Large clumps of named cultivars and most species can be increased by division in early spring, although many professional growers prefer to divide Oriental hybrids (Helleborus × hybridus) in September.
For the best results, ensure that clumps are split into several pieces of a reasonable size, with at least one growth point, and water well until they are established in the soil. The new divisions may be slow to establish, due to the lack of fine roots, and flowering may be poor in the following year, but they are likely to settle in given time.
Exceptions: Helleborus foetidus and H. argutifolius are not suitable for division, but can be raised by sowing fresh seed early in the year. It may take two to three years before plants reach flowering size.