Vegetative Propagation of Ornamental Shrubs
When propagated by vegetative means the new plants retain the exact characteristics of the parent plant. In vegetative propagation growing parts are separated from the plant and when they have put out roots of their own they develop into new, separate individuals. Shrubs may be propagated by cuttings (hardwood, softwood or root cuttings),, stooling or division. Grafting and budding are also vegetative means of propagation. Here pieces of two different plants, one the scion (graft or bud) taken from the particular species to be propagated and the other the stock, are united by one of several possible means, whereupon the graft (or bud) receives nourishment from the rootstock and grows into a new plant. Grafting and budding are very important in plant breeding.
Propagation by hardwood cuttings
This is one of the simplest vegetative methods of propagating woody plants. No special equipment is required and the cuttings may be inserted directly in the nursery beds. Themust be well prepared in autumn by forking to a good depth. It should be of a light, sandy-loamy consistency; sand may be added to heavy soils to make them lighter. In the case of dry soils peat or leaf mould should be worked in to help retain moisture.
Hardwood cuttings are taken from the parent shrub in late autumn after the leaves have been shed but before the first frosts. They are cut with a sharp knife or secateurs from well-developed one-year-old shoots, the bottom cut sloping as much as possible, the upper cut straight across. The bottom cut should be made just below the bud, the upper cut may be up to 2cm (½ in) above the bud and the length of the cutting should be 10 to 20cm (4 to 8in) but may be up to 30cm (12in) long for some shrubs.
Hardwood cuttings are generally taken in October and November. They can be inserted out of doors without protec tion, though with some, especially cuttings of evergreen shrubs and trees, it is advisable to provide the protection of an unheated frame. They are usually nodal cuttings, although it is sometimes argued that heel cuttings root more readily. With cuttings of evergreens the lower leaves should be removed. The cuttings are placed upright about 5 to 10cm (2 to 4in) deep and 5 to 8cm (2 to 3in) apart in a straight-sided trench with a little coarse sand scattered along the bottom, and the soil is then filled in and well firmed. There they can be left until the following autumn, one year later, when they should be ready for transplanting.
Of the ornamental shrubs appearing within this website those that can be increased by hardwood cuttings are: buddleia, calluna, clethra, deutzia, caragana, colutea, cornus (Corpus alba), forsythia,, lonicera, philadelphus, ribes, sambucus and spiraea.
Propagation by softwood cuttings
The method of propagation by softwood cuttings, which is suitable for most ornamental shrubs, is the one most widely used by gardeners nowadays, for these may be taken practically any time during the growth period and the work entailed in propagating various shrubs may be evenly distributed throughout that period.
For amateurs, however, increasing shrubs by softwood cuttings is not an easy task. First of all it requires knowledge of the right time to take the various types of cuttings. True, softwood cuttings may be taken practically any time from spring till autumn but in some species it is better to do so earlier, before the shoots begin to show the slightest sign of turning woody at the base (greenwood cuttings), in others later, when the shoots are partly woody (half-ripened cuttings). In certain species cuttings should be taken with a sliver of the old wood where the shoot arises (cuttings with a heel) and some softwood cuttings must be inserted in the soil immediately after they are taken, whereas others can be kept until they have wilted.
Amateurs may also have little success with rooting cuttings directly in beds. Apart from a few exceptions, softwood cuttings will root only under certain conditions in a greenhouse, propagator or frame. They may be inserted in peat, pure sand, vermiculite, perlite or a compost such as the John Inns cutting compost, but it must be remembered that raised in pure sand or vermiculite the cuttings will need to be moved into ordinary compost as soon as roots have formed or they will soon starve. Cuttings of this type quickly lose moisture, so it is essential that a very damp atmosphere can be maintained and that they should be encouraged to form roots quickly. Dipping them in hormone rooting powder or liquid and if possible providing some form of bottom heat are aids which assist rooting. Ideally, softwood cuttings are best inserted in a propagating case or closed box within the greenhouse and shaded from direct sunshine, and they may also be succesfully rooted under mist.
The cuttings should be sliced cleanly just below a node or joint and the bottom leaves trimmed off. The length of the cutting will vary between 2cm (¾ in) and 8 or 10cm (3 or 4 in) depending on the type of plant. Using a small dibber, the cuttings should be inserted to about a quarter of their own depth and the rooting medium firmed round their bases. If inserted in earthenware pans or pots, the cuttings thrive best if placed round the edge of the receptacle. The start of new growth indicates that roots have formed. The first roots appear in two or more weeks, at which time routine care changes. Water is applied to the soil instead of being sprayed on the cuttings, ventilation is increased and the cuttings are shaded for shorter periods. Once they have rooted the cuttings are transferred to flowerpots but should be left in the frame or greenhouse, to be put out in beds the following spring.
Shrubs that are readily multiplied by softwood cuttings are actinidia, akebia, berberis, buddleia, chaenomeles, clematis, colutea, cotoneaster, cytisus, deutzia, erica, forsythia,, , , hydrangea, , , kerria, , lonicera, magnolia, , ribes, rubus, sambucus, spiraea, vaccinium, viburnum and .
Propagation by root cuttings
Propagation by root cuttings is a little-used method, reserved chiefly for those species that have adventitious buds on the roots and cannost be reliably propagated by other methods (rubus). The roots are dug up in the autumn before the frost and stored in sand in a spot protected against frost. To differentiate between the top and bottom of the root cuttings, which should be about 5 to 12cm (2 to 5in) long, a straight cut is made across the top and a sloping cut across the bottom of the cuttings. They are inserted in a mixture of peat and sand and covered with a layer of about 2 cm (¾in) of light soil mixed with sand. Further care is the same as for hardwood cuttings.
Propagation by layering, stooling and division
Layering is not often used in commercial gardening for it produces only a very limited number of new plants, but for the amateur gardener this method is well worth consideration. The procedure consists of taking well-ripened annual shoots in spring and bending them down to the ground. At this point the stem is slit halfway through, pegged to the ground and covered with soil. Roots and new shoots will grow from the incision, which should be kept covered with good soil. As a rule, the layered shoots produce a good root system during the first year and can be separated from the parent plant in the autumn and transplanted. This method is used with success in increasing magnolias and akebias.
Stooling is a method similar to layering. In early spring the parent plants are cut back close to the ground so that they will put out as many young shoots as possible. When these are about 20 cm (8 in) long they are covered with a mound of soil up to about half their length. This is repeated about two or three times during the growing period. The shoots are generally well rooted by the autumn and can then be cut away from the parent shrub and transplanted. This method may be used to propagate calycanthus, clethra, cornus, magnolia, staphylea and viburnum.
Propagation by division and by means of suckers is a less commonly used method. In the first instance either a clump is separated from the parent shrub or else the entire shrub is divided into sections which are then grown on like seedlings or else put in their permanent sites. Those shrubs that put out a great number of new suckers from the base are easily increased by cutting away these suckers in the autumn and transplanting them. They are transferred to their permanent sites when they have developed a good root system, as a rule in one or two years.
Whenby grafting a part of the plant to be propagated (the scion) is transferred and attached to the rootstock of another plant. There are several methods of attachment. Where the scion and stock are of the same thickness whip or tongue grafting is employed. When the diameter of the scion is smaller than that of the stock then saddle grafting, slit or notch grafting, rind grafting or cleft grafting are used. The principal methods of grafting are shown in the illustration below. The scions for grafting may be taken from woody or green branches.
It is advisable that the stock should be more advanced in growth than the scion, and for this reason the scions are cut during the winter before the growing period begins and planted, right side up, in a really cool, shady position such as the north side of a wall where they will remain dormant until the stock is already coming into leaf. The successful union of the scion and rootstock depends on the correct tying and treating of the cut surfaces. Raffia is still traditionally used for tying even though it is being replaced of late by plastic tape. The cut surfaces should be spread with grafting wax leaving no spot uncovered.
Shrubs may be grafted also in winter in the greenhouse. In this case the stocks must be made ready in time, that is potted up during the preceding growth period and then stored in a frame or bed. About a month before grafting their growth is accelerated by transferring them to a greenhouse with a temperature of about 20° C (68° F). The scions are grafted on when the stocks begin to bud.
Budding is used to propagate only certain species of ornamental shrubs, chieflyand lilacs. The usual time for budding is late June, July or early August and it is done out of doors. The bud is taken with a sliver of wood behind it from a well-ripened annual shoot. The wood shaving is removed and the bud inserted inside a T-shaped incision made in the stock, which is in full growth. It is then tied in place with a piece of soft material such as raffia. The tie should be slit in time so that the bud and rootstock are not constricted. In early spring of the following year the stock is cut away immediately above the bud in the case of roses, whereas with lilacs it is cut off about 15 to 20cm (6 to 8in) above the budding point.