When I dug my yellow flag I didn't even know I had any blue flag in the plants but they did show up, but after 15 years they are still almost non existent. I've got a couple of plants on each side of the pond, one side I planted on and the other side one just showed up. So in 15 years of growth I doubt if I can find 6 plants around the pond. Now the yellow flag's are much more numerous but not what I call invasive. The word invasive would be in the eye of the beholder I guess. The strange thing about the flags is I planted most of them the on the west side of my pond and I'm not sure I can find one on that side to this day but most of the rest of the pond has yellow irises. So I am guessing it was seeds from the original planting before those plants died out that populated the rest of the pond. The prevailing wind is from south west and I have always thought that is what spread the seeds mostly to the north and west of the pond.
