Usually, ornamental grass foliage is kept on the plant to protect the crown (main growth points at soil level) from weather extremes and is pruned back in late winter or early spring (around early March, more or less). With many species, the winter foliage can also be decorative, adding texture to the landscape when other perennials have less prominent foliage. When pruning, don't cut flush with the ground, as this might injure ready-to-emerge new growth; rather, leave a few inches (perhaps 6-8, though this can depend on the species of grass) of "stubble", which will soon be covered-up by growth later that season.

Miri