This section is from the book "Culinary Herbs", by M. G. Kains. Also available from Amazon: Culinary Herbs, Their Cultivation, Harvesting, Curing and Uses.
Several of the perennial herbs, such as sage, savory, and thyme, may be easily propagated by means of layers, the stems being pegged down and covered lightly with earth. If the moisture and the temperature be favorable, roots should be formed in three or four weeks and the stem separated from the parent and planted. Often there may be several branches upon the stem, and each of these may be used as a new plantlet provided it has some roots or a rooted part of the main stem attached to it. By this method I have obtained nearly 100 rooted plants from a single specimen of Holt's Mammoth sage grown in a greenhouse. And from the same plant at the same time I have taken more than 100 cuttings. This is not an exceptional feat with this variety, the plants of which are very branchy and often exceed a yard in diameter.
Layering is probably the simplest and most satisfactory method of artificial propagation under ordinary conditions, since the stems are almost sure to take root if undisturbed long enough; and since rooted plants can hardly fail to grow if properly transplanted. Then, too, less apparent time is taken than with plants grown from cuttings and far less than with those grown from seed. In other words, they generally produce a crop sooner than the plants obtained by the other methods set in operation at the same time.