Grasses from cool climates (such as Carex, Calamagrostis, Chasmanthium, Deschampsia, Festuca, Hakonechloa, Helictotrichon, Molinia and Stipa) require frequent division, perhaps every three years, to prevent root congestion and loss of vigour.
Grasses from warm climates (such as Arundo, Cortaderia, Imperata, Miscanthus, Panicum, Pennisetum, Phalaris and Spartina) require only infrequent division.
Sometimes ornamental grasses can fail to flower. Although this may indicate that the clump is congested and in need of division, the most likely explanation is lack of light. Try moving the plant to a sunnier position, or cutting back overhanging foliage to increase light.
Ornamental grasses rarely suffer from pests and diseases. Occasionally rust can be a problem, but regular division is likely to reduce the risk of this disease developing by improving air flow around the leaves. Chemical controls are not usually necessary, but fungicides containing myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), penconazole (Westland Fungus Attack) and triticonazole (Scotts Fungus Clear Ultra) can be used to control rust on ornamental plants. The products permitted under organic regimes have little effect on rusts.