Layering is one of Nature’s own methods of plant reproduction. In a natural state, shrubs that grow long arching stems, such as those of the blackberry family, layer themselves. Where the tip of the long arching stem touches the ground, as it does always when allowed to grow unchecked and unsupported, roots form, and these gradually set up a fresh stool,” from which new stems rise.
In the same way, some herbaceous plants, such as the strawberry, send out “runners,” and at the tip of these runners roots also form, and establish themselves as new plants.
In each case the roots form while the stem is still attached to the old plant; therefore each young plant can draw for a time on the food supply of the old plant.
When gardeners wish to root cuttings that are “ shy rooters,” they frequently adopt variations of this method. Side stems of carnations, and long stems of various shrubs such as rhododendrons and forsythia, are deliberately bent down to the soil level (or, if more convenient, a pot of soil is raised to the level of the stem), and a slight incision is made in the stem. This cut portion is then bent down under the soil surface, and fastened into position with a peg. When the roots have formed, the stem dividing the two plants is cut through, and as soon as convenient the new plant is moved to a permanent position elsewhere.
This method of propagation can be adapted by the individual gardener to varying conditions. In the case of a plant of border carnations, for instance, the procedure is this. About the end of July, when the plant has a number of strong side growths round the flowering stem, a little fresh sandy compost is prepared, and strewn thickly round the carnation. Some crushed mortar rubble or lime should be added to the compost. Each side growth is taken in turn, stripped of one or two of the lower leaves, cut half through with a slanting upward cut, and then pegged down so that the cut is open under the gritty compost, while the tip is held firmly in a normal position.
It takes usually about three weeks for roots to form, and then the new plant may be severed from the old one. It is generally advisable to leave it where it is for a further three weeks, so that it becomes firmly established on its own roots before it has to stand up to the shock of transplanting.
One or two indications of the times suitable for each operation so far described have been given, but it is not possible to give any absolutely constant rule. The gardener must learn by experience.
A rough guide, however, is this. Hard wood cuttings are best taken in the open in autumn, when rains are probable, and no leaf growth is expected for some time. This allows the cutting to make roots during the dormant season.
Soft cuttings, of new growth, can be taken under glass at any season when such growths are available on the plant in question. They are generally available towards the end of the flowering season, or at the beginning of a new season of growth; but much depends on the individual plant.
Soft cuttings will root in the open only during warm weather, and then only in shade and when kept moist. The best general time for taking such cuttings is in August, when the growths are not too soft and new, and when soil, air, and rains are all likely to be warm.