Spring is a great time to divide ornamental grasses, as it gives them an entire season to settle back in and root themselves well. The way I tell whether to divide is how they performed last year and what the plants look like: Are there areas of each clump that didn’t send up leaves as thickly as others? Are there dead-looking areas in the middle perhaps? If so, definitely divide…cutting out the unproductive parts…if not, your choice. Maybe you want more plants for the new area, and so you are motivated to divide, even if it means smaller plants for awhile. I love big, full perennials and to my eye most ornamental grasses just get better as they get bigger, so frankly I rarely divide most of my grasses. But some kinds do need it more than others (and we don’t know which yours are).