The technique involves initiating, developing and establishing a root system on a stem either before or after that stem has been removed from the parent plant, and this can be done in two ways: by layering and by stem cuttings.
With layering, the stem is encouraged to produce roots before it is severed from the parent plant. The main problem is to establish the rooted layer after it has been removed from the parent.
With stem cuttings, the stem is removed from the parent plant before it is encouraged to develop roots. With this method, it is difficult to keep the cutting alive until its roots have grown and established. Its advantage is that, usually, it takes up much less space than propagating by layering.
Always propagate from stems that have a high capacity to produce roots. The gardener must learn to judge this capacity in plants, as it has a significant affect on the eventual success of the layer or cutting.
The age of the parent plant, as well as that of the actual stem to be propagated, both have major influences on the capacity of a stem to produce roots.
A seedling that has recently germinated cannot immediately flower and produce
seeds, as it is not sexually mature. Therefore it is described as being juvenile, and the only way it can reproduce is asexually, from vegetative parts. Many juvenile plants have a very high capacity to regenerate vegetat-ively. The juvenile stage is not transient but fixed in some ornamental plants, such as some varieties of ivy (Hedera helix) and the Lawson's cypress varieties 'Ellwoodii' and 'Fletcheri', and these have a greater capacity than their adult varieties to produce roots on stems.
Juvenile plants, however, do not have much value in the garden as they do not flower and produce seed. Most of them will eventually mature into adult plants, when they will flower. In this condition a plant is sexually mature, capable of regenerating by producing seeds, and so it does not need to reproduce vegetatively.
A plant's capacity to regenerate from vegetative parts declines with age. Thus the gardener wishing to propagate vegetatively from a mature plant encounters a problem. This can be overcome by growing a mature plant that does not produce flowers, so that its capacity to regenerate vegetatively may be increased, and thus the rooting capacity of its stems.
To prevent the plant, or at least the stems used for propagation, from flowering, prune them rigorously so that strong, vegetative (that is non-flowering) shoots are produced. The harder the stems are pruned, the faster they will grow and the more they will produce roots. The first flush of growth on a stem always has the greatest capacity to develop roots. Really fast growth is achieved by a combination of hard pruning and forcing the plant in a high temperature, at least 16°C/ 60°F. Under these conditions, roots may be induced on otherwise quite intractable stems.
The capacity of a stem to root will also be dependent on the age of the parent plant. The older the plant, the less it will be able to produce roots on stems, even if the stem to be propagated has been severely pruned. If the plant has been grown from a seed by the gardener, then he will know how old it is, and if he can successfully propagate from it.
If the plant to be propagated has been produced by a vegetative technique quite recently, it should root from its stems quite readily, but this is not always so. Many plants, however recently they were vegetatively propagated, may in fact be quite old! If the original plant was grown from seed and displayed attractive ornamental characteristics, it would always have been propagated vegetatively to maintain those characteristics in subsequent generations, but its inherent capacity to produce roots on stems will have declined, despite a temporary resurgence after pruning.
Thus new (that is, young) varieties are generally easier to propagate from stems than older varieties. This is particularly evident with plants such as deciduous azaleas, in which the old Ghent varieties are over 140 years old, the newer Ham varieties are only 20 years old, and the Exbury varieties are 40 years old. The last two groups are relatively easy to propagate from stems.
Before propagating from stems, it is also necessary to consider whether there is a best time of year in which to propagate a particular plant by a particular technique, as it is pointless to do so in the wrong season. The capacity of many plants to produce roots on stems varies considerably during the different seasons, whereas in others there is no distinct seasonal variation. The index at the back of this book will tell the gardener when, where and how to propagate a particular plant.
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