The last week of January to first week of February is the best time to prune ornamental grasses.
New spring growth will not emerge properly and abundantly if plants are not cut timely and severely. You can cut with sharp power or manual hedge shears, or a serrated bread knife.
Cut straight across, parallel with the ground, 3” to 4” above ground. Do not leave a nub, mound, cone, or pouffe, just a straight-across flat top, close to the ground.
You can follow up in late February or early March with a small amount of fertilizer, but be stingy, as excess fertilizer will cause ornamental grasses to grow too succulent and too fast, and they will flop over instead of standing up.