The Vegetative Propagation of Plants

Although the propagation of plants on any substantial scale is best left to the professional nurseryman, there are occasions when a few extra specimens of a particular stock are required. It then becomes of great interest and pleasure to increase one’s stock of a favourite variety.

Many plants may be raised or increased from seed, which is nature’s way. This can be very satisfactory in the case of natural species and in strains of mixed colours, but in most cases, in fact nearly all, seed is not a satisfactory or reliable way to increase stock of named varieties, of garden origin.

Plants raised from seed, whilst having some of the chief characteristics of the parent plant, are liable to vary in many details and to a considerable degree. To reproduce a plant entirely like its parent it is therefore necessary to propagate it vegetatively. There are a number of ways of doing this, but we are concerned only with those methods which are easy to carry out and within the reach and ability of the average gardener. The chief of these are: simple division, or the splitting up of the roots of healthy, established plants; layering; stem and root cuttings; budding, and grafting.

Division is the simplest; the word is self-explanatory. Yet it is not always done with sufficient care, and it is of little use attempting to divide a plant which naturally forms a single tapering root.

The easiest of all perennial plants to divide are those which form a mat-like root system and which are of a spreading habit. These include michaelmas daisies, Chrysanthemum maximum, achillea, solidago or golden rod, phlox and rudbeckia. Very often it is easy to detach strong portions without actually lifting the entire plants. It is important to secure the outer healthy portions for growing on, and not those towards the centre of the clump, which will be old, weak and possibly hollow-stemmed and affected by disease. Such older cores are best discarded, and either placed on the compost heap or burned if suspected of disease.

Once the plants are lifted, many can be torn apart with the hands. In some instances this does not work well,so that a safer method should be adopted, particularly where stock is limited or in the case of a special plant.

It is possible to chop up the clump with a spade, but this is not the most efficient way, for the divided pieces of root may very well contain a woody core.

To avoid damage, the safest way is to lift the plants carefully and deeply insert two garden forks, or even hand-forks, back to back, and then lever one against the other until the clump is pulled apart. A knife may also be used to sever portions, and its use does prevent a jagged wound, especially in the case of tough-rooted subjects.

Once the selected portions have been secured, they should be planted immediately, so that they do not dry out. Treated well, they will soon settle in their new positions and make good young plants. If really choice plants which make only small growth are divided in the autumn, it is quite a good plan to put them into small pots of compost and keep them in a cold frame until the spring.

Rather more care is needed with plants which do not produce offsets but form a thick, close root system. These can be carefully cut into portions, making sure that each one has a strong bud or two, for without these there is little point in attempting to divide. In nurseries, this type of root, which includes lupins and delphiniums, is usually washed, so that it can easily be seen where to cut.

Some perennials have rhizomatous roots, including the popular Liatris pycnostachya, Iris germanica varieties, the ornamental grasses and lily of the valley. These divide easily by cutting or cleanly breaking the roots, making certain that some buds are included on each separate piece planted up.

Plants such as heucheras, lythrum and paeonies, which have thickish and sometimes tuberous-like roots, usually have to be eased apart with a knife. An important point here, which in fact should always be borne in mind, is to see that each division for planting has some fibrous roots. Care should be taken to remove all jagged or torn tissues, for if left they may very well become a starting place for decay, which once it gains a hold will soon ruin a young plant.

Dahlias and Salvia patens also have tuberous roots which can be divided by severing them at suitable points, so that a cluster of smallish tubers can be made into a number of individual plants. Providing there is a strong bud on each section and the portions are potted up, a good sturdy plant will develop.

There are a few subjects, such as physostegia, which produce stolons or underground stems which run out from the centre of the plant. These can be severed at suitable points and only need laying in drills.

With fine soil pressed around and over them, they will soon make nice flowering plants.

None of these methods of division should present any difficulty to the average gardener. The points to observe are carefulness in pulling or cutting the pieces apart, leaving them free from jagged edges, paring the wounds clean whenever necessary, and selecting young vigorous portions for growing on, rather than large, old, worn chunks.

Some herbaceous plants and shrubs throw up suckers which on occasion will appear quite close to the lower main stem, and at other times at points on the roots, which may be some distance from the centre of the plant. The latter can sometimes be a nuisance—in fact it is just what happens when a plant occupies more than its allotted space and becomes invasive. The helianthus or sunflower is an example, while lilacs frequently throw up these unwanted growths. With herbaceous plants it is, of course, quite easy to sever the suckers below ground level, and plant them up when they will soon make good individual plants. This can be done in both the spring and autumn, when all old, woody or unwanted roots and weak shoots should be removed.

Another means of propagating some plants is by detaching young offsets. Each of these is really a tufted development or extension of the plant’s stem at soil level. They can usually easily be detached either with the fingers or with a knife and planted up separately. Auriculas, primroses, hardy geraniums and some myosotis are good examples of plants which make offsets, although the latter vary in size and shape.

Apart from the perennial plants, many bulbous plants can be increased in the same way. Narcissi and tulips often produce many new young bulbs alongside or on top of the parent or main bulb, and the same is true of crocuses, gladioli and montbretias. Particularly in the case of gladioli the number of offsets formed, in addition to the new main corm, is very large. Often they fall off, or they can be detached and planted up separately, when they will flower after a year or two.

The layering method is especially useful in the case of hard-wooded subjects, which are not easy to divide, and from which cuttings do not root readily. We need not go into detail concerning the various ways that layering may be done; aerial layering, for instance, is not likely to concern the grower of flowers for indoor decoration. The hardy border and Malmaison carnations are usually layered towards the end of July, before the stems become too tough. Selected plants in good healthy condition are secured and the sturdiest stems, which have not carried a flower, are the ones to choose. First place some fresh soil mixture, preferably consisting of equal parts of loam, silver sand and moss-fibre litter, on the ground around the plants, so that it is at least an inch higher than the surrounding ground.

Remove the leaves from the part of the stem which will be under the ground. Then insert the point of a sharp knife just below a joint from which the leaves have been removed. Draw the knife up at an angle, through the joint, to about in. beyond it. When the knife is removed this will leave a tongue, of which the end should be trimmed clean. Then bend down the shoot and press it into the soil mixture, so that the cut is kept open and is not damaged. Peg the cutting down firmly into position, so as to encourage good and quick rooting. If desired, more of the soil mixture can be put over the layered stem, and if it remains moist, rooting will occur within 6 or 7 weeks.

Among the shrubby subjects which respond to layering are azaleas, comus, forsythia, rhododendrons and lilacs, of which the branches are simply pulled down into the soil and firmly pegged in. Some fresh earth should be placed where the layers are to be fixed. A U-shaped incision is made in the stems with a sharp knife and the split is kept open by placing a small stone in it.

There are just a few flowering plants, such as lythrum, which throw out little stems or runners which root when they touch the ground.

The most common method of vegetative reproduction is by means of cuttings. These may be from stem, root or, in some cases, from a leaf. The majority of hardy perennials grown for cut flowers are propagated from strong young fresh growths. Nearly all are secured in the spring, for then the sap becomes active and the rooting process is quicker; even so, many plants often yield good cuttings during the summer and will still have time to become rooted before winter.

The taking of cuttings is the securing of a portion of the plant it is desired to increase, and inducing it to form roots of its own, so that it can grow on as a separate entity.

The precise manner of taking and rooting the cuttings, and the correct season for the job, will vary according to the subject. The majority of hardy perennials are secured from young growths, preferably taken in the spring. This gives them time to root and develop a little before the growth lessens its activity in readiness for its winter rest. Many hard-wooded plants are taken in the autumn, although evergreens and some half-hardy perennials, including dahlias, are secured in the early part of the year. The majority of tender greenhouse plants can be propagated from stem cuttings during the spring and summer. In all instances the exact time will depend on when suitable growths become available.

Some cuttings will root more easily than others, indeed many young shoots will root with no preparation at all. It is possible in some cases to take cuttings with rudimentary roots, and these will normally become established more quickly than those without this advantage.

The conditions required for good rooting include a moist atmosphere in which loss of moisture through the leaves on the cutting will be reduced to a minimum. Once roots have formed, they will be able to make good any such loss. For the majority of cuttings of hardy plants a suitable rooting soil mixture consists of one part each of loam and peat and two parts of coarse silver sand, placed over a layer of good drainage material at the bottom of the tray or pot being used. If the surface of the soil is covered with a layer of silver sand, some will fall into the holes as they are made, and the base of the cuttings will rest on it.

For some cuttings, pure sand or even all peat or sphagnum moss can be used, while vermiculite is finding favour with some growers. If a propagating case is available, it will provide humidity, so valuable for good root formation.

As to the actual taking and making of the cuttings, the most usual way is to secure them from the top of strong stems when the plant is in full growth. It is unwise to make them too long, but they should be of sufficient length to allow them to be firmly inserted in the rooting medium. The base should be cleanly cut at the lowest joint on the cutting, without actually bruising the joint. If the base of the cutting is left torn or ragged, energy will be wasted by the healing of the injuries at the expense of the urgent necessity of root formation. In some instances it is possible to take cuttings with a heel, which is a side growth, separated by a firm, gentle downward pull from the main or larger stem.

Such cuttings are more usual from shrubby plants than from herbaceous perennials. Whatever cuttings are being taken, soft, sappy shoots should be avoided and those which are ripe, or half-ripe, and fairly firm used. In all cases the cuttings are stripped of their lower leaves before being inserted, which should be done very soon after they are taken from the parent plant. Some authorities recommend that zonal pelargoniums or geraniums should be left on the bench for some hours to dry before planting.

There are a number of hormone rooting powders now available, and in some instances they appear to have been of help in encouraging roots to form, especially the rather more difficult subjects.

Although where substantial quantities of cuttings are concerned, they can be planted in trays or under a frame, if a small number is involved the formation of a callus and roots is induced more quickly

if the cuttings are placed round the edge of a flower-pot. If such pots are kept plunged in a bed of sandy peat, drainage, aeration and warmth will be assured.

Some cuttings, such as those of impatiens or balsam, succulents, roses and oleander, will root freely in water, though there is the danger of breaking the roots when planting them in soil.

There is a different way of taking cuttings of pinks. The tops of the shoots are pulled out of their ‘sockets’, and are then known as pipings. Yet another type of cutting is known as the mallet stem-cutting. This is when a section of the main stem from which the shoot arises is removed, the whole then being planted in the usual way. The hederas or ivies, and some other climbing or vine-like plants, are often treated in this way.

Whether inserted in boxes or pots (or out of doors in the summer) it is essential to make each cutting firm at the base. In many cases it is a distinct advantage to cover the receptacles with a sheet of glass if indoors, so as to provide the necessary humid atmosphere. The glass should be turned at frequent intervals, preferably each morning, so that condensation moisture can be wiped away. The boxes can also be stood in frames, where after to days or so a little ventilation can be given on all days when the weather is reasonable. Bell-glasses, cloches or other glass covers are useful in the open ground. Whether indoors or out, the sun must be kept from shining directly on the glass, or some shading can be given, and this also applies to cuttings which are inserted into the open ground.

The compost or soil around the base of the cuttings must never be allowed to become dry, although water must always be applied with care to prevent damping off, mildew and similar troubles. From the time when the cuttings have rooted, which will be seen from their appearance, they must be kept under cool conditions so that a lot of top growth is not made without a corresponding good root system. With hardy perennials, the rooted plants can be moved to their permanent quarters as soon as they are large enough.

Apart from securing cuttings from the top growth of a plant, stock may be increased from root cuttings, although it is now a comparatively little-practised art. Some herbaceous plants readily respond to this method of propagation. The method is similar for all subjects, although, naturally, some make thicker and more easily obtainable cuttings than others.

Although not essential, I have found it best to cut the upper end of the cutting straight across, and the lower one on the slant. This will be a reliable guide as to the right way up to insert the cuttings, although it is quite satisfactory to lay the root cuttings horizontally in pans or boxes, especially some of the smaller, thinner-growing subjects.

With many plants it is possible to secure cuttings up to 2 in. long which can be planted upright, while in other cases I in. long is the maximum, with the thickness down to in. These pieces of roots are buried in boxes or pans so that their tops come just level with the surface, the best rooting medium being a sandy soil to which peat or leaf mould has been added, and which should be kept nicely moist. Some growers use a layer of pure sand so that the base of the cutting just rests in it. The sand has a stimulating effect, causing an early formation of roots.

Spring and early summer are the right seasons for taking cuttings, and before inserting them it is best to remove the fibrous roots, leaving the thicker ones.

Among the plants which can be increased in the manner just described are Anemone pulratilla and its forms, which really are best propagated in July, Anemone japonica, anchusas, some of the hardy geraniums such as subcaulescens, gaillardias, mertensias, oriental poppies and Primula denticulata.

Although some plants are lifted to secure the cuttings, where small quantities are cultivated the soil can be scraped away from round the plant, until the roots are bared. Haller the cuttings are secured the soil is returned to position, the plants will not be harmed.

After placing in pans or boxes, the cuttings are stood in a sheltered cold frame or cool house and covered with glass, which should be turned daily. Rooting will soon take place and top growth commences. A flower-spike should not be allowed to develop the first season, for it will definitely weaken the young plants if not removed.

Many plants can be propagated from leaf cuttings, but this method is chiefly employed for plants such as begonias, gloxinias, saintpaulias and the like.