Many years ago I received a flat of Iris rhizomes from a friend when she was dividing clumps that had outgrown the area in which they were planted. I planted them in the flower border that runs across the back of our yard. For many years they have bloomed, each year putting on a larger show. The past two or three years I have thought that it may be time to divide and replant them. Unfortunately that thought happens when the iris is in full bloom. When the appropriate time comes to divide and replant I have usually moved on to the many chores of summer not the least of which are the high demands of the vegetable garden. But not this year. Iris can be divided any time from late July through September so I ask: is September too late to keep a resolution? I don’t think so. This year I divide (and conquer) the iris.
There are more than 200 species of iris but the most common in our area are the bearded and Siberian iris. Iris are easy to grow hardy perennials but if they become crowded then they are more disease and insect-prone and flower production is reduced.
I already know that the flower bed in our yard is a good spot for the iris as it is receives full sun and is well-drained. Selecting new areas to put the divided rhizomes into is not a problem. It is always best to have the new site ready to go so that the rhizomes aren’t drying out in the sun while a new plot is dug. The new hole should be about 5” deep with a small mound in the middle of the hole (you will see why in a bit).
Also, have ready a few marked trays or buckets to put the rhizomes into if you would like to keep track of the different varieties/colors. I have plenty of pictures of the Iris in bloom so that I have a good idea of where the colors currently are in the bed even though all that I can see right now are the leaves.
So, what are tools needed for this job? A spade or a digging fork, a pair of shears, a sharp knife, and a bucket of 1:10 bleach/water solution. The first step in the actual dividing process is to cut the leaves back to a third of their height using a pair of garden shears, trimming them into a fan shape. Be sure to dip the shears into the bleach solution often to avoid spreading diseases.
Next, lift out the entire clump that is going to be divided by getting underneath it with the spade or digging fork. You may need to work around it in a few areas to get under it.
Place the entire clump on the ground and take a look at it to decide where it should be divided. It is easy to see which rhizomes have new growth and which are no longer supporting any foliage. Using the sharp knife cut away each section of rhizome that will be replanted, dipping the knife into the bleach solution often to disinfect it. Each new rhizome section should have plenty of roots and a fan of leaves.
At this time it is important to check for iris borers and the bacterial soft rot that often accompanies them. Each rhizome should feel firm when pressure is applied. If the rhizome gives way easily then that is a sign that there may be a borer. The adult iris borer is a brown and grey moth. It lays its eggs on the iris leaves and plant debris at the base of the iris in the fall where they will overwinter and hatch into tiny caterpillars in April and May. The new caterpillars will crawl up the foliage, chew pinprick sized holes and begin to tunnel their way back down toward the rhizome. Signs of their feeding are streaks that appear tan or water-soaked. By mid-summer the borers can be up to 2” in length and have reached the rhizome. Their feeding allows the entry of the bacterial soft rot that turns the rhizome into a smelly, mushy, mess. Late summer will see the borers moving into the soil to pupate, emerging as adult moths in the early fall. There are some varieties of the Siberian iris that are more tolerant to a borer attack.
Scouting and sanitation practices can be the most useful controls. Look for the tell-tale signs of chewing damage and water-soaked streaks in the spring when it is easy to crush the insect while it is still in the leaf or remove the leaf entirely. If the plant has above ground symptoms in July then dig up and examine the rhizome for signs of borer activity, discarding any that are infested. In the fall remove all plant debris where the eggs might overwinter. If you have an infestation that you feel is severe enough to warrant an insecticide then acephate (highly toxic to bees when freshly sprayed and as a residue) and spinosad (non-toxic to bees when it is dry) are generally recommended. Here is a link to the Virginia Cooperative Extension fact sheet which also includes some images: Iris Borer
And now back to the rhizomes. It’s time to place them into the prepared holes, putting one rhizome section on top of the mound in each hole. Spread the roots out and down and fill the hole with soil. Be sure that you don’t bury the rhizome; it should still be visible from the surface.
The fans of leaves are usually planted so that they all face in the same direction. Water each plant thoroughly and in years such as this one, more than once. Newly transplanted iris may need a winter cover of straw to keep the newly planted rhizome from coming out of its new location due to the thawing/freezing cycles that can happen. Just be sure to remove the straw in the spring. These new plants may not bloom much the first year but after that they should be back to their normal, showy selves.