Answer: Based on your description I am not certain what is happening, partly because I am not sure what kind of iris you have. Generally speaking bearded iris look somewhat disheveled by this time of the summer and can look pretty shabby especially after being dug and divided. The foliage should be cut back to about six inches when you replant; the rhizome should be at or very close to the soil surface. The soil should be kept moist but not saturated or sopping wet until they are rerooted. If covered with soil and kept moist as a result, the rhizomes will rot and then you would see yellowing and dieback in the foliage. There are also some pest and disease problems that can cause discoloration but if they were healthy up until you divided, then I would suspect watering/planting depth to be the problem.
Siberian and Japanese iris should be replanted at the same depth as they grew before. If too deep they could suffer rotting at the crown and this would show as discoloring foliage. The soil should be kept moist while they become re-established; if too dry they will not root well and drying foliage will be a symptom of that. (These are usually better divided in the fall when there is naturally more rain and temperatures are cooler.) If it is one of these, you should trim back the discolored foliage and make sure you are watering correctly.
You water to supplement rain, if needed, to keep the soil moist but not sopping wet. Use your finger to dig into the soil and see if you need to water. If it is still damp, do not water yet. When you water, apply it slowly at the roots so it soaks in, watering deeply helps them root deeply as soon as possible. After watering, wait a few hours and then dig down to see how far the water went; sometimes it can be surprising. It is better to water deeply less often than to sprinkle lightly daily.
You can also use an organic mulch in a flat layer several inches deep over the root areas but not touching the crown or covering the rhizome. This will help keep the soil evenly moist, hold down weeds, and also feed the soil slowly as it breaks down over time.
I hope this helps you trouble shoot. Good luck with your iris!
Q&A Library Searching Tips