TRAINING YOUNG HASS AVOCADO TREES INTO A CENTRAL LEADER FOR ACCOMMODATION IN HIGHER DENSITY ORCHARDS
P.J.C. Stassen, S.J. Davie & B. Snijder
Institute for Tropical and Subtropical Crops,
Private Bag X11208, Nelspruit, 1200,
Republic of South Africa.
Abstract
Avocado trees should be shaped in the nursery and
trained immediately after planting to a central leader. It is necessary to
balance the side shoot growth at specific times and remove strong upright water
shoots. The resultant conical or pyramidal tree shape will allow better light
interception by the leaf canopy and improved light penetration into the canopy
for maintaining bearing wood and avoiding unproductive bare areas. Small
timeous manipulations will make later major pruning, which could stimulate
deleterious vegetative growth, unnecessary.
Early attention to tree shape will make higher
density plantings manageable and highly efficient trees a reality.
Key words: tree structure, manipulation techniques, Persea
americana
1. Introduction
Avocado producers in South
Africa have traditionally believed that the ultimate size of an avocado tree
determines the initial planting distance. This has led to plantings of 8 x 8 m
and mostly 10 x 10 m made some 15 years ago, resulting in the giant trees found
today. The orchards become light-impenetrable caverns with vast unproductive
surfaces developing within and between the trees. The bearing surfaces shi-ft
ever higher in the tree and further from the centre of the tree. The practise
of removing alternate rows or certain identified trees brings only temporary
relief and the trees rapidly fill the space created resulting in even larger
trees.
Currently orchards are being planted at densities of
5 x 5 m or 400 trees per hectare (Köhne &
Kremer-Köhne, 1992 and Köhne,
1993). This, however, accelerates the overshadowing problem and results in
encroachment by about the sixth year with the necessity of a tree thinning
strategy. Alternatives to tree thinning, which is regarded by Stassen, Davie
& Snijder (11995) as illogical when the trees have only just reached their
bearing stage, are:
i. to prune trees to a pyramidal
form just after harvest and to follow this up with certain summer pruning
manipulations (Snijder & Stassen, 1995). In this way light interception and
light penetration is improved and die-back at the base and within the tree is
prevented.
ii. to saw back alternate
rows and develop into a central leader structure (G. Martin, University of
California, Riverside - personal communication, 1995). When these trees come
into production the remaining trees are corrected.
Köhne &
Kremer-Köhne
(1992) found that the break-even point higher yield, especially in the initial
years, is reached sooner with plantings of 800 trees per hectare as opposed to
400 trees per hectare.
Stassen et al. 0 995)
recommend that new orchards for higher density plantings be spaced, orientated
and structured in such a way that they can be maintained within their allocated
space for as long as possible. Instead of a grow-as-you-wish approach it is
recommended that certain manipulation inputs be made to achieve the most
suitable tree structural form, based on the classical recommendations of Cain 0
972). A planned framework is obtained with the use of manipulation techniques
and adherence to certain principles, such as a hedgerow system orientated in a
North-South direction (Stassen et al., 1995).
The purpose of this paper is to provide certain
provisional guidelines by which young "Hass" avocado trees can be
shaped to a central leader and maintained in an orchard of about 800 trees per
hectare. This study is still in the initial stages but there are promising
indications that this structuring together with some other manipulation tools
as listed by Wolstenholme & Whiley (1990), will supply the means to
achieve:
i. an earlier break-even point
ii. more manageable trees
iii. more efficient orchards
iv. continuous renewal of future bearer shoots
2. Materials and methods
Nursery, newly planted and
young bearing Hass on Duke 7 trees in the Kiepersol area were used to test the
principles expounded by Stassen et al. (1995) on a semi-commercial
scale. Information, observations and reactions obtained in the past 24 months
will be discussed.
3.
Results and discussion
In this paper we are limited
to discussing guidelines for developing "Hass" into a central leader.
The first trees trained are now 24 months old and have retained a an adequate
fruit set (figure 1).
The
following principles must be borne in mind:
i. avocado trees can be
manipulated but the timing and intensity, as well as the type of cuts made,
will influence reactions
ii. tree manipulation is not a one-off treatment
especially in the initial formative stages where drastic cuts should be the
exception.
iii. young trees must be strong-growing during the
formative stage.
iv. early fruiting, from the second year, is a
requirement and must be induced. Various methods as discussed by Wolstenholme
& Whiley 0 990) should be applied where necessary.
Manipulation guidelines for
successfully shaping a central leader tree are briefly as follows:
i. develop
the tree as far as possible in the nursery by selecting or forcing a strong
vertical shoot just above the bud union. If the shoot is allowed to develop
rapidly, lateral shoots will develop more horizontally with a natural balance
relative to the leading shoot. A strong root system is essential. The selected
shoot must be supported by wire or a thin rod which does not restrict side
shoot development. A strong tree from the nursery that already has only one
leader and a few well balanced side shoots can be easily trained further in the
orchard (figure 2).
ii. plant trees before a
flush period to perpetuate strong growth, preferably between July and October.
Fertilise monthly with 30 g nitrogen (LAN) per tree and irrigate.
iii. if the trees from the
nursery have the wrong growth structure the tree must be pruned accordingly.
Where the tree has no side shoots or has developed crookedly a strong lower
vertical shoot must be forced into a central leader. The original part must not
be immediately removed but should be bent till the new shoot starts to develop.
iv. side shoots that are more
than a quarter of the thickness of the leader must be bent horizontally or cut
back 50% to a bud or preferably to a flatter lateral side shoot. Shoots more
than half the thickness of the leader must be removed at their juncture with
the main shoot.
v. side shoots must be tipped
at 200 mm to force lateral shoots if they are not developing normally. These
laterals develop into the future bearers.
vi. do not unnecessarily remove leaves but remove
unwanted shoots early.
vii. remove strong vertical
shoots and shoots in unwanted positions early, especially those with narrow
angles of attachment. Keep side shoots reasonably horizontal by cutting to
flatter laterals in late summer. viii. remove very low shoots (to a height of
50 cm) that will hang on the ground or will hamper irrigation and cultivation
processes during the first winter. Also remove shoots that are too dense or
close together. Continue with the structural forming of the tree throughout the
spring and summer. Trees which are 20 months and older should be forced to set
fruit by restricting growth during autumn. Scholefield (1985) found that
avocado flower initiation took place in autumn. As this is the period when
decisions on the trees' reproductive development are made the process must be
supported for a good fruit set. In places with lower soil potential, growth
will be easily controlled with normal practices. Cincturing can be used in
early autumn to force strong growing trees to set fruit.
At a fruit size of about 30
mm, fruit thinning should be done by pruning so that the tree potential is not
exceeded and fruits of an acceptable size are produced. About 80 fruit/tree
should be left on trees of this age. Further tree manipulation entails cutting
back side shoots to laterals, especially to the more horizontal, and keeping in
mind the width limitations and the pyramidal shape requirement. This pruning
should take place in the post harvest period and with the removal of vertical
water shoots in spring and summer. Bearer shoots will be renewed by cutting
back branches in the post-harvest period. Vigorous water shoot development indicates
excessive nitrogen application in the vegetative stage, which should be
avoided.
References
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