Q: I have planted many types and colors of iris over the years. However, a problem arises after I have separated the roots. The next year when these flowers bloom, they have lost their color and bloom white. Do you know of any reason why the vivid colors have turned to white?
A: You are not the first person who has reported this phenomenon to me. In fact, there is a long discussion regarding it at www.hort.net.
One theory is that white iris are more vigorous than other colors due to requiring less energy for coloring their flowers. Eventually they might overwhelm the other iris in a bed and the flower color scheme would become predominantly white.
You probably selected the strongest rhizomes when you divided them, inadvertently favoring vigorous white iris.
Another theory is that your iris could be forming seed and genetically reverting to their parents’ color: white. This is not likely since iris do not readily produce viable seed.
Genetically speaking, the rhizome of an iris that produces a colored flower will continue to produce that same color no matter how many times you divide it.