My neighbor did that and the bottom of the pot eventually broke in the ground and she didn't know until it started taking over the whole side yard one spring.
I live next door and it spread into my yard as well and I am trying to rip it up and keep it contained as well.
I cut the bottoms out of 5 gallon joint compound cans, sink 'em in the ground with about 2 or 3 inches sticking up, and plant my mints in those. I have had one for six years with no problems. Every few years you have to refresh the soil and I pull up all the mint, throw away the dead stuff, trim all viable parts and replant. BYW, my lemon balm is not contained (didn't realize at first it was a mint) and I am keeping a close watch on it. I cut it back and cut away at the base of the plant regularly.
I have a friend that just brought me the mint that she promised me.. I am going to put it in a plactic pot and sink it into the ground...
:rose :dance :tools :weight
Just as an aside, the biggest flora-pest we have in our yard is the bronze fennel my wife has in her herb garden ... self sows like crazy. Or just maybe it is the silver maple trees in my neighbors yard that now have saplings growing out of every orifice in our house and yard. Talk about real pests.
Our apple mint is planted near some of my wife's invasive prennial flowers and it is interesting to watch them all fight it out. I keep thinking I should plant some Japanese knotweed there to add to the fray to see who wins.
Best applemint I ever had used to grow in my septic leach field ... and we ate it for years with no harm to us (and that was years ago). Just luck I guess.