Bearded irises (Iris × germanica) are tall, elegant additions to the flower border, however they require dividing to reduce the chance of soft rot and borer damage. Regularly dividing iris rhizomes every two to three years also keeps bearded irises performing and blooming at their best. If left undivided, flowering will decrease and the rhizome might be subjected to pests and damage.
You can divide bearded irises any time after flowering, through the month of August.
Equipment / Tools
- Gardening gloves
- Pitchfork or hand fork
- Garden hose
- Disinfected gardening scissors
- Disinfected pruners or sharp knife
- Garden trowel
Dig and Lift the Rhizomes
Using a pitchfork, carefully dig around the plant, starting about a foot away from the outer-most edge. Be careful not to pierce the rhizomes with the fork. Work the fork around the plant and gently lift the rhizomes out of the soil. Because bearded iris rhizomes grow very shallow near the soil surface, this is one of the easiest plants to lift.
Once you have lifted the bearded iris rhizome, 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.
Trim the Leaves
Once the rhizomes are clean and separated, use a pair of disinfected, clean gardening scissors to trim down the foliage into six-inch fans. Cutting down the foliage 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 might be damaged and will need to be cut shorter than six inches.
Inspect the Rhizomes
Inspect the rhizomes for signs of small to medium holes. These are telltale signs of borer damage. If the bearded iris leaves have dark streaks in them, the plant probably is infected iris borers, so look closely. Also look for soft spots, another common iris problem called soft rot.
Using a clean, disinfected 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 be at least three 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.
Prepare the Planting Location
Choose a full sun location and use a hand trowel or shovel to dig a shallow trench that will be wide enough to spread out the rhizome's roots. Make the trench about two to three inches deep, then create a soil mound in the center that reaches just about soil level.
Water the planting area thoroughly. Then, place a rhizome division in the center of the mound. Spread the roots around and down the soil mound. Cover the roots with soil, being careful not to bury the rhizome. The upper half of the rhizome must be exposed to sunlight in order to produce blooms. The rhizome will probably settle a bit lower as you water it, so be sure to lift it closer to the soil surface if you see that happening. The bearded iris will rot and won't bloom if the rhizome is buried too deeply.
Water well and do so weekly until you start to notice new growth. Once new growth starts, the foliage will grow back with the middle leaf growing tallest.
Fishburn, Jennifer. “Growing Irises: How to Plant, Grow and Care for Iris.” University of Illinois Extension, https://extension.illinois.edu/news-releases/growing-irises-how-plant-grow-and-care-iris