by S.C. Mason
Cyclopedia of American Agriculture, 1903
Layering is the process by which a part of a plant stem is made to produce roots while still attached to and nourished by the parent plant, so that it may be able to maintain an independent growth. The tendency, under favorable conditions, to produce roots from the cambium zone of some part of the stem is manifested by many plants, especially in the tropics. It may be noticed in the species of Ficus cultivated in the greenhouse, in Epigæa and Rhus Toxicodendron in the woods, in tomato vines in the garden, in grape canes lying on the ground, and frequently in young apple trees when the trunk becomes covered with earth to an unusual depth. With most such plants, rooting by detached parts is easily accomplished, and this being more convenient, layering is generally practiced only with those plants which do not root readily from cuttings.
The mode of root production is essentially the same in either case. The right conditions as to moisture, temperature, food supply, etc., seem to stimulate the formation of one or more growing points in the cambium zone. The multiplying cells force their way through the bark, and if favorable soil contact is secured, supporting roots are soon developed. The same results may come, sometimes more readily, from or near a callus formed in the effort to heal a cut surface. It is when the food supply is deficient, or the cell action is so slow that the detached part would perish before supporting roots could be established, that rooting while the parts are still attached to and nourished by the parent plant need be employed.
The different methods of Layering are simply matters of detail adapted to the varying natures of the plants to be dealt with. Usually branches are selected of rather young wood, which can easily be brought under the soil and which, when rooted, can be removed without damage to the old plant. The most favorable season is generally the spring or time of most rapid cell growth.
The methods of Layering may be represented in the following diagram:
As shown in Fig. 1248, a suitable branch is bent to the ground and held in place by a forked pin, so that a portion of it is covered with 2 or 3 inches of rich earth, the end being bent to an upright position and fastened to a stake. The bend and consequent rupture of the bark may be all that is needed to obstruct the movement of food material and cause the development of roots at this point. If not, a tongue may be cut not deeper than one-third of the thickness of the branch from below upwards and near a bud or node. In Fig. 1249 a layered branch is shown with a ring of bark removed, a good practice with thick, hard-barked species.
For many low-branched shrubs, mound- or stool-layers are prepared (Fig. 1250) as follows: In the spring, head the bush back to a series of stubs, which will produce a large number of vigorous young shoots. By midsummer, in some cases, or the following spring, a mound of earth is thrown around the old stool and the base of the new shoots, and from these latter abundant rooting is secured, so that by the following autumn or spring they may be separated and set in nursery rows.
When a branch cannot be brought to the ground, sometimes the earth is brought to the branch by clasping the halves of a broken or specially made pot around a tongued or girdled branch and filling in earth and sphagnum moss to retain the moisture; or the moss may be held in place by a cone of strong paper (Fig. 1251). It may be necessary to support the pot with a light stand of stakes. Where a moist atmosphere is retained, as in a conservatory, merely a ball of sphagnum bound around the branch with twine will serve an equally good purpose with less trouble. This kind of propagation is known as air layering, Chinese layering or circumposition.
In the case of vines, a cane may be laid horizontally in a shallow trench, covering a few inches to induce rooting, and leaving a node or two exposed for growth, and so on to the end, as shown by Fig. 1252. After young shoots are well started from the uncovered buds, the earth may be filled in to the level of the dotted line.
In Fig. 1253 is shown what is often called the serpentine layer, in which the cane is bent, portions being covered and the intervals left above the ground. It is said that by this means the tendency of the sap to flow to the extremity and there make the strongest growth, is overcome, and even rooting secured the whole length of the cane. This method is often used with quick growing vines like clematis and wisteria, from which it is possible to secure a succession of layers from the annual growth during spring and early summer.
All of the foregoing operations will be found more readily successful in the more moist situations; more successful in the nearly saturated atmosphere of the southern states, for instance, than in the comparatively dry conditions of the prairie states.