I have one growing in an old cast iron pot out front. When I stuck it there it was only going to be until it grew a little. I forgot about it, it bloomed the following spring and has bloomed for me every yr since. I think it's been there about 4 yrs now.

littlebit

I have some non-historic TB's in 20" containers and their performance is not very good after 3 years. They travel too fast to the edges.

I have iris in pots. I find that initially they are OK. They do not get brown spot, borers, and rot is rare. They will even bloom in pots, but eventually, they don't do as well.
Perhaps a large pot would solve the problem. The root system is quite extensive, and you can tell from the root ball in a pot that they reach the edges pretty quickly.
Laetitia

I have some dwarfs in a containor and they are doing fine. They are in a very large containor though and have plenty of room for roots and rhizomes to spread for at least another year. Use the largest pot you can.

Mike