How to Divide Irises With Rhizomes
Bearded irises are tall, elegant additions to the flower border, but they are also relatively high maintenance. You can help cut down on the incidence of soft rot and borer damage through regular division of the iris rhizomes every two to three years. This will also keep bearded irises performing and blooming at their best. If left undivided, flowering will decrease and the rhizome will be subject to more pests and damage.
Dig and Lift the Rhizomes
You can divide bearded irises anytime after flowering, through the month of August. Using a pitchfork, carefully dig around the plant, starting about a foot away from the outer-most edge. Try not to pierce the rhizome with the fork. Work the fork around the plant and gently lift the rhizomes out of the soil. Since bearded irises are grown at soil level, this is one of the easiest plants to lift.
Once you have the bearded iris rhizomes lifted, shake off any loose soil. Rinse any remaining soil with a garden hose. If you don't have space to do this in the garden, it is sometimes easier and neater to do it on a tarp. Rising off the soil will allow you to better see the rhizomes and roots to inspect for damage.
Once the rhizomes are cleaned, separate the individual rhizomes from one another. Don't break them apart; just loosen the already separate sections.
Cut the Leaves
Once the rhizomes are clean, cut the foliage to about 6 inches. Cutting the fan of leaves connected to a lifted iris rhizome makes the plant easier to work with when dividing and replanting and helps prevent water loss while the plant is becoming re-established. The fan does not need to be cut symmetrically. Some leaves may be damaged and will need to be cut shorter than 6 inches. The leaves will start to grow back, with the middle leaf growing tallest.
Inspect the Rhizomes
Once the rhizomes are clean, look for small to medium holes. These are telltale signs of borer damage. If your bearded iris leaves have dark streaks in them, you probably have iris borers, so look closely. Also look for soft spots, another common iris problem called soft rot.
Using a sharp knife or pruners, remove any traces of either iris borer damage or soft rot and dispose of these segments of the rhizome. Soft rot spreads easily, so disinfect your cutting tool with denatured alcohol between cuts to prevent further contamination.
Divide the Rhizomes
Find natural places to make a split, such as where the rhizome has forked. Study the rhizome and make sure each section you have chosen will wind up being at least 3 inches long and will have healthy roots growing from it. Then make a clean cut through the rhizome, using the same sharp, disinfected knife or pruner.
Get the Garden Ready
Choose a full sun location and start by digging a shallow hole that will be wide enough to spread out the rhizome's roots. Make the hole about 2 to 3 inches deep, then create a mound in the center of the hole to just about soil level.
Replant Bearded Iris Divisions
Soak the soil in the planting hole. Then take a rhizome division and place it in the center of the mound. Spread the roots around and down the mound. Cover the division with soil, being careful not to bury the rhizome with more than an inch or two of soil. Remember, it will probably settle a bit lower. The bearded iris will rot and won't bloom if buried too deeply.
Water well and do so weekly until you start to notice new growth.