In my experience, the ring of protection would have to be quite deep. I've found the rhizomes 12 inches or more below soil level in my garden. Don't know if this is uncommon behavior but it is what I've personally witnessed. I originally planted my common mw in huge pots but the roots eventually found their way through the drainage holes and into the garden soil. I cut off the top growth of shoots that come up in unwanted areas but I'm sure this is not ideal for many gardeners.

Mary