Layering, one of the processes in horticulture by which plants are multiplied. In the propagation of plants by cuttings, a portion of the stem is removed and inserted in the soil, where it takes root and becomes a new plant; in layering, the portion of the stem which is to form the new plant remains attached to the parent plant while the roots are forming. In the cutting there is sufficient nutriment accumulated within its stem and leaves to enable it to throw out roots, through which it can derive sustenance; but in the layer the portion that is to become a new plant is sustained by the old one until it is provided with the means (roots) of self-support. Many plants which it is almost impossible to propagate by cuttings are readily managed by layers. The operation of layering was no doubt suggested by the manner in which some plants multiply themselves in the wild state; many, as the red raspberry and blackberry, throw out stems just below the surface of the earth; these run along underground, sometimes for several feet, when the end turns up and seeks the light; abundant roots are formed, and when the connection with the old plant is severed by decay or otherwise, the new one grows on and in time repeats the same process.
This propagation by suckers, as it is called, is illustrated in fig. 1. In close imitation of this, the gardener selects a branch which starts above ground, and bends it down so that a portion can be buried in the soil; roots are soon thrown off by that part of the stem covered by the earth, and when these are sufficiently developed the communication with the parent is cut off. Many plants, the stems of which strike root easily, such as the wistaria and the grape, maybe easily multiplied by this simple process. Other subjects are more difficult, and require special treatment. One of the common methods is to form a "tongue" on the buried stem by cutting halfway through it in a sloping direction; the old practice was to cut the tongue on the under side of the branch, but it is much better to cut it upon the upper side, as shown in tig. 2. If measures are not taken to keep this tongue open, the wound will sometimes heal, and no roots be formed; hence a sliver of wood or other matter is put in to keep the cut open; or if the nature of the plant will allow, a slight turn of the stem will separate the parts. In this method the uninjured portion of the stem serves to keep the extremity alive while roots form.
The end of the stem is usually turned up and held in an erect position by tying it to a stake. Some plants thus treated will form roots in a few weeks, while others require the whole season, or even longer, before the new plant may be removed. This kind of layering is practised on herbaceous stems (as the carnation), or upon woody stems two or more years old. Shoots of the grape of the current season, if layered as soon as the wood has become firm enough to handle, will form good plants by autumn. Very flexible vines are treated by what is called serpentine layering, in which successive portions are put below ground, alternating with other portions exposed to the air. In very moist summers florists resort to what is called " layering in the air " for the propagation of geraniums (pelargoniums); in such seasons the stems of these plants are so succulent that cuttings made in the usual way would decay before they took root; in this case the stems are tongued as if they were to be layered, but are left without contact with the soil; the cutting checks the luxuriant growth, causes the soft wood to harden, and in some seasons the cut surface will not only be calloused but even emit aerial roots.
Among the modifications of layering is one employed when the stem cannot be made to reach the earth, but the earth must be brought to the stem; a pot with a slit in the side is made to include the stem and then filled with earth; some contrivance is supplied for keeping the earth in the pot in a properly moist condition, and when sufficiently rooted the layer is detached. Mound or stool layering is largely practised by nurserymen to procure quince stocks for dwarfing the pear, and paradise and doucin stocks for dwarfing the apple. The plants when well established are in early spring cut off near the surface of the ground; numerous shoots spring from the remaining portion of the stem, which are allowed to grow during the season; in the following autumn or spring earth is heaped up around the plant so as to cover the base of these shoots for three or more inches with good soil, and this shaped to form a neat mound as shown in fig. 3; the buried bases of the shoots throw out roots into the soil of the mound, and in autumn the earth is removed, the rooted shoots severed from the parent plant, or stool as it is called, and set out by themselves; if properly managed, the stools will produce successive crops of stocks.
Fig. 1. - Propagation by Suckers.
Fig. 2. - Tongue Layering.
Fig. 3. - Mound Layering.