Hi Canuckgardener, - I live out in Ontario and have much the same soil as you do. Over the past few years I have noticed that all of my purple Irises have transformed into yellow ones. Given that I am not particularly fond of yellow irises, I am sure that I have not planted any yellow ones, so this is not a result of any of the commonly proposed issues. Digging deeper into the biology, chemistry and genetics of flowering plants - I believe I can explain what has happened, but as to why - I am still searching. Flower colours are the result of different levels of 2 different "chemicals" - anthocyanins - which produce blue, purple, burgundy/red pigments and Carotinoids which produce yellows/oranges. If for some reason, the plant is not able to produce adequate amounts of anthocyanins, then flowers which were originally purple or burgundy will loose these pigments and become yellow if carotinoids are still being produce or white if they are not. If this is the case, then it is unlikely that yellow Irises will change colour. Most of the reports I have found are of Purple Irises becoming yellow or white. I would be interested to hear if anyone knows of Yellow or white Irises changing to purple or other colour transformations.
The question is - why do the flowers stop producing anthocyanins? This is the question that I am still searching for answers to. Given that we are both struggling with clay/alkaline soil, perhaps it is due to pH but other options include temperatures, water, trace minerals etc. If I find out anything else, I will let you know.
