Thanks for the question. Quick question and a request for you. What were the colors of the irises you planted near your house after you removed the yellow irises?   Also if the irises by your house are now blooming, could you please send us two or three pics of them?

Many thanks!

Thanks for your response.

You came up with the same conclusion that I was going to present to you. The vast majority of irises that come up in the spring arise from underground rhizomes. Irises have perfect flowers, which means that in each flower there are both male and female reproductive structures. Such flowers can pollinate themselves, usually when an insect pollinator enters the flower. Sometimes pollinators carry on them pollen from flowers of other plants of the same species. In such cases cross pollination could occur. But regardless of the type of pollination, this will only affect the resulting seeds and not the rhizomes. So even if cross pollination took place, it would not have changed the plant being pollinated. So I concluded that when you transplanted the yellow irises, one of more pieces of their rhizomes were left behind and that is what you are now seeing.

Interesting question and congratulations on solving it yourself!