Window boxes ought to keep even mint out of trouble.

I'm reminded of a discussion on some bamboo vendor's website about how "easy" it really is to contain running bamboo.

"Just" excavate a moat around it and pour a foot-thick concrete barrier with no openings ...

Then there are those solid stainless steel canisters that they ship plutonium in ... one of those might contain a mint ...

Great suggestion! I'll keep my eye out for one at the second hand store.