Vegetative Propagation Methods
There are two basic systems of increasing plants: one, known as seminal, meaning from seed, the other vegetative, meaning from some portion of the plant that is not a seed. A seedling, whatever its source, is a new individual carrying virtually nothing from its parents except two sets of genes, one from the female, the other from the male, plus a minute quantity of cytoplasm from the female egg. In contrast, a vegetatively propagated plant is not a new ‘individual’, but an extension of another plant in another place deriving all its characteristics from those of its single parent.
This is not just a matter of academic interest but of great practical importance since, even of species, always show some variation from each other and their parents whereas plants vegetatively increased from a single original parent will usually be identical in every detail. Collectively such plants constitute a clone and if they are of special merit are often given a distinguishing clonal name, eg. ‘Peace’ is the clonal name of all roses vegetatively propagated from a seedling rose of that name raised in 1945.
The advantage of growing plants from seed is that it gives this possibility for change, that it maintains vigour and generally produces a generation free, at the outset, from diseases acquired by the parent plants; it is also often the cheapest method of multiplying stock. The advantages of vegetative methods of propagation are that they enable even minute variations to be carried on, allow plants that are sterile or produce little good seed to be increased and in some instances provide very simple methods of increasing stocks of plants that anyone can use.
Gardeners use four principal methods of vegetative propagation: division, cuttings, layering and grafting.
Division is the simplest and is done by breaking the plant into several pieces. It is particularly suitable for. Those that are loosely structured can be pulled apart with the fingers or, if tougher, levered apart using a pair of forks back to back. Others, such as peonies, and , make tough or woody crowns which may need to be cut through with a knife.
The essential feature of a division is that it must have roots and at least one growth bud. When big old plants are divided, it is the younger outside portions that usually provide the best specimens for replanting. Division of most hardyis best done in March to April, but it can be carried out at any time of the year provided sufficient care is taken to re-establish the divided plant by suitable protection and watering. In spring neither may be required if the weather is mild and showery.
Cuttings differ from divisions in starting life with either top growth or roots, but not both. There are different forms of cuttings. Stem cuttings are made from a piece of stem,from a single leaf usually with its leaf stalk attached, and from a piece of root. The cutting, whatever its original character, must be induced to form the missing parts.
Stem cuttings may be prepared from young growth, semi-mature growth or end-of-season growth — these three types are known as soft, half-ripe and hardwood. Soft cuttings are taken mainly in spring for herbaceous or sub-shrubby plants, including dahlias and. Half-ripe cuttings are usually taken in summer for a great many ornamental shrubs. Hardwood cuttings are taken mainly in autumn for increasing some shrubs and also for bush fruit, such as currants and gooseberries.
As a rule cuttings are severed just beneath a leaf or joint, because this is the point at which roots most readily develop. Sometimes half-ripe and hardwood cuttings are pulled off the parent plant with a sliver of older branch attached, and these are known as ‘’. Soft cuttings must be solid flesh right through, not hollow at the base; half-ripe cuttings need to be firm at the base, neither too immature nor too hard, which takes a little experience to gauge well, and ripe cuttings must be really firm and mature without being too woody.
Soft cuttings must be kept in a very humid, still atmosphere, or they will flag and quickly die. Firstly, insert them in cuttingin a propagator, warming the to maintain a close atmosphere and stimulate the fastest possible root formation, ie in two or three weeks.
Half-ripe cuttings are just a little more resistant to collapse but also require damp, still air. Root them either in a propagator, a pot placed inside a polythene bag or in mist, a special device which sprays the cuttings with water automatically and keeps them constantly wet. Cuttings of this type usually take a month or so to root.
Hardwood cuttings have little or no tendency to flag and if they are of deciduous plants are likely to be bare of leaves when (or shortly after) they are taken. You can plant them outdoors if they are hardy, or in a frame or greenhouse if they are tender, but they take many months to root well.
Many different materials are used as cutting composts, the favourites being sand, Perlite, peat and leafmould. A mixture of equal parts peat and either Perlite or sand is a good basic formula.
Various chemicals, often referred to as root-forming hormones, can hasten or increase root formation. They are sold in the form of powders combined with a fungicide which prevents diseases from attacking the cutting while it is rooting. They are sometimes offered in three strengths, weak for soft cuttings, medium for half-ripe and strong for hardwood.
For soft and half-ripe cuttings it is usual to remove the lower leaves completely; large leaves may be cut in half to reduce the demand for moisture. When this has been done, place the base of the cutting in water, then dip it in the hormone rooting compound and finally press it gently into the rooting mixture in a pot, pan, tray or bed, whichever is most convenient. It may be necessary to use a dibber (a pencil-like piece of stick) to make holes for soft cuttings, and hardwood cuttings are usually lined out in little trenches chopped out with a spade, but half-ripe cuttings can usually simply be pushed in about half their length and made firm with the fingers. Water soft and half-ripe cuttings well, unless you are using mist which will take care of that automatically.
Once cuttings are rooted, they start to grow again but it may be worth lifting one or two carefully to see just how good the root formation is. When it seems satisfactory, pot the cuttings individually in a potting mixture and keep in a moist, unventilated atmosphere until they are established and air can be admitted. This can sometimes be the trickiest part of the whole operation. Hardwood cuttings can usually be replanted into nursery beds the autumn following insertion.
Layering might be described as taking cuttings without separating them from the parent plant until they are rooted. In some plants, strawberries for example, it is a natural means of increase, long runners being produced with plantlets along them. Where these touch the ground they root and form new plants and so the colony extends rapidly.
In many plants the rootings of young stems can be promoted by wounding, dusting with hormone rooting powder and covering with soil. This is the usual method of increasing border carnations, the non-flowering stems being partially slit through a joint in summer and pegged to the soil around the parent. If this is done in July sufficient roots are likely to have been formed by September to allow the layers to be severed from the parent plants and then, a few days later, lifted and replanted elsewhere.
Many shrubs andcan be layered by bending a flexible stem and inserting it beside the parent plant in a shallow trench. Make a small slit on the underside of the stem to allow for rooting. Cover with cutting compost and hold securely in place with a hoop or peg. If this is done in spring it is possible that the layers will have made enough roots to be separated and transplanted by the autumn, but frequently shrub layers require at least a year, possibly two, before they are able to exist on their own. Brambles of most kinds will root from the tips of young canes if these are bent over and pegged to the soil.
It is also possible to layer shrubs and climbers without actually bringing stems to ground level. Instead, after suitable wounding and dusting with hormone rooting powder, they are wrapped in wet sphagnum moss, a sleeve of polythene film is pulled over this and tightly tied at each end to keep in the moisture. When sufficient roots have formed in the damp moss, the layer is severed and potted or planted. This is known as ‘air-layering’.
Grafting is the most complex method of propagation and consists of uniting two plants, one of which, known as the rootstock, will provide the roots and the other, the scion, all or most of the top growth. Each plant retains its separate identity and it is important to stop shoot growth from the rootstock because, if retained, it would have the characteristics of this and not of the plant grafted on to it. Nevertheless each can have some influence on the other and rootstocks are often chosen to control the vigour or precociousness of fruit trees.
Two of the many methods of grafting will illustrate the principles involved: splice-grafting and budding. Splice-grafting is suitable when there is not a great difference in thickness between rootstock and scion. Both are taken from year-old plants. The plant providing the rootstock is beheaded and a slice of bark and wood is removed at the top of the stump. An exactly similar cut is made at the base of the scion, and the two cut surfaces are placed together and securely bound so that there can be no movement while they are uniting. As a rule the wound is further covered with grafting wax to prevent loss of moisture from the cut surfaces. This kind of grafting is usually done in early spring when growth is just starting and it is an advantage if scions can be a little retarded by being pre-cut in winter and partly buried in soil.
Budding, by contrast, is usually done in summer when plants are in full growth. It is the main method used for the commercial increase of roses. The scion is a single growth bud cut with a shield-shaped portion of bark and the rootstock is prepared to receive this by having a T-shaped incision made in its bark. The flaps of bark on each side of the T are gently raised and the bud bound in. No wax covering is required.
Buds are cut from well-developed young stems and it helps if you remove the sliver of wood which is cut with the bud as this will expose more of the active layer of cambium beneath the bark which has to unite with the cambium of the rootstock. It is usually possible to tell in a few weeks whether the union has occurred and the following spring the bud should start into growth. Before that date, cut off the stem just above the inserted bud so that rising sap is concentrated on it. The bud is now the source of all further top growth.