Answer: Ornamental grasses are attractive in the winter landscape so I'd leave them alone until spring. Not all ornamental grasses are winter-hardy and you won't know until spring whether or not your plants have survived so I'd recommend leaving them alone until spring. At that time you can cut the old foliage down or do as I do: comb through them with gloved hands, pulling up as you run your fingers through the foliage. The old blades will pull of easily, without harming the crown of the plant.
Best wishes with your ornamental grasses.
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