Has anyone just used rocks in a pot on a shelf in the pond for iris, pickerel rush or cardinal flower?
Welcome @SET to the GPF!
The answer is absolutely! Or just skip the pot altogether and plant directly on the shelf. A few rocks to hold the plant in place and maybe a handful of gravel to give the plant something to get started rooting into, and you'll soon have a naturalized plant.
The caveat is you may WANT some plants in pots - iris for example will grow large and set some massive roots down. If you fear you may not be able to control it, keep it in the pot.
I find cardinal flower grows much better at the edge of the pond on the land side - it likes damp, but not necessarily wet.
Go to the New Member forum and tell us a bit about your pond!