Garden Workshop: Vegetative propagation
Vegetative propagation is a method of propagating plants other than by growing from seed. Most fruit, and some vegetables, cannot be conveniently increased, or ‘propagated’, from seeds, usually because the resulting plants are not quite true to type or because they do not produce viable seed.
If you wish to propagate named varieties of some fruit and vegetables, for instance Majestic potato or Cox’s Orange Pippin apple, you will have to increase these by means other than by seed.
With the example above, although both varieties came from chance fertilization of flowers resulting in viable seeds of outstanding merit, the likelihood of producing plants having all the characteristics of the particular variety by growing them from second generation seed is remote. The reason for this is that, when raised from seed, the offspring contain some of the characteristics of each parent. The genetical make-up of seeds, although similar, is not absolutely identical with one parent or the other. However, this problem can be overcome by vegetative propagation, detaching and growing on part of a plant of the desired variety. This part may be a shoot, stem, leaf or root cutting; a tuber, sucker or runner; a bulbil, division or offset; a bud or graft.
Whatever part is used for propagation, the resulting plants will be identical to their parents; thus the desirable characters of the variety remain unaltered. All tubers from the potato Majestic have the same genetic constitution as the plant from which they came, and similarly all the shoots of Cox’s Orange Pippiti have the same composition as each other.
A sport is ‘two plants in one’; it is a bud variation that grows into a shoot different (usually in one character only) from the typical growth of the plant that gave rise to it. For example, the potato variety Up-to-date, which has smooth tubers, gave rise to a plant bearing rough-skinned tubers. This was then vegetatively propagated and given the name Field Marshal. New varieties produced from sports must usually be propagated vegetatively.
Sometimes, partial sports arise, as when areas of the skin of peaches are smooth like the skin of a nectarine, while the rest of the fruit has the typical downy skin of the peach.
The basic methods
The growing of vegetables by vegetative propagation is really quite simple and can be quite profitable. The raising of fruit may prove more difficult to master, but patience and perseverance should prove very rewarding.
The principles of vegetative propagation are quite straightforward, although there may seem to be a bewildering number of methods of doing it. Vegetative propagation occurs quite naturally with some plants, such as, raspberries, strawberries and blackberries. The task of the gardener is to ensure that the individual plants that are propagated are given adequate space, care and attention, to enable them to grow and become established.
Crops such as fruit trees are slightly more demanding, as areof one type or another. Stem or leaf-bud cuttings, for instance, are taken and rooted in containers of suitable (equal parts by volume of moss, peat and sharp washed grit); sometimes the wounded end is first dipped in hormone rooting powder or a liquid hormone preparation to ensure quicker and more certain rooting, but this is unnecessary for most cuttings of fruit or other crops. The unrooted cuttings are placed in a greenhouse, propagator, or frame and kept warm and m6ist, to encourage root formation and prevent them from drying out. Cuttings of some semi-ripe shoots, and all ripe shoots, such as those of gooseberries, can be rooted outdoors. Citrus cuttings, which are semi-ripe, need warmth and protection in temperate climates.
and are more complicated, but the simpler types can be performed by the gardener, provided that he or she is prepared to take time and trouble over the operation. Failures are, more often than not, due to lack of understanding of the basic requirements for success.
The stems of woody plants consist of three layers of tissue, the outer bark, the inner heartwood, and the vital cambium—an extremely thin layer, only a few cells thick, separating the heart-wood from the bark.
When a bud or wood graft of the desired variety, known as the scion, is attached to the rootstock, the cambium layers of rootstock and scion must be in contact for scion to unite. This is why it is vital to use a really sharp knife and make clean, firm cuts when grafting or budding.
The other important consideration is to ensure that the point of union does not dry out and that the budded or grafted portion does not move.
You do not need much special equipment for successful propagation. To cover the methods in this article the basic essentials include: a well constructed and carefully maintained garden frame; secateurs and a pruning and budding knife; some pots or boxes and sterile cutting compost; raffia or clear polythene tape; grafting wax; a watering can with a fine rose; a garden syringe and some clear polythene sheet or bags to blow up and put over cuttings that need humid conditions. Some gardeners also like to have hormone rooting powder or liquid to hand for less hardy cuttings. Other necessities include: healthy, disease-free plant material; and clean, weed-free, well-drained and prepared, to receive the plants; plus as much care and attention as possible.
The foregoing list is not as formidable as it sounds, and most gardeners will already have many of the items tucked away somewhere.
The following methods of propagation all exploit the natural habits of the plants involved and are all relatively simple for the amateur to carry out.
Runners and rhizomes
Runners, or stolons, are stems which grow above ground, and produce one or a series of tufts of leaves at what would otherwise be an ordinary leaf joint. The stems at these points form roots on contact with moist soil. Strawberries may be propagated by this means. The best method is to raise new plants by pegging the runners, one each into a pot of compost and stopping each runner from producing more than one plantlet. Rhizomes are creeping stems growing at or below the soil surface, as with mint. They provide a simple and easy means of vegetative propagation. Both the above-ground runner and the underground rhizomes are forms of stems modified to perpetuate the plants, and do so naturally; the problem with mint is often not how to propagate it, but how to stop it from propagating itself naturally and becoming too invasive.
The tips or extremities of the ‘rods’ or arching stems of such fruit as black-berries and loganberries will form roots on contact with the soil and develop into new plants. You can assist nature by pegging the tips onto or into some prepared soil and covering them with fine soil. This is known as tip. Layering involves the pegging down of suitably-placed shoots, branches or stems onto loose friable soil. To assist and speed up rooting a slanting cut 2.5-4 cm (1 – 1-1/2”) long is made half-way through the underside of the stem, opposite a leaf-joint; the stem is pinned down onto the soil at that point. Quince and cobnut can be increased by layers pegged down in autumn. Stool layering is another natural means of propagation which has been modified by man to accelerate the process. The method involves planting the crop to be increased, such as apple and pear rootstocks, at an angle of 45° to ground level, and cutting back the central stem to about 10 cm (4”); then, when the resultant shoots appear, these are earthed up successively like potatoes. In autumn the rooted shoots are exposed to view by carefully removing the soil around them and then severed and planted out.
Other forms of layering exist, but are not considered here because they are needed only in very exceptional cases.
Suckers are shoots which arise naturally from buds situated below the soil surface, and this phenomenon is common to many fruits and nuts. Healthy young rooted suckers are separated from an older plant, lifted and replanted as 6 required. Globe artichokes are vegetables that can be propagated from | suckers. Suckering is sometimes so c successful that the numerous growths are an annoyance—plums and raspberries are cases in point.