huge, ruffled and flat. Japanese bloom about a month after TBs.
4. Louisianas (LA) are native to the U.S. Gulf Coast area. They perform best in somewhat acid soil and prefer to be wet in the spring. Blooms have brightly colored style-arms and also signal-crests; they are usually wide open.
5. Pacific Coast Natives (PCN) are normally grown in the far western part of the U.S only due to climatic conditions. They are usually one to two feet tall and are graceful, dainty flowers.
6. Species iris have 2 classifications: I. confusa and I. missouriensis. The latter prefer wet springs and dry summers; we often refer to them as Blue Flag or Rocky Mountain iris. The former want conditions similar to azaleas in a frost free climate.