I put some apple mint on my allotment in a pot, sunk into the ground with the rim just above the level of the soil. I find that slows down the spread of plants with similar habits, but doesn't stop them spreading. Loganberry in particular is just about impossible to prevent from spreading. Anyway, this mint seems to grow slower than "normal" mint, although a stolon has still managed to grow over the rim and rooted in the surrounding soil, so make sure you leave a bit of pot rim as a little barrier. Is it normal for speciality mints to grow slower than standard mints?