After three or four years in the ground many herbaceous perennials start to die out in the centre. As well as making new plants, dividing them keeps them young and strong.
Begin by digging-up your plant and removing as much soil as you can. Often this is just a question of shaking the soil off the plant but if your soil is thick and sticky try digging the soil away with an old kitchen fork. Aiming a jet of water from a hose pipe at the roots is messy, but effective. With plants that have been in the ground for a year or two you can now pull apart pieces to replant. The pieces should have two or three shoots above the crown and pieces of root attached to it. Older plants will have become woody and will need to be cut apart. An old bread knife is ideal for cutting away sections of plant. With very large clumps you may need to use your spade to cut the plant into pieces.
Replant your new plants immediately wherever you want them to be in the garden and water them. Evergreen plants, such as the black grass, Ophiopogon planiscapus ‘Nigrescens’, need a bit more mollycoddleing so plant them in pots and grow them on in the shelter of the house or in a cold frame or greenhouse. Plant them out in the garden when their roots have filled the pot, usually by the beginning of summer.