This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
By cutting up the roots of certain plants into pieces 2 or 3 in. long, and covering them with about 1 in. of gritty soil, it is possible to raise new plants. This method of propagation may be practised about October and November, or in February and March, the root cuttings being inserted in a hotbed of moderate temperature. Some plants like Horse-radish and Sea-kale are easily and generally raised in this way; while such weeds as the Bearbind, Dock, Thistle, Dandelion are also increased quite as readily by chopping up the roots. Other plants that may be raised by means of root cuttings are Anemone japonica, Acanthus mollis, Bocconia, Dictamnus Fraxinella, the Sea Hollies (Eryngium), the Globe Thistle (Echinops), the Oriental Poppy (Papaver orientale), Statice latifolia, etc. Many kinds of trees and shrubs like Hawthorns, Plums, Apples, Pears, Quinces, Roses, Poplar, Mulberries, False Acacias (Robinia), Sumach (Rhus), Paulownia, Sophora, etc, may be propagated from root cuttings.