ABSTRACT
Vegetative propagation is an aspect of gardening that requires knowledge and skill and can with experience bring great pleasure and enjoyment. Asexual multiplication ensures that the favoured characters of garden plants are conserved as clones, which in the vast majority of cases preserve without variation desirable traits. Skilful vegetative propagation is an ancient art which has been practised by civilisations worldwide as they required stable supplies of food and ornamentals. From the 16th century onwards, plant hunters delivered their collections from around the world into Europe and soon afterwards North America. These were taken from the environments for which they were fitted. Propagation of this material in alien conditions required the development of knowledge, expertise and skills by the gardeners who were charged with safekeeping these novelties. Today, the International Plant Propagators’ Society (IPPS) provides a network for the exchange of such information.
Simple vegetative propagation usually involves encouraging root formation by sections (cuttings) of plant stems and other organs. Cuttings are of three basic types: softwood, where there is little or no secondary thickening; semi-hardwood cuttings, where secondary thickening has commenced; and hardwood cuttings, which are heavily thickened. Propagation processes are described and illustrated for softwood cuttings of Pelargonium, semi-hardwood cuttings of Hebe salicifolia, Autumn Glory, and woody cuttings of blackcurrant (Ribes spp.). Techniques are described for preparing cuttings, treating with root-stimulating compounds and subsequent husbandry which maintains the cuttings until roots have emerged and they become self-sustaining new plants. Aspects of vegetative propagation techniques are also described and illustrated in Chapter 5 for the runners (stolons) of strawberry, Chapter 6 for the swollen roots of Agapanthus, and in Chapter 7 for entire plants of Campanula spp. The propagation and subsequent husbandry of swollen stems is further described and illustrated for rhizomes of Iris germanica varieties and for the leaf scales of Lilium spp. The latter in particular requires the careful cultivation of fragile leaf scales until rooting and the formation of photosynthetically active leaves has taken place.
Understanding the roles of growth-promoting substances in plant cell division and multiplication was a key step in making propagation more reliable and widely applicable. Plants produce several growth-regulating chemicals as part of their control and management systems. Synthetic rooting preparations use auxins and their derivatives as means for encouraging root cell differentiation, expansion and growth. These induce the development of root initials from the meristematic cells within stems. In some plants, transformation of meristematic cells into root primordia is a frequent happening. This is discussed and illustrated by reference to the winter-flowering Jasminum nudiflorum in which rooting is triggered naturally where stems are in contact with soil.
Carefully controlled vegetative propagation utilising in vitro root and shoot production is now used worldwide in the micropropagation industry, which commercially produces a huge range of food and ornamental plants. Micropropagation is illustrated with plants initiated from cells extracted from undifferentiated callus tissue and grown under sterile conditions until root and leaf formation are initiated. Thereafter, the plants are sustained in humid conditions using mist propagation. Control equipment required for mist propagation is illustrated and discussed. Learning achievements are listed at the end of this chapter.
