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.

• Cool climate grasses are best divided as they come into growth in late winter to early spring.
• Warm climate grasses don’t come into growth until late spring, so are best not divided until late spring (May).

• Lift grasses from the ground with a spade.
• Split them using two forks inserted back to back in the centre of the clump, and then prize them apart. Alternatively, cut the clump through the middle with a spade. 
• Some grasses have tough roots that may require a sharp knife, saw or even an axe to split them apart. 
• Small grasses, such as Hakonechloa, can be split by tearing lifted clumps apart by hand. 
• Cut down the foliage of Cortaderia selloana (pampas grass) before dividing - making sure you wear stout clothing and gauntlet-style gloves. 
• Keep divisions as large as possible.
• Divisions can either be potted up in John Innes No 2 compost for growing on, or if they are more than 12.5cm (5in) in diameter, they can be planted back out into the garden.

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.