Agriculture Reference
In-Depth Information
fruits. In addition, hormones produced by the fruit
inhibit flower development and with many species,
the maturation of fruits will considerably reduce
the plant's ability to continue producing flowers. By
removing dead flowers, a pruning technique called
dead-heading, the appearance of a garden border
can be maintained. An added bonus is that plants
that have been dead-headed may continue to flower
for many weeks longer than those allowed to retain
their dead flowers. Examples of species needing this
procedure are seen in bedding plants which flower
over several months including
Tagetes erecta
(African
marigold); in herbaceous perennials,
Delphinium
and
Lupin
; in small shrubs,
Penstemon fruticosus
; and in
climbers,
Lathyrus odoratus
(sweet pea) and
Rosa
'
Pink Perpetue'.
Many species such as
Begonia x
semperflorens-
cultorum
(wax begonia) and
Impatiens x walleriana
(busy lizzie) used as bedding plants have been
specially bred so that flowers do not produce fruits
containing viable seed. In such cases, there is not
such a great need to dead-head, but this activity will
help to prevent unsightly rotting brown petals from
spoiling the appearance of foliage and newly produced
flowers.
walls or fences. Similarly, fans and espalier
forms can be developed. However, regular
trimming of hedges produces a mass of
laterals with no single leader making a dense
well-shaped hedge.
X
The pruning cut
should be made just 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. 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.
X
Pruning should remove any shoots that are
crossing
as they will lead to dense growth
and may have damaged bark which could be
a point of entry for disease.
X
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.
X
Root pruning
used to be done to restrict
over vigorous cultivars, especially in fruit
species, but this technique has been largely
superseded by the use of dwarfi ng rootstock
grafted on to commercially grown scions.
Root pruning is still seen, however, in the
growing of bonsai plants.
5
Pruning - some general principles
Pruning affects the shape of the plant, through
a property of plant called
apical dominance
.
This is where the apical bud inhibits growth
of buds further down the stem. By removing
the apical bud, lateral shoots are released and
develop. The success of such pruning depends
very much on the skill of the operator, so a
good knowledge of the species habit is required
together with an appreciation of the purpose of
pruning.
X
Young plants
should be trained in a way
that will refl ect 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 fi rst few years forms a bush apple. A
cordon is a plant in which there is a leader
shoot, often trained at 45 degrees to the
ground, and where all side shoots are
pruned back to one or two buds. Cordon
fruit bushes are usually grown against
Senescence and death
The term '
senescence
' in the plant's life cycle refers
to the period between adulthood and death of the
plant. It is the stage after flowering and fruiting
where growth has ceased and a gradual deterioration
occurs. This is most obvious in ephemeral, annual and
biennial plants which flower and fruit only once before
senescence and death.
In perennial plants, the same cycle of seed, juvenile
growth, adult growth, senescence and death occurs
through the plant's lifetime, as in all plants, but
the term 'senescence' is also used to describe the
changes that take place through the year and are
repeated each season in leaves and fruits. In deciduous
trees and shrubs, changes in leaf colour associated
with autumn are due to pigments that develop in
the leaves and are revealed as the chlorophyll (green
pigment) is broken down and absorbed by the plant
(Figure 5.11) and waste products accumulate.
Pigments
are substances that are capable of
absorbing light; they also reflect certain wavelengths
