Springtime colour is one of gardening’s greatest joys. Colourful bursts dispel the long darkness of winter with its depressing wetness and cold. Social research is clearly showing the physical and mental benefits obtained from the emergence in spring of bright garden colours linked with lengthening daylight. As with most gardening pleasures, this requires advanced financial outlay and an understanding of the rhythms of plant growth.
Planting bulbs such as daffodils, tulips and hyacinths in autumn is the necessary investment. In return, plant breeders now provide a huge array of colours, shapes, sizes and seasonal sequencing with bulbous plants.
Geoff Dixon: February Gold daffodils
Bulbs are large pieces of vegetative tissue which come pre-loaded with immature leaves and flowers, safely wrapped inside a dry coating of protective scales. Essentially, bulbs are large flower buds which are stimulated into growth by planting in warm, moist soil or compost. These conditions trigger the emergence of roots from the base of each bulb. Because bulbs are nascent plants, they require careful handling and are safest once planted.
Many bulbous species originate from higher altitude mountainous pastures and are naturally evolved for dealing with fluctuating periods of heat, cold and drought. Once safely planted at depths which should equal twice the length of each bulb, they will survive the freezing, thawing and fluctuating soil water- content delivered by winter weather.
Geoff Dixon: Bulb structure showing the flower bud embedded in the bulb
Warming soils of spring encourage growth and emergence of the leaves and flower buds contained within each bulb. Speed of emergence is governed by interaction between the genetic complement of bulbs and an interaction with their environment. Identifying and understanding the impact of this interaction formed the basis for Charles Darwin and Alfred Wallaces’ theory of natural selection. For springtime gardeners it is expressed in the multiplicity of bulbs on offer. Choosing a range of daffodil varieties for example, provides colourful gardens from February through to late May.
Geoff Dixon: Technique for planting bulbs using hand trowel and some sand for drainage under the bulb
Conserving the joys of spring pleasure over years can be achieved by naturalising bulbs. This means planting them in grass swards. This works effectively for daffodils, provided the foliage is allowed 8 to 10 weeks of uninterrupted growth and senescence after flowering. During this period, photosynthesis produces the chemical energy needed for replacement growth, which provides bulb multiplication and flower bud development for the following year. Tulips are much less easily naturalised in British gardens. This is because the leaves mature and senesce much more quickly after flowering, hence, less energy is produced, therefore, regrowth is less, and replacement flower buds are not formed.
For most gardeners the policy should be one of enjoying each springtime’s show and replacing bulbs with new ones every autumn for a relatively modest outlay.
A lavender field near Provence, France.
Flowering is the process by which higher plants transfer male gametes to female organs thereby uniting two sets of chromosomes and increasing natural diversity. During the formation of male and female gametes, slight changes take place in chromosome structure. Consequently, the resultant next generation differs slightly from its parents. That is the stuff on which natural selection operates.
Useful variations increase the survival fitness of some offspring, while individuals with disadvantages wither and die. Charles Darwin recognised the power of natural selection for the environmentally fittest individuals and how that leads eventually to species evolution. Succeeding generations of scientists have discovered details of the processes involved and how these may result in more useful plants for humankind by plant breeding.
Transferring the male gametes (i.e. pollination) happens by a variety of mechanisms which are suited for the environment in which particular plants grow. At its simplest, pollen which consists of cells containing male gametes is transferred within the same flower. That is suitable for plants growing in for example, alpine environments where few other options exist.
Pollen grains contain both reproductive and non-reproductive cells.
Cross-transfer of pollen from one flower to another is achieved either by physical means such as wind or water, or by partnerships with animals – particularly insects and especially bees. Wind transfer is suitable for trees such as hazel, birch and willow, which flower ahead of leaf formation in the early spring when it is too cold for insect flight. Biologically, it is a wasteful mechanism because much of the pollen does not reach its target.
Cross-pollination by insects produces by far the most colourful and exuberant flowers. These have evolved brilliantly colourful displays and intricate mechanisms suitable for either general interaction with insects or as means for partnership. These relationships have co-evolved and converged over numerous generations meeting the needs of both parties.
Sexual reproduction in plants. Video: FuseSchool - Global Education
Plant scientists are presented with intriguing questions in understanding how these relationships could have developed. On the practical side, plant breeders are presented with enormous opportunities for developing massive arrays of new varieties, particularly with ornamentals such as the garden favourites like dahlias, chrysanthemums, lilies and roses.
Enormous international trade has developed over the last hundred years exploiting increasingly colourful flowering plants.
An estimated 24% of Europe’s bumblebees are threatened with extinction.
Cross-pollination is absolutely vital for many field vegetable crops, especially peas and beans and the top and soft fruits. A reduction in beneficial insect populations now presents dire threats for natural biodiversity, our food supplies and the enjoyment of ornamentals.