I have 3 copies of the cultivar 'Peppermint Stick' growing in 5 gallon pots, and they will be put into the ground next month. From all I've read, it expands clump-size by rhizomes, but not to the extent of being "invasive."

Keep in mind, it's a 10'+ tall plant, and the girth of the clump will likely exceed 5' fairly quicky. You'll need to give it space.

So far, my potted plants have sent up new shoots, and all have been within 2" of the crown.

Thanks for the feedback. I have planted it in a corner of the garden behind the pond as a backdrop along the fence. I had visions of it filling 1/2 of my back yard. I feel a little better now.