I have my own mint (regular and spear) kept in a plastic container on bricks off the ground to ensure it doesn't take over the garden. But we had some building work done last winter and the builder put (quite a small amount - maybe 2 wheelbarrow loads) of fresh topsoil in one area to fill in after works were finished. That builder is no gardener because the soil (I reckon removed from the next project just started) has proven to be full of couch grass roots and spearmint shoots as well which was only discovered after we had put some new plants into it. So have to do a big dig this autumn to clear and just pull all mint shoots as they poke their heads out inthe meantime.