August and September are good months to divide your bearded iris. As irises grow they make huge clumps of plants and as they get crowded your blooms will get less. Many, old crowded iris beds have only a few blooms each spring, generally on the edge of the clump. Iris should be divided every 3-5 years to improve blooming and keep the garden bed from being overtaken. You may also want to divide iris to give some to friends or make another garden bed. If you need to move iris to another location in the garden August and September are also good times to do it.
Irises make new plants on rhizomes. Many people think these are roots, but the roots actually grow from the underside of the rhizome. Rhizomes are actually underground stems and you will see joints or nodes along the rhizome with leaves (or “fans” in iris jargon), popping up along each node. Rhizomes are tan, knotty looking and can branch off in unusual ways. There should be roots on the bottom of younger rhizomes. Each rhizome section with its “fan” can become a separate iris plant.
It’s pretty easy to divide iris and any gardener should feel competent to do it. You’ll need a good sharp knife to help divide rhizomes, some scissors to cut the iris leaves and something to put your divided rhizomes in. You may also want some common household bleach for disinfesting rhizomes.
Decisions about divisions
Start by deciding what you will do with the divided plants. If you need to prepare a new site you should get that done. Irises will “hold” outside of the soil for a few weeks but it’s best to get them planted as quickly as possible. Contact your gardening friends to see if they want some divisions if you think you’ll have too many.
New iris divisions will have flowers exactly like the plant you divided. If you want different colors or varieties of iris you may be able to trade your divisions for them. Place an ad or contact local garden clubs if you don’t have gardening friends to trade with.
If you need to hold the divided rhizomes for a few weeks wash the soil off them, let them dry in a sunny place for a few hours then put the rhizomes in a cool dry place. You don’t want new growth to start until you have them replanted so don’t store them in plastic bags or anything that retains moisture.
Here’s how to do it
Dig the whole clump of iris up. Irises have shallow roots and this is easy to do. Make a circle around them with a spade and then lift them up. If the clumps are packed together in a bed you may cut through some with your spade as you lift them but you will generally have plenty of good rhizomes left.
Put the clump of iris on a tarp, a board, or on cement and gently wash all the soil off the rhizomes with a garden hose so you can see what you have. Cut the iris leaves back to about 3-4 inches; it doesn’t matter if they are cut on a slant or straight across.
After a rhizome section blooms it will never bloom again. Examine the clump you lifted carefully. To determine which rhizomes sections are old you can look for the flower stem. Old rhizomes may also be devoid of roots and have tiny holes on the underside where the roots fell off. In a clump old rhizomes are generally in the center.
You’ll want to divide irises between joints, leaving each section with one or two sets of leaves and a healthy section of rhizome consisting of 2 or more joints. You can start new plants from a single node or section, but they will be smaller and may not bloom for 2 years. Sometimes you can snap the joints apart with your fingers but cutting is more precise.
If the old iris rhizomes have new sections of rhizomes with no leaves or very small leaves on them you can save the old rhizome and replant it with the young daughter plants. It will provide food for them until they grow more leaves. Otherwise discard old rhizomes that have bloomed.
Examine the rhizomes you are keeping looking for mushy areas or large holes in the top side of the rhizome. Large holes may indicate iris borers and there may be a large pink worm inside the hole. Those pieces should be discarded in the trash, not the compost pile. In the compost pile the pink “worm” can mature into the adult iris borer moth and infect your other plants. Soft, mushy areas indicate bacterial rot and should also be discarded.
Next add one cup of common household bleach, without scent added, to a gallon of water and soak the good rhizomes for 10 minutes. Remove, rinse with clean water and and allow them to dry in a sunny place for a few hours. This removes disease organisms. You can re-use the bleach solution for several batches on the same day. If you know the name or color of the iris you are dividing you can write that on the leaves of the divided pieces with a marker or add a label held on with a rubber band to the piece.
Replant the divided sections of rhizome shallowly, root side down, leaves up, with the surface of the rhizome just under the soil. Plant the rhizomes 1 foot apart. Iris bloom best in full sun positions. If the weather is dry water the replanted rhizomes once a week.
If you buy irises to plant or hold your divided rhizomes try to get them into the ground by mid-September in zones 5 and lower, the end of September for zones 6 and higher. This will make it more likely that they will grow well for you and bloom the first spring. Larger rhizome sections will probably bloom in the spring. Smaller sections may take two years to bloom. Fertilizing with a slow release granular fertilizer in early spring helps make strong plants and large blooms.
If you take good care of your irises and divide them faithfully you’ll have them around for years.
Here are some additional articles you may want to read.
Black spot on roses
Edible landscaping that provides fall color
How to grow heritage garden flowers
If you want to contact the author or sign up for her free weekly garden email newsletter you can write her at firstname.lastname@example.org