ABSTRACT
Bulbous plants offer gardeners a simple and straightforward means for achieving colourful and interesting flowering displays over a long season. There is an enormous array of these plant forms which extends the availability of flowering virtually year-round. Bulb types such as daffodils (Narcissus), tulips and hyacinths, together with corms such as crocus and gladioli, are most popular. These are used in this chapter as key examples for describing and illustrating form and function in bulbous plants. Also included in this group of plants are those which produce tubers, rhizomes and swollen roots. Most bulbous plants are geophytes and have evolved naturally in dry, arid zones, consequently their husbandry must permit sufficient post-flowering periods for the restoration of reserves of resources and that the soil conditions provided allow for senescence and secure dormancy of large masses of tissue. In particular, types of bulb and corm divided by gardening categories are introduced and discussed. Most bulbs may be planted as “dry bulbs”. But planting “in-the-green” is especially valuable for small early spring types. The nutrition, long-term establishment (naturalising), use in containers for external and internal decoration, problems with pests and diseases and over-season care are considered and illustrated. Biological differences between bulbs, formed of leaves and corms, an expanded stem base, are considered. A range of specialist bulbous plants are discussed. These provide a sequence of flowering from late winter round to the following late autumn. The effects of climate change are affecting bulbous plants such that they are flowering earlier than was previously the case. Bulbous plants propagated by asexual vegetative means are influenced by the soil conditions and climate where they are grown. Consequently, although they may be the same variety, those plants produced in more northerly zones have greater degrees of hardiness compared with those coming from further south.
Bulbous plants are good examples of source and sink relationships. Gardeners manipulate these relationships as a means of achieving colourful displays and maintaining colonies of some bulbs over several seasons. The internal and external structure of bulbs and corms are described and illustrated. These plants present examples of modified stems and leaves which have evolved and fit into environmental niches, which gardeners replicate. Processes that stimulate and inhibit flowering are considered together with the diversity of bulbous plants. Flowering and vegetative growth may occur in different seasons, as with autumn-flowering Colchicum, that makes them especially valuable garden components but does require gardeners’ understanding of their growth cycles.
The biological diversity of bulbous plants has resulted in forms of rhizomatous, stoloniferous and tuberous types which gardeners cultivate and which can provide colourful displays. Some bulbs have diverse root structures where contractile forms pull the whole structure deeper into the soil, feeder types which anchor the plant and root hairs which form an absorptive surface. Some bulbs lack root hairs and their function is fulfilled by the feeder types.
Flower initiation and extension take place in some bulbs while they are still quiescent or just beginning growth, this is illustrated and its bearing on husbandry considered. The multiplication of well-established bulbous types is illustrated by the vegetative division of Agapanthus, which over years produces dense mats of swollen moisture-retentive perennating roots. This provides a simple example for gardeners of the manner by which the asexual multiplication of garden plants is achieved. Learning achievements are provided at the end of this chapter.
