METHODS FOR INCREASING PLANTS
Plants are increased in many ways but all can be grouped under one or other of two main headings, one known as sexual. Which means increase by , and the other known as asexual or vegetative propagation, which means increase by some part of the plant other than a .
A plant that is raised from seed is a new individual in a sense that is not true of a plant raised vegetatively. It is the result of the union of a male and a female cell, each contributing its own set of the genes which control inheritance. If these sets of genes are markedly different, as they well may be in plants of hybrid origin, theresulting from the union will be different from their parent or parents (for in plants the male and female elements may be supplied by one plant or by two plants). By contrast, a plant increased by vegetative means is not the result of the union of male and female cells and its genetical make-up. Except in rare cases, is exactly the same as that of the plant from which it came. It will. Therefore, resemble this plant in every detail. Whereas a usually varies in some way. Even if only minutely. Often the variation is great and the plant may be a distinct improvement on its parent.
Because a seedling is a new individual, it has its own lease of life. It usually, though not invariably, starts life unburdened by the diseases that have attacked its parents and it often shows a vigour which old plants have begun to lose.
The vegetatively increased plant cannot be guaranteed these advantages. It may or may not be free of inherited diseases and though it may sometimes show at least a period of greater vigour this, too, may tend to decline if the process of vegetative increase is continued from generation to generation.
Seed or Vegetative Increase?
So we see that seed gives the possibility of variation which may be useful and represent progress. And that it maintains vigour. Vegetative increase is desirable where it is important to preserve every detail of the parent plant unchanged.
The principal methods of vegetative increase are by division,, layers and grafting.
Division is the simplest of all. It simply means that the plant is lifted and split up into several pieces, each of which can, if desired, be replanted to form a new plant. Many herbaceous plants can be split up very easily with the fingers but some make such thick, lough clumps ofand shoots that some extra leverage must be applied. One way of doing this is to push two hand forks or small border forks back to back through the middle of the clump and then pull their handles outwards, in opposite directions. With really big, old clumps this can be repeated several times with successive pieces until they are re-duced to a size that can be tackled by hand.
Some plants make solid fleshy crowns which cannot be broken easily and are better cut through with a knife, but take care that theare not also severed in the process.
It is nearly always the younger outside portions of the plant that will grow into the best new plants. The centre of an old clump is often starved and half dead and is best discarded. The best time to divide plants is generally just as they start to grow, usually in the spring but clusters of bulbs can be split up into separate bulbs when they are dormant, which is often in summer.
Cuttings may be prepared from, or roots and each type has its uses. Whereas the division starts life with shoots and roots, the has either a shoot or without roots or a without shoots. It must be induced to produce the missing part.
Stem cuttings may be prepared from young growth, when they are often referred to as soft cuttings; from older growth that is beginning to get firm, when they are called half-ripe cuttings: or from stems at the end of a season of growth, when they are called ripe or hardwood cuttings. At whatever stage they are taken the method of preparation is similar but the methods of making theproduce roots are different.
Acutting may be anything from 1 in to 1 ft (2-5 to 30cm) in length according to the kind of plant from which it is taken. It usually has several leaves or leaf buds and it is often cut off just below one of the leaves or buds. This may be referred to as a joint or node and such a cutting may be called a nodal cutting. Occasionally cuttings are severed half way between leaves or nodes and such cuttings are called internodal. Quite often cuttings are pulled oil’the parent plant where the shoot joins a main . Such cuttings are often called slips or, since they come away with a small piece, or heel, of the older stem attached, are called heel cuttings. Before insertion the heel is trimmed with a sharp knife or razor blade.
The problem with cuttings is to keep them alive while they are making roots. Soft cuttings die very quickly but they also tend to make roots quickly. Cuttings of firm growth lake longer to die but are usually slower in forming roots. Hardwood cuttings die most slowly but also root most slowly. With all kinds rooting can be speeded if the base of the cutting is dipped in special. Different strengths can be obtained to suit soft, half-ripe and hardwood cuttings.
Cuttings are inserted in sandy soil, pure sand, peat, vermiculite, perlite and various other materials: some gardeners favouring one. Some another. Soft and half-ripe cuttings are often placed in a small frame, a box covered with glass or in a pot in a polythene bag, the object being to cut down loss of water and keep the cutting fresh while it is rooting. A little artificial heat, especially from below (known as bottom heat), will also speed rooting. Ripe or hardwood cuttings are often inserted outdoors, and since they do not lose moisture so readily, are not so much in need of covering.
Leaf Cuttings These are mainly used for a fewplants, such as African violets (saintpaulias) and rex . Well-grown leaves are carefully removed and either inserted like cuttings, stalk first, into the soil, sand or peat, or placed flat on the surface and held there by hairpins or little stones. When this latter method is used, cuts are usually made in the main veins of the leaf to encourage rooting. Almost always some bottom heat is advantageous. Leaf cuttings are usually taken in summer.
These are used for a few plants, mostly with rather thick roots, such as anchusa, verbascum, hollyhock, limon-ium and romneya, but phlox and gaillardia which have relatively thin roots can be propagated in the same way. The roots are cut up into pieces an inch or so in length and are either laid horizontally or inserted right way up in sandy soil, usually in a frame or. This is usually done in winter or spring.
The object ofis to induce stems to form roots before they are severed from the parent plant, which sustains them with sap during the period of rooting. Some plants adopt this as a natural method of increase. Both the strawberry and the violet throw out long runners in summer along which are little clusters of leaves and shoots – incipient plants. Where these touch the soil they form roots and actually grow into new plants. With plants such as this the gardener merely assists nature by holding the runner firmly to the soil with a piece of bent wire or a stone and by limiting the number of runners and the number of on each so that the plant is nol overstrained.
Plants Without Runners
There are many other plants which do not produce runners and yet can be layered. It may be possible to bend down pliable stems to soil level and cover them with a little soil, holding them in place with bent wire, a forked stick or a stone. Rooting is likely to be made more certain if the stem, where it is to be buried, is first wounded and the wound dusted with hormone. A slit may be made through a joint or an encircling cut made in the bark just below a joint, or the shoot may simply be twisted or bent sharply so that some of the tissues are bruised and the sap flow restricted. It is from the callus which forms to heal such wounds that roots are most readily formed.
Air-If no stem can be bent to ground level a system known as air-layering may be used. It is mainly applied to shrubs and young stems are most suitable. The chosen stem is wounded in one or other of the ways just described and the wound is dusted with . Then damp sphagnum moss is wrapped quite thickly around the wound and a sleeve of polythene film pulled over it and tied on each side so that the damp moss is completely enclosed. In time roots will grow into the moss and when there are plenty of these the layer can be severed and potted or planted to grow into a new plant.
When to Layer
can be done at any time of the year but for shrubs is usually most successful in spring or early summer. Border carnations are layered soon after mid-summer.
This is a surgical operation whereby a shoot or bud, known as the scion, of one plant is joined to the root system of another plant, known as the stock. It is used to increase many plants which cannot readily be increased by cuttings. Commercially produced roses are nearly always raised by budding, a form of grafting, and apples, pears, plums and cherries are also increased by some form of grafting.
Budding is in many ways the simplest both to describe and carry out. It is usually done in summer when plants are in full growth. The buds, which are dormant growth buds, not flower buds, are cut from firm young stems. They are usually cut out with a shield-shaped slip of bark and wood I \ to 2 in (4 to 5 cm) in length and usually the sliver of wood cut with this bark is removed. The stock is prepared by making a T-shaped incision in the bark of a young stem and prising this open so that the prepared bud can be slipped under the flaps of bark. It is then bound in place with raffia, soft string or special budding strips. In a few weeks the bud should have united with the stock and the following winter the stock is cut off immediately above the bud which thereafter channels all further growth above ground. The growth will partake of the character of the bud and not of the stock, though the latter may influence rate of growth, ultimate size and anchorage in the soil.
A simple form of grafting, which is not unlike budding, is known as rind grafting. The stock needs to be fairly stout and the top is cut cleanly across. A downward incision, about 2 in (5 cm) in length, is made in the bark from this point of severance. A firm young stem is cut from the plant to be increased and a long, sloping, wedge-shaped cut is made at the lower end.
The shoot is then pressed gently beneath the bark of the stock with its cut face against the wood of the stock. It is bound in place with soft string and the wounded area is completely covered with grafting wax.
Whip and Tongue Grafting
Slightly more complicated is the whip and tongue graft. This is most suitable when scion and stock are similar in thickness. The scion is prepared much as for rind grafting except that a second cut is made in the opposite direction to form a thin tongue on the cut face. The stock is prepared with two similar cuts in reverse, so that the two tongues can be fitted together and will hold the two cut surfaces in exact register. The scion is then firmly bound to the stock and the whole wounded area covered with grafting wax.
When to Graft
Grafting is usually done in late winter or early spring and growth from the scion should start within a few weeks. Once a good union has been made, the stock must not be permitted to make any growth of its own, a stipulation which applies to all forms of grafting.