Sometimes, taking root cuttings is the only way to increase your stock of certain plants, including some herbaceous perennials with fleshy roots. The following are a few that can be multiplied in this manner: Eryngium (sea holly), Romneya coulteri (the Californian tree poppy, discovered by Irishman Thomas Coulter), acanthus, verbascum, Pulsatilla (Pasque flower), some phloxes, mint, horseradish and comfrey. When the plant is dormant (now) dig it up carefully, and cleanly cut away a few, vigorous, unblemished roots from near the crown (where the green shoots sprout from). If the roots are thinner than a pencil, cut them into about four-inch lengths and lay them on a tray or shallow pot of well drained, gritty compost. Cover with a thin layer of compost.
If the roots are thicker, cut into two-inch pieces, making a diagonal cut at the lower end, and a straight cut at the upper end (this distinguishes the top from the bottom). Insert the pieces into a pot of compost with the top end just below the surface. Label the containers and put into a cold-frame or greenhouse until the new growth appears in the spring.