Answer: Although these plants are rated hardy into zone 5, it may be difficult to overwinter them in containers as opposed to when planted in the ground. The reason for this is that the top portions of plants are often more cold hardy than their roots, and the roots of container plants are exposed to much more cold than they would be when planted (and insulated) in the ground.
Probably your best option would be to keep them sheltered in an unheated cold frame or polyhouse over the winter. If that is not possible, I would suggest setting the pots into the ground up to the rim.
Alternatively, you could try to insulate the pots by plunging them to the rim in mulch, or wrapping them with bubblewrap, or some similar insulating material. The top of the plant should be left open however to avoid trapping condensation. It is also a good idea to set the pots in a location that is shady so that once they are frozen, they stay at an evenly cold temperature. (In a sunny spot, the alternating freezing and thawing on warm or sunny winter days can cause damage to the plants.)
Finally, and this is probably the best option in terms of survival, you could plant them.
I'm sorry I can't be more encouraging, unfortunately your climate is just very cold for container plants to try to withstand. Good luck with your grasses!
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