The bearded irises are a common old-fashioned flower found in many gardens and landscapes throughout Kentucky. They are very easy to grow perennials that do best in full sun and well drained soils. There are several classifications of the bearded iris from miniature dwarf, standard dwarf, intermediate and tall. The tall varieties are the largest group having thousands belonging to it.
While they are easy to grow, they still can have a few problems if not cared for properly. The iris borer larvae can invade the rhizome by tunneling through it allowing for bacteria to enter. This bacteria usually will result in bacterial soft rot, a very pungent smelling disease.
To prevent bacterial soft rot, it is important to use an insecticidal spray of Sevin or Malathion in the spring when the plants are about 3” tall and repeat spray weekly for 2 weeks.
Sanitation is also key to keeping this disease under control. Foliage should be cleaned up in the fall to prevent laying of eggs by the adult iris borer moth for the next year. It is also important to note that the iris prefers to be grown in a bed without mulch covering it, so it would be very beneficial to use a pre-emergent weed control regularly.
To help keep the iris rhizomes healthy it is important to remove declining blooms to keep seed from forming. If seed is allowed to form then the rhizome will have less production of stored food that can decrease the bloom production the following year.
Irises should be fertilized yearly in the spring when the foliage starts to grow. A general rule of thumb is to fertilize with a complete fertilizer such as 10-10-10 or 12-12-12 at a rate of 1-2 pounds per 100 sq.ft. This spring fertilization is the only one needed to help have a healthy rhizome.
Spring blooming irises can be divided in August/September if they are getting to thick in the area you have them.
If you are an iris enthusiast check your area to see if there is a local iris society or a Master Gardener group that may have a member that is into raising irises to get some varieties you may not have – if they are willing to share their rhizomes. Also, many Master Gardener groups and local garden clubs sponsor spring and fall plant exchanges so you may want to check those out to see if there are any irises at these events to add to your garden collection.
Submitted by Lori Bowling, Agent for Horticulture, Boyd Co. Cooperative Extension