The best time to cut back your ornamental grasses is any time before new growth sprouts in the spring. If you haven’t done this yet, plan to do so in late February!

Ornamental or warm season grasses in the landscape
I thoroughly enjoy ornamental grasses in my landscape for their interesting foliage textures amidst my evergreen shrubs and perennials, and for their lovely seed heads in fall. I even like seeing their golden-brown leaves dotting the landscape in winter. My yard is home to a variety of ornamental grasses of various sizes and shapes—blue fescue (Festuca glauca ‘Elijah Blue’), pink muhly (Muhlenbergia capillaris), dwarf fountain grass (Pennisetum oriental ‘Hameln’), little blue stem (Schizachyrium scoparium), prairie dropseed (Sporobolus heterolepis), and switchgrass (Panicum virgatum ‘Shenandoah’). These warm season grasses are mostly maintenance-free with one exception: the dead foliage needs to be cut back in the late winter to showcase the thick flush of new growth in spring.
Tips for cutting back foliage
I choose a warm, sunny day in late February, retrieve hand and electric pruning shears and some bungee cords from the shed, and head to the yard. For more delicate, short grasses, such as blue fescue and dwarf fountain grass, I use very sharp hand pruning shears and cut the foliage to 1 to 3 inches above ground level. For larger grasses, I wrap a bungee cord around the center of the foliage of each grass clump. Then I use electric shears to cut the foliage below the bungee cord, about 3 to 6 inches above the ground. I’ve discovered that electric shears make quick and easy work of the cutting-back process on these thick, tall grasses. And the bungee cord holds the foliage bundle together in a nice, neat package that I simply pick up and toss in the compost pile!
Article written by Beth Leonard, Extension Master Gardener Volunteer.
Resources:
https://plants.ces.ncsu.edu/plants/category/ornamental-grass/