CHARLESTON, W.Va. -- In the spring, there's nothing like the tall, showy iris to add that punch of excitement to the garden.
Irises come in many different shapes, sizes and colors. There are two main forms: bearded and beardless.
Bearded irises are identified by thick, bushy "beards" on each of the lower petals of the blossoms. Originally, most bearded irises were native to Central and Southern Europe.
Beardless irises are mostly native to Asia and include Spuria, Siberian, Japanese and Louisiana irises. They all bloom after the tall bearded irises, extending the iris bloom season even longer.
According to the American Iris Society, now is the time to separate and to plant bearded irises for spring bloom. I found this information at www.irises.org:
"This digging and separating is best done between one and two months after bloom season, usually in July or August. Soon after this the irises grow roots that help to hold the plant firmly during the winter in areas where freezing and thawing can result in heaving the rhizome out of the ground."
Irises do best in sunny spots with well-drained soil. If the soil is clay, add very coarse sand and humus. Bone meal and a good garden fertilizer, low in nitrogen, are good for irises. The roots must be buried firmly to hold the plant in place, but the rhizome (the thick, fleshy root) should be near the surface. If you have several plants, plant them at least a foot and a half apart, "facing" the same way. The rhizomes will then increase in the same direction, without crowding each other too soon.
According to the website, culture of the beardless irises differs somewhat from culture of the bearded irises. They should be transplanted in the fall or in early spring. The roots should never be allowed to dry out while they are out of the ground and they should be watered heavily after transplanting. They should be set slightly deeper than the tall bearded.
Japanese iris should be planted in a distinct "depression" in heavy soil to assist in supplying moisture to the plant. Siberians and the Pacific Coast natives can tolerate light shade but the Spurias, Japanese and Louisianas demand full sun.
Louisianas and Japanese require moist conditions during the summer months. All, except Louisianas, should be planted in a permanent spot where they can remain for many years as they resent being disturbed. Louisianas tend to "creep" and therefore, should be tended to every few years. All are heavy feeders and need to be fertilized regularly.
Here's some good information from About.com on dividing irises: