Dividing Iris by Ashley McNamara

Unknown bearded iris cultivar. Photo by Tina Ligon.

Here in the arid West, as far as landscape plants go, it's hard to beat iris. The numerous species and endless cultivars have flowers in virtually every shade of color imaginable (except for bright reds and fuchsias); the distinctive foliage adds texture and interest to the garden space even when the plants aren't in flower; and those big ol' rizomes at the base are the plant kingdom's answer to a camel's hump, storing moisture and nutrients for the hard times. Iris, once established, are almost carefree. They should be deadheaded once they are done flowering, and they like a bit of bone meal worked into the soil and some mulch in the fall. Oh, and one other thing: every once in a while, individual clumps of iris should be lifted up out of the ground and divided.

This is because iris, as they grow, have a tendency to grow out from the center of the plant, leaving a clump of dead rhizomes in the middle. The good news is that it's easy to do, and it gives you a chance to move and spread more iris into different areas of the landscape, and also to share and trade different cultivars with your gardening friends. The best time of year to do this is after the plants are done flowering but at least two months before the ground typically freezes. Here in the higher elevations of Colorado, that generally means any time from the end of June until the end of August.




Dividing iris requires a few simple steps:
1. Select a large clump in which there is little vegetative growth at the center of the plant. Use a pitchfork or broadfork if you have one, as you are less likely to damage the plant by slicing through the rhizomes. Dig down several inches and lift the entire plant (or as much as you can get at once) out of the ground.

Overgrown iris clump. Note dead leaf tips due to drought.

Carefully dig down several inches and lift the clump. 

2. Shake the clump free of loose soil and carefully examine the rhizomes (theses are the beefy, tuberous parts at the base of the plant. With a sharp knife, cut off and discard any parts of the rhizome that seem hollow or mushy. It is okay to leave parts of the rhizome that do not have any vegetative growth emerging directly from them. These are referred to as "mother rhizomes" and even though they will never bloom again, as long as they aren't dead or diseased, they will continue to supply nutrients to the plant.

Large clump of iris rhizomes. 

Trim off dead rhizomes. 

3. Dust any cuts with fungicide powder (such as copper sulfate), and trim the leaves down to a fan about 6 inches long to prevent the wind from blowing them over once they are planted again. Leave as many of the roots (the long stringy things under the rhizomes) as possible. 

Dust any cuts or breaks with anti-fungal powder. 

Trim the foliage down to 6 inches. 

4. Plant the rhizomes in well-worked garden soil. Dig a hole and spread the roots well within it, then back-fill the hole, covering the rhizomes about halfway up with soil (about half the rhizome should be sitting on the surface of the soil when you are done). Water in well to help the soil settle, then add more soil and firm in well if necessary. Space the rhizome pieces at least 6 inches apart. 

Whether you are planting your rhizomes in a pot or in soil, water them well. 

Alternatetively, if you want to give away some of your iris rhizomes, plant them in a pot. Don't forget to water them and keep them moist as long as they are in your custody!