Answer: Most ornamental grasses have a clumping habit rather than a running habit so your fears may be unfounded. (Bamboo is a running grass and is quite invasive, which might be where the rumor started.) Some grasses spread by seed, so cut the seedheads down before they have a chance to spill their seeds in the garden.
If you're still concerned about containing the roots of your plants, you can plant them in 3- to 5-gallon pots and bury the pots in the garden. Make sure the pots have adequate drainage holes in the bottoms, and bury the top edge of the pot under 2 to 3 inches of soil so they share in the water and fertilizer you apply to the rest of your garden. You can dig and divide the plants every three to four years so they won't become rootbound in their subterranean homes.
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