|Iris germanica can bloom from April thru June. Check your local Farmers Market for fresh stems.|
Durham Co. Extension Master Gardener
Irises are native to the Northern Hemisphere and grow to their southern limit on North Africa's coast. The northern limit of irises is in northeastern Asia and Alaska. Drawings of iris have been found in Egyptian palaces. The word ‘iris’ is derived from the Greek word for rainbow and the Greek Goddess Iris was the Goddess of the rainbow. In addition to wheat and olive oil, the ancient Romans traded irises and other ornamental plants throughout the Roman Empire. The Fleur-de-lis, the recognized national symbol of France is a representation of the iris.
Iris germanica, native to central Europe, is a perennial with thick, fleshy, underground rhizomes and sword-shaped, evergreen leaves. Each year underground buds (new rhizomes) develop from the original rhizome. These new rhizomes produce a large fan of leaves and several flower stalks. Each rhizome will send up flower stalks one time and afterward will concentrate on producing new rhizomes.
The flowers have six petals. The three upright petals are called ‘standards’ and three hanging petals are called ‘falls.' A fuzzy line running down the middle of the basal portion of each fall gives the flower its name, “bearded” iris. There are thousands of cultivars with a rainbow of colors including blue, pink, purple, red, white and yellow. Most bearded iris bloom in April, May and June but the time of blooming is dependent on the cultivar. There are certain cultivars that bloom again in summer and fall. Irises are classified into miniature dwarf, standard dwarf, miniature tall, and tall ranging from 8 to 38 inches in height.
Bearded iris are best planted in July through September with rhizomes spaced 8 to 10 inches apart. The rhizomes should be planted at or just below the soil surface. The roots are underneath and penetrate into the soil below. The soil should be loamy and well drained with a pH between 6 and 7. Poorly drained soil or planting rhizomes too deep can cause them to rot. While iris can tolerate partial shade, they do best when planted in full sun. Excessive shade will reduce or even prevent flowering.
Fertilization should be done in moderation. While nitrogen, potash, and phosphorus are essential for iris, excessive nitrogen promotes lush growth that makes the plant more susceptible to rot diseases. A soil test should be used to determine fertilizer amounts. Usually a low-nitrogen fertilizer such as 5-10-10 is recommended. Fertilizer can burn the rhizomes and should not be applied directly on them but around them. Plants that are growing well usually do not need fertilizing. Flower stalks should be cut back to an inch or two above the rhizome after the blooms fade. In early fall, cut the leaves 6 to 8 inches from the ground.
Because the rhizomes continue to bud and propagate, iris generally become crowded and need to be divided every 3 to 5 years. A decrease in blooming or rhizomes that are being pushed out of the ground are signs it is time divide and replant. Iris are best divided 4 to 6 weeks after the flowering period but may be divided any time. The leaves are cut to about one-third their length, the clump is dug up and the rhizomes washed free of soil. The rhizomes should be examined and any section not having firm white roots should be discarded. The rhizomes are cut apart so that each section has at least one healthy fan of leaves and healthy roots.
o Bacterial soft rot: Use of fresh manure or excess nitrogen, coupled with poor drainage, contribute to soft rot development. Bacteria enter through injuries or cuts to the rhizome. Rhizomes become mushy and have a disagreeable odor. Diseased rhizomes need to be removed and destroyed.
o Mosaic: A viral disease that causes a mottling of leaves and flowers. Since it is transmitted by aphids, aphid control may be a preventive measure. Infected plants should be removed and destroyed.
o Iris borer: The larval form of a moth, which chew into the leaves, eats its way down inside the leaves to the rhizomes, reaching a length of about 1 to 1 ½ inches. Borers often will hollow out whole rhizomes causing fans to collapse and the remaining tissue to rot. Iris later develop loose, rotted bases and holes in rhizomes. Bacterial soft rot readily attacks borer-infested plants. Old leaves, stems, and plant debris should be carefully removed and destroyed in the fall. A registered insecticide can be applied to the rhizomes in the spring as new growth occurs.
o Crown rot fungus: A fungal disease causing rot at the base of leaves where they join the rhizome causing them to fall over. Reddish-brown "mustard seeds" which are produced by the fungus are the diagnostic finding. Trim leaves to admit more sunlight and air movement to the rhizomes; carefully remove and destroy all diseased leaves.
Dried rhizomes of I. germanica are traded as orris root and are used in the production of perfume. Iris essential oil from flowers is sometimes used in aromatherapy as sedative medicines. Gin brands such as Bombay Sapphire and Magellan Gin use orris root and sometimes iris flowers for flavor and color.