When herbaceous perennial plants have finished flowering or lie dormant in the spring, large, long established clumps can be lifted and divided. The method of division will depend on the size of the plant and the type of root system. The general rule is to never divide plants when they are in flower.
Regular division of clump forming perennials is actually very beneficial to the plants, as it provides the opportunity to remove any old, tired and diseased sections, allowing the plants room to regrow and refresh themselves.
The goal is to split the plant into two or more separate self-contained portions, each with its own leaves and root system. Care should be taken to prevent as little damage as possible to the roots and leaves. Any dead or damaged parts should be cut away with a sharp knife and discarded. There is not real limit to the number of divisions that can be made so long as the individual plantlets have sufficient roots and leaves to survive, however the smaller you make the divisions the longer it will take the plants to recover and regrow to normal proportions.
Cut irises, lupins and red hot pokers into portions with an old knife. Only replant the young vigorous pieces and discard the old worn-out parts. Large clumps will often need tougher treatment as follows:
Christmas roses and other hellebores can be divided after they have finished flowering. Choose a fine day, lift the plants carefully, teasing the roots apart with your fingers. Old tough clumps may need the aid of a knife. Plant them 45 cm (18 in) apart in a moist, partially shady spot. For really good blooms, they need shelter from the morning sun and wind. Water them if the soil is dry, until they are established.
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