To trim ornamental grasses,trim four to six inches in the early spring hence this promotes the growth after the last frost. When you trim and clean the grass of its old growth you allow it to receive more sun at the crown of the plant. Sun helps the plant to produce leaves. Frost and heavy snow can damage your ornamental grass. Inspect the plant during early spring. You can use a pair of shears to remove any infected leaves.Don't trim the ornamental grass before the winter, the old grow will help to insulate the plant during bad temperature.