Use this method to propagate most herbaceous perennials and to rejuvenate large, tired clumps that no longer flower well.
In early spring, select a healthy clump of plants and water them well. Cut back any top growth to the ground. Using a fork, lift the clump, taking care to keep the whole root ball intact.
Cut solid crowns into portions with a spade or old knife. If you can't pry other pieces apart by hand, use two forks back-to-back to split the clump into smaller sections, ready for replanting.
Discard dead central portions of overgrown clumps. Replant healthy hand-sized pieces with strong buds in soil improved with well-composted organic matter. Water in well.
Excerpted from How to Grow Practically Everything
© Dorling Kindersley Limited 2010