Dividing Bearded Irises

  • Difficulty Rating: Beginner
Dividing Bearded Irises

The bearded iris is a favorite around our house. It's unanimous that the very large, unique blooms are beautiful especially in large groupings. And our son, who tends to the needs of all of our flowers, loves the incredibly easy maintenance, from the basic watering/feeding to the dividing of the plants when it is needed.

The bearded iris is a favorite around our house. It's unanimous that the very large, unique blooms are beautiful especially in large groupings. And our son, who tends to the needs of all of our flowers, loves the incredibly easy maintenance, from the basic watering/feeding to the dividing of the plants when it is needed.

The bearded iris is a perennial so it comes back year after year without the need for you to plant anything. It is extremely cold tolerant, especially when you consider that its rhizome, the thick stem-looking part that the plant grows from, lies just beneath the surface of the soil. It is also extremely heat and drought tolerant. It typically blooms in during the spring months, from April to June, but some varieties rebloom later in the summer. The care outlined in this article refers to those that are not reblooming, or remontant, irises.

The basic needs of the bearded iris couldn't be simpler. It loves sunlight. Plants that are too shaded may not bloom, so full sun is best. The rhizomes will rot if they are exposed to too much moisture so they prefer well-drained soil that is just moist enough to provide water to the roots. And feeding is optional. We do not fertilize our irises; however, light fertilization with a fertilizer that is lower in nitrogen and higher in phosphate can benefit them.
The most time intensive task in caring for the bearded iris is dividing the plants when they become too thick. Even this job requires very little of your time, per plant.

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Bearded irises need to be divided every 3-5 years. A bearded iris rhizome will send up a flower stalk one time and afterward will concentrate on producing new rhizomes. Ideally, most of the rhizome will lie just below the surface of the soil but those in this photo were completely exposed. This allows you to see what they look like when it is time to divide. When the rhizomes are not visible, you can envision how many there are radiating from a "mother" by how many fans, or groups of young leaves, there are radiating from a central location where there is no growth. Other signs it is time to replant is a decrease in blooming or rhizomes that are so crowded they begin pushing one another out of the ground.

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Dig the clump of rhizomes with a spading fork. If using a shovel, take care not to cut the rhizomes. I neglected to warn my son that he would be my photo subject so he was not prepared with proper footwear for digging!

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This is what the clump looks like after all the soil is broken away from the roots. The rhizome in the center of the clump has already produced a flower. There is no fan growing from it and you can see it has other rhizomes growing from it. The rhizome to the right of it is one that has rotted, as has the rhizome below it. See how they are shriveled. I checked them for iris borers and, fortunately, found no evidence of them.

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These rotten rhizomes need to be removed and discarded. Some people just break the rhizomes apart. We like to cut them to be sure no unhealthy parts are left behind. The Fiskars Powergear Bypass Pruners are my tool of choice for this. If you discover rotten rhizomes, it is a good idea to clean the blades with bleach water to prevent spreading any disease to healthy rhizomes.

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Another way to identify which rhizomes have already bloomed is to look at the bottom side. Those that have yet to bloom are covered with thick, healthy roots. The roots of the rhizome that is working on producing more rhizomes have shriveled up.

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Again, I used my Powergear Bypass Pruners to separate the new rhizomes from the old. Once all the new rhizomes have been removed, the central rhizome can be discarded. If there are any rotten rhizomes or those infested with pests, don't put them in your compost pile.

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While it's debatable that this is necessary, the fans of the rhizomes can be cut off 6 – 8 inches above the rhizome to prevent them from drawing as much moisture from the rhizome but still allow them to feed the rhizome through photosynthesis.

The ideal time to divide and replant bearded iris rhizomes is between July and September. As long as they are kept sufficiently watered while the weather is still hot and dry, newly planted rhizomes can tolerate heat. The rhizomes can also be divided and then stored until you are ready to plant them later in the season.

When planting bearded iris rhizomes, start by digging the soil 10 – 12 inches deep to loosen it. Dig a shallow hole in the loosened soil to accommodate the rhizomes. The hole should be wide enough to allow the roots to spread out. Rhizomes can be planted in a triangle formation to provide the look of a grouping. Just envision each side of the triangle being 12 inches long when spacing the rhizomes this way. Or the rhizomes can be planted in a line with 12–18 inches of space between them. Create a small mound in the center of the hole and center the rhizome over it so that the top of the rhizome will be just below the soil surface (or even slightly peeking through) and the roots will be buried deeper. Planting rhizomes too deeply can prevent them from blooming or cause them to rot. So unless you live in a climate with extremely hot temperatures, it's really OK for them to be slightly exposed! Attention should be given to the direction the fan is pointing so the iris grows pointing in the direction you desire.

Diligence in dividing your iris rhizomes every 3-5 years will leave you with big, beautiful beds of irises, or plenty of them to share with friends.