Native to Argentina, pampas grass (Cortaderia selloana) is a dramatic warm-climate ornamental grass. The plants vary in their ability to produce thick, showy plumes, so observe yours for a season to see what they do. In the meantime, in late winter or early spring before new growth appears, cut them back to about 12 inches from the ground. This is a yearly maintenance chore with large ornamental grasses. Keeping the plant intact through the winter makes it look good and enhances its ability to tolerate cold. Use pruning shears or hedge clippers to cut back your pampas grass.
To make trimming and cleanup easier, first bind the old leaves together with string, packing tape, or a bungee cord. Whatever tools you use, wear gloves and a long-sleeved shirt because the grass blades can have sharp edges that will slice through exposed skin.