Because sea holly have a taproot--one large, central root from which other roots grow out laterally--you can't propagate sea holly from stem cuttings. In order to propagate blue sea holly, you'll need to propagate from the root. In autumn, dig up the sea holly you wish to split, and cut away thick-ish sections of the roots: choose roots that are about the thickness of a pencil. Don't take too much; we recommend cutting no more than one-third of the sea holly's total root mass. Replant the parent clump immediately, and plant the individual root sections in a container in compost or well-drained soil. Make sure that the cut end of each root section is facing up, and covered with soil but just barely below the surface. Keep your sea holly container in a sheltered area over the winter. In spring, the root segments will take off, growing new roots and green shoots. After all the threat of frost has passed, transplant your new sea holly into the garden, and like the parent plant, be sure to water them regularly for the first growing season.