Parts of plants can be pruned (removed) to reduce the competition within the plant for the available resources. In this way, the plant is encouraged to grow, flower or fruit in a way the horticulturist requires. A reduction in the number of flower buds of, for example, chrysanthemum, will cause the remaining buds to develop into larger flowers; a reduction in fruiting buds of apple trees will produce bigger apples, and the reduction in branches of soft fruit and ornamental shrubs will allow the plants to grow stronger when planted densely. Pruning will also affect the shape of the plant, as meristems previously inhibited by apical dominance will begin to develop. The success of such pruning depends very much on the skill of the operator, as a good knowledge of the species habit is required.
A few general principles apply to most pruning situations:
- Young plants should be trained in a way that will reflect the eventual shape of the more mature plant (formative pruning). For example a young apple tree (called a 'maiden') can be pruned to have one dominant 'leader' shoot, which will give rise to a taller, more slender shape. Alternative pruning strategies will lead to quite different plant shapes. Pruning back all branches in the first few years forms a bush apple. A cordon is a plant where there is a leader shoot, often trained at 45 degrees to the ground, with all side shoots pruned back to one or two buds. Cordon fruit bushes are usually grown against walls or fences. Similarly, fans and espalier forms can be developed.
- The pruning cut should be madejust above a bud that points in the required direction (usually to the outside of the plant). In this way, the plant is less likely to acquire too dense growth in its centre.
- Pruning should remove any shoots that are crossing, as they will lead to dense growth. Some plants, such as roses and gooseberries, are made less susceptible to disease attack by the creation of an open centre to produce a more buoyant (less humid) atmosphere.
- Weak shoots should be pruned the hardest where growth within the plant is uneven and strong shoots pruned less, since pruning causes a stimulation of growth.
- Species that flower on the previous year's growth of wood (e.g. Forsythia) should be pruned soon after flowering has stopped. Conversely, species that flower later in the year on the present year's wood, e.g. Buddleia davidii, should be pruned the following spring.
Root pruning was used to restrict over-vigorous cultivars, especially in fruit species, but this technique has been largely superseded by the use of dwarfing rootstock grafted onto commercially grown scions. Root pruning is still seen, however, in the growing of Bonsai plants. Pruning is largely concerned with creating the shape of a plant, and controlling apical dominance, but the removal of dead, damaged and diseased parts is also an important aspect.
The flowering plant
The progression from a vegetative to a flowering plant involves profound physical and chemical changes. The stem apex displays a more complex appearance under the microscope as flower initiation occurs, and is followed, usually irreversibly, by the development of a flower. The stimulus for this change may simply be genetically derived, but often an environmental stimulus is required which links flowering to an appropriate season.
Photoperiodism is a day length stimulated developmental response by the plant.
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