Hi Dee, I have kept my ornamental grass contained by cutting a large plastic trash can, making a ring of it (you should only need a 7" bands)
When I plant grasses I plant the grass in the band which I also included in the hole and leave the band about 1" above the ground it keeps it in the band no matter how big it gets. Also it keeps a space between the grasses planted near one another. Now remember you can use any size container you can cut.... wide, narrow or even square if you can cut a band from it you are styling.
Much depends on whether the plant spreads by seed or by root runners or by rooting aboveground stems.
For northern sea oats (seed) I pull unwanted seedlings whenever seen. For horsetail rush (roots) I plant it in a bottomless container like Bonnie suggests.
For mint (stem rooting) I plant it in containers or pull it out when it spreads too far.
Good advice. I would add that not all ornamental grasses are invasive but the maiden grass (Miscanthus) can spread fast and furious. If you grow annual grasses like some of the newer selections of Pennisetum
or fountain grass, cut off flowers once they begin to set seed. This will prevent them from spreading too much.
Thank you for all your help. I have to get out and do some work now! haha
I'm trying Stipa " Ponytails" and Carex "Prairie Fire" this year, never thought they might be invasive since I had such a hard time getting them going from seed.