I was recently asked both of the following questions:
“I have a bed of what was purple bearded iris. All of them have become very light in color. Is there an amendment I need to add to the soil? Why this has happened?”
“I planted a new bed of different colors of bearded iris a few years ago and they were beautiful. But this year they are looking as if all have turned to a whitish blue.”
I have had the same experience, not only with iris, but also with gladiolus. I did some research and confirmed what I was thinking. The flowers are not changing color. So why do beds of say, purple iris often become predominantly a less desirable color? What most likely happened is that the purple irises were overtaken by much more vigorous colors such as the white or light blue iris. Also the more vigorous white and blue irises will tend to bloom earlier than other colors, giving the impression that a whole garden bed of iris have changed color. A few weeks later, when the other colors start blooming it is more apparent what has happened.
Also, you can’t change the color of an iris by changing the soil ph., or anything else in the soil. That sometimes works with hydrangeas, but not with any other plants that I know of. The color of an iris, for example, is a result of its genetics. The rhizome of an iris that produces a purple flower will continue to produce that same color.
The best way to fix this problem if you want to return your bed of iris to the colors you prefer is to address the problem when they are blooming. Dig out and discard the white and light blue iris that you don’t want while they are blooming and easy to tell from the other irises. When the colors you prefer start to bloom, mark these with a wrap of green gardening tape, a twist tie, or some other method that will allow you to identify them after the blooms are gone. Several weeks after blooming has finished, did up and divide the whole bed, saving and replanting the iris you marked as desirable.
For the next few years you should have the iris colors you prefer. Bits and pieces of the more vigourous white and blue iris are sure to remain, however and after a few years you may need to repeat the process.