During college I spent almost two years living on an iris farm. It was the perfect, country setting with abundant blooms every May that attracted hordes of spectators. I spent hours weeding the acres of iris (Iris spp.) rows. Living and working on the farm led me to a better understanding of the facts about these plants and the common misconceptions about the plant structures.
One of my first lessons was the term rhizome. If you look at the soil surface at the base of an iris you will likely see a lump protruding from the ground. These is the storage structure known as a rhizome which is sometimes mistakenly called a bulb. Many use the term “bulb” for any underground plant storage structure. True bulbs contain five major parts which make up an entire plant within this structure. A rhizome is technically an enlarged, underground stem that develops new plants on it and roots that grow underneath.
Some examples of plants that develop from rhizomes, aside from irises, include lily of the valley (Convallaria majalis) and canna lily (Canna indica). Plants that grow from rhizomes can be quite invasive. Bearded irises grow from rhizomes and have blooms with petals that cascade. The leaves are flattened in a fan-shape. The Dutch iris have similar care requirements to the bearded iris, but grow from bulbs. Dutch iris blooms tend to be smaller and lack the beard.
The bearded iris typically benefits from being divided every three to five years. An indication that it is time to divide is when you notice the number of blooms decreasing. Dig the entire clump of iris. Using a knife separate the rhizomes. A healthy rhizome will be firm and should have green leaves fanning out from it. Trim the leaves to about one-third of their height. The original rhizome can be discarded because it will not flower again.
Irises need sun and well-drained soil. Plant the healthy rhizomes no more than five inches deep. If planted too deep the rhizomes will rot. Rhizomes grow horizontally and may still be barely visible at the soil surface after planting. Space the rhizomes 18 to 24 inches apart with the leaf “fans” facing the same direction.
The most common pest for irises is the iris borer. The juvenile is a caterpillar which will appear in late July. It eats its way through the rhizome and then into the soil as it develops into an adult moth. When dividing irises, you can identify pest damage by pressing on the rhizomes and examining for soft spots. If you notice a pink caterpillar you should discard the rhizome to avoid the further spread of the iris borer. In the fall, remove the old iris leaves. This is where the iris borer lays eggs to overwinter and hatch in the spring.
For the first winter, cover the rhizomes with a layer of mulch. Remove the mulch in the spring to avoid rotting.
While most varieties of irises bloom once during summer, there are reblooming varieties that will give you a second show in early fall. With almost every color available, irises bring a great show to any landscape. Combined with the long history, these plants are a timeless classic.
Cynthia Domenghini is an instructor and coordinator for K-State’s horticultural therapy online certificate program.