My whole area tends to be a sort of honey hole where exotic plants grow wildly all over the place. It used to be a massive grass farm eons ago. Several years ago I became aware of a large patch of Giant Japanese Iris (japonica) that was growing in a ditch around the corner from me. I only notice because I saw the large purple flowers on top of 5' tall stalks, and knew where was no "weeds" around here, let alone anything native, that should put out a flower like that. Investigation led me to believe they were Giant Iris. So I dug up about a dozen of them, brought them home, and planted them in a little bed out back. They've since, apparently, spread.
A few weeks ago I was exploring my back property for a place to set up a new target shooting rig I had built, when I saw something familiar over on the edge of the property. It looked like long blades of grass but I knew it wasn't grass. It had an elegance and daintiness about it that looked familiar. I stuck my hand down in the mud and pulled one up, suspecting it was in fact one of my Irises. And so the root system said it was. I replanted it back where I found, keeping the area in mind. Yesterday I saw that my own Irises were blooming like crazy so I went to look at my new, young, wild patch. To my disbelief, they were flowering at a small age and size already.
Hard to see in this pic because of the vegetation, but look closely and you'll these long blades growing up out of the mud and water. It's a pretty massive patch! Edited: patch circled in yellow.
Some close-ups of a smaller patch that I closer to:
I have no idea how they got there, but there they are. Dead-ringers for their supposed parents, which are located about 100 yards closer to my house than these are. I can only assume a rain storm that caused minor flooding last year must have carries some plants/roots/seeds to that place in back where they now spread. This is exciting! These are rare and beautiful and tend to sell for a premium in shops!