My opinion is to use a wide 10 gallon pot to start, but eventually you will need to either dig up and divide, or repot. I divide my Leymus every two years, without a pot to constrict the growth. Never had a problem with it running on me to the point where I needed a pot. Either a bigger pot for you or try using bamboo shield, where the plastic liner is very thick and not subject to a quick biodegrade.

You could just chop out unwanted rhizomes each year when the ground is workable...it's unlikely to run REALLY fast if its only avenue of escape is the hole in the bottom of the pot. I have my Leymus planted in a large terracotta pot and its foliage looks great with the red of the pot. A beautiful garden addition if used with awareness, caution and inventiveness!

I think I will try moving it into a larger pot then. Maybe that will slow it down at least. I think the pot it's in now is only two or three gallons. I thought the smaller pot would keep it little but it has other ideas.