An easy method of propagation is layering, though this, too, produces only a limited number of new individuals. This method is based on the fact that if the young shoot or shrub is bent and inserted in the soil it will put forth roots. Some shrubs multiply in this manner in the wild, e.g. the blackberry, dogwood, etc.
In gardening layering is used mainly for those shrubs that are not easy to propagate by cuttings’ or grafting. Layering may be carried out either on the spot where the shrub is growing or else in a special bed to which the shrub to be increased is transferred. One advantage of the latter is that it is possible to prepare a soil mixture suitable for propagation, i.e. a lighter, sandy soil, and that the shrubs can be spaced far enough apart. Young one-year shoots root best.
Vegetative reproduction is a method commonly used in the propagation of shrubs because most species produce prolific shoots or root suckers. As a rule this method yields plants for setting out in permanent quarters in a shorter time and also makes it possible to propagate shrubs that do not set seeds. It is the only method that ensures that the new plants shall be true to type. Vegetative reproduction includes propagation by division, layering, cuttings, grafting and budding.
Some seeds, even though stratified, will not germinate until the spring of the second year, e.g. the cornelian cherry, sometimes also the bladdernut, dogwood, wayfaring tree, English holly, staghorn sumach, etc. Such seeds should be stratified another year and in spring the pots in which they are stored should be put out in a shady spot and sunk in peat halfway to their rims, the peat being watered regularly to keep the substratum and seeds from drying out. In spring the seeds are sown either into a frame or in pots filled with a mixture of equal parts loam, peat and sand.
To promote the growth of roots a tongue-like incision is made in the wood at the point of contact with the ground or a narrow strip of bark is removed. It is beneficial to expose the layered branch to the sun because warmer soil promotes rooting. As a rule the branch puts out a good root system during the growing period so that it can be separated from the parent plant in November or early spring and transplanted to the bed. This method can be used for the propagation of various species of Viburnum, Cydonia, Exochorda, Magnolia, Rhododendron, and other shrubs not easy to propagate by cuttings.
A similar method, known as stooling, is used to increase choice varieties of the hazel or to produce a greater quantity of rootstocks for budding or grafting fruit trees. Shrubs to be increased are cut back close to the ground in early spring so that they will put out as many shoots as possible. As these shoots grow they arc covered with a mound of soil up to about 30 centimetres high. During the summer the shoots put forth new roots and in the autumn the soil is removed and the rooted shoots are cut away from the parent shrub and planted out. This method is sometimes used to propagate certain varieties and species of Philadelphus, Deulzia, Hydrangea, and, Syringa.