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Iris do not need dividing every year. When a clump definitely appears overgrown or does not flower as profusely as it did the previous year, then it is time to divide. I divide my iris about once every three to four years.
Bearded iris and their relatives with rhizomes (underground stems) tend to be evergreen. Thus, they do not provide a handy guide for when they should be divided. Check your local nurseries; when iris rhizomes are for sale bareroot — late summer to late fall, depending on your climate — that is the time to divide the iris already growing in your garden. If you divide when the soil is still warm from summer but the air has begun to cool for the approach of winter, you will be replanting just at the peak time for disturbed roots to re-establish themselves without placing a severe demand on them to supply abundant moisture to the leaves.
For best results, do the whole task in a single day. Obviously with a large planting of iris, you can't. Then do one clump at a time, but finish that clump all in one day. Yes, the rhizomes can be stored for a while; but the survival rate is greatest if freshly divided plants are replanted immediately.
Use a spading fork to dig and lift a clump of rhizomes. You might have to break the clump apart to get it all out of the ground. I drag the clump onto a tarp, where I divide the rhizomes with pruning shears. Discard any old or mushy growths, any that smell bad, and any that have obviously bloomed already. Try to locate the newer side growths that have not yet flowered but that have about 2-4 inches of plump rhizome. Trim any broken or torn roots; trim the leaves to about 3-4 inches. (This is the only time the leaves should be cut. You cut them to reduce the demand for moisture, which the disturbed roots would not be able to meet.) Dust the cut end of the rhizome with sulfur to inhibit rot (even though iris prefers an alkaline soil). If I am going to store the rhizome without immediately replanting, I shake the whole plant in a bag of sulfur to dust it thoroughly.
I generally replant the divided iris in the same spot from where I just dug it. Spit the area where the rhizome is to be planted (dig it to the depth of the tines of the spading fork). Iris are shallow-rooted; double spitting is a waste of effort. Add some compost and stir into the soil. Avoid peat moss, which is acidic; iris prefer alkaline conditions. Dust the bottom of the planting hole with a generous amount of bone meal and stir it into the bottom soil; then cover that mix with enough soil so that the iris roots will not be in direct contact with the bone meal. Add no other nutrients that might burn the roots or promote the growth of foliage before the roots are ready to supply abundant moisture. (My soil is heavy clay, so I also stir gypsum into the soil.)
Plant the rhizomes at the surface of the soil. They can be covered with a slight amount of soil, but you should see the rhizome if you lightly brush the soil with a fingertip. 
The rhizome should be horizontal, which means the fan of leaves will be at an angle. The fan is at the end toward which the plant will grow; position the rhizome accordingly, aiming it in the direction you want future growth (not directly at the growing end of another rhizome). I generally plant three rhizomes — all the same variety — in each hole. I arrange them in a triangle, spaced so that the sides of the triangle are about a foot long, with the growing end of each rhizome pointing at the cut end of another. This might cause crowding and the need to divide sooner, but it also ensures that at least one plant will survive in that location. The roots should be spread out in the soil under the rhizome. After a hole is planted, carefully step on the soil around the rhizomes to tamp the soil. Then water thoroughly.
Aside from the bone meal at planting time, I give only a little fertilizer during the growing season after flowering is done. Iris are not heavy feeders. Since bearded iris are evergreen, I never remove foliage that is not dead. I do remove the flower stalks after all flowers have faded.
23 May 1999
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