How to Take Root Cuttings
From: DK Books - How to Grow
With the exception of variegated plants, which produce only green shoots from root cuttings, many perennials can be propagated using this method. Never take more than a few roots from each plant, and quickly replace the plants in the soil. Thin roots, such as those of garden phlox, should be laid horizontally on the soil to root.
Flowering kale (Crambe)
Sea holly (Eryngium)
Tree poppy (Romneya)
Trim Off Healthy Roots
In winter or early spring, lift the plant, scraping away soil from larger plants to expose the roots. Cutting close to the stems (crown), remove three or four fat, healthy roots, avoiding brittle, damaged or woody pieces. Seal in a plastic bag.
Cut Top and Bottom
Cut each root into 2- to 3-inch segments with a sharp knife. Trim the top end (nearest the crown) straight across and the bottom at an angle, to make sure you plant them right end up. You don't need to do this with thin roots.
Insert the root cuttings vertically in pots of soil (with the blunt end at the top), spacing them 2 inches apart. Lay thinner roots on the surface, covering them with 1/2 inch of coarse sand or grit. Water the roots with a diluted fungicide and move pots to a sheltered spot outside.
Cover pots and trays with a plastic tent, or place them in a cold frame, and keep the soil damp. When weather warms, be sure that the cold frame or plastic tent doesn't get too hot. The cuttings should be well rooted in about six months. They may produce shoots beforehand but wait until roots appear at the holes at the bottom of the pots before transplanting.