Ornamental grasses are meant to grow and spread without requiring frequent trimming, like grass in the lawn, but they do require an annual clean-up to remove dead foliage. Flowers, seed heads and old blades that die must be cleared out to make room for new growth in spring. You can clear out the dead growth in late fall or early spring. Grasses that produce large, showy plumes, such as pampas grass (Cortaderia selloana), offer feathery texture to a bleak winter garden, so you might want to wait until spring to cut them.
Pull the living foliage back to gain access to the dead material within the ornamental grass. Have a helper hold the live foliage out of the way if the dead foliage is difficult to get to.
Grasp a bundle of dead growth in your hand and use hand shears or a sharp knife to cut through the dead grass blades. Cut about 6 inches above the ground or as short as 2 inches when dealing with low-growing ornamental grasses. Hold the bundle together while you cut so you can easily discard the dead grass without having to pick through the living foliage.
Gather another bundle of dead growth in your hand and cut it back to 6 inches. Repeat this process until you remove as much of the dead growth as possible.
Pick through the live foliage to find any remaining individual blades of dead growth and cut them back as you work your way to to opposite side of the plant. Add the dead plant material to a compost pile or a green materials waste bin.