A
pruning option currently being trialed in various countries is
'monoconical' pruning. This term simply means that the tree will
have one (mono) central trunk and will be pruned into a
Christmas tree type shape (cone). The idea was first suggested
in 1936 by Roventini and has been trialled just about every decade
since (Morettini 1944, Marinucci 1956, Montanari 1961, Jacoboni
1962;1981, Scaramuzzi 1968).
Recent
trials have used monoconical pruning to produce a tree which by
its shape and growth habits will allow plantings of up to 1,250
trees to the hectare. Such dense plantings have not been
able be mechanically harvested with 'modern' tree shakers, however
new designs are being introduced including 'over-the-row' type
harvesters which actually straddles the young trees. It is also
hoped that the need for pruning will be somehow reduced and/or
mechanised. Due to the upright shape of the tree, the fruit
will be easier to remove with a mechanical harvester but all aspects
of the pruning method must be assessed. Earlier crops have
also been recorded in a number of monoconical trials with
specific varieties, however as outlined below, the lifespan of
the grove may be shorter than with other traditional pruning methods.
The total orchard may need to be replaced in about the twelfth
to fifteenth year or earlier due to reduction in crops or
difficulty of maintenance and grove health management.
Although
dense orchard trials are not proven at present, a summary by
Fontanazza on how the monoconical shape is achieved follows. (Summarised
from IOOC publication 'Olive Pruning')
First
Year - A stake protruding 2m (6ft) above the ground should
be positioned either at planting time or soon after. It is important
to keep the main leader tied up to the stake to keep the trunk
erect and upright. This leader (trunk) acts as a pump as
outlined below. If for some reason the leader is badly damaged
then it should be replaced by a lower vigorous shoot. The lower
branches (up to 30-35cm off the ground) should be eliminated to
encourage upward growth. If pruned correctly, the conical
shape should be somewhat apparent at the end of the first
year.
Second
& Third Year - Pruning should be kept to a minimum, by
removing only very low branches (up to 40-50 cm off the ground)
and any upright growing shoots that might compete with the leader.
The leader must always be taller than all other branches. Side
branches must come from the trunk in a spiral fashion from the
ground up to use maximum light. The conical shape will be quite
apparent by this stage.
Fourth
Year - When the first fruit has been picked, pruning is done
in a similar manner to the preceding years. The aim is still to
have the leader dominating the tree's growth. At this stage, any
branches below 80-90 cm can be removed for access by a mechanical
harvester.
Young
Bearing Period - During the fifth to seventh years the tree
will have a definite monoconical shape. It is during these years
that the tree reaches a balance between vegetative growth
and fruit bearing. This balance must be maintained with
regular pruning throughout the life span of the tree. At maturity,
the tree should stand no higher than 4 m (13 ft) and should be
pruned back to this height when it tries to go further.
A
problem which researchers are coming up against is the fact that
olive trees do not naturally grow in a Christmas tree shape and
therefore must be heavily pruned to keep them in this shape as
the trees reach maturity. To combat this problem, research is
being done into the possibility of mechanically pruning
the trees, however little research data is available to date.
To
state it simply, olive trees often don't respond well to the inaccuracies
of mechanical pruning. Monoconical trees are very difficult to
climb when mature and therefore hand pruning and leader
training/staking can become cumbersome. The most feasible
option is to have a machine which can lift a person up to the
top of the tree (3-4 m off the ground) as ladders cannot be well
positioned on a monoconical tree with a thin leader towards
the top. Another method being used in the Italian trials
is to set up 3 metre tall vineyard like trellises which
hold the stake and the tree.
The
size of the harvester needed to drive over the top of such a dense
orchard can start to cause problems as the trees continue to gain
height (or if the trellisses get in the way). Such tall harvesters
are also limited by the slope of the ground that they can safely
work on. The combination of 'dwarf' type varieties combined with
tall over the row harvesters is currently being assessed. Future
reports from the monoconical trials will be interesting to
follow.
A
study by Preziosi and others was done to compare a number
of growth and yield factors between monoconically and vase pruned
trees of three Italian varieties. In general, they found little
difference between the two methods however the following
extract from their conclusion is worth noting.
"The
vase training system gave a greater tree growth with respect to
monocone, favouring a rapid completion to the structure of the
tree; moreover the vase training reduced by about 30% the
time for winter pruning with respect to the monocone. Greatest
crown development, obtained with vase training, gave highest production
with respect to monocone in Frantoio, while in the other
cultivars there were no particular indications of one system being
better than the other."(These results were presented
at the Second International Olive Growing Symposium and also published
in Acta Horticulturae 356, Jan 1994.)
The
density of closely planted monoconical orchards can lead to increased
olive scale and sooty mould problems in mature densely planted
monoconical orchards. This is due to reduced light and air penetration,
both essential for the health of olive trees.
It
should be noted however, that monoconical pruning may be suitable
for less dense groves where a standard 'tree shaker' type
umbrella mechanical harvester can access the trunks of the
trees. The most recent harvesting machinery, being manufactured
by Korvan and AgRite in California, works freely
on trees in a hedge type layout.
There
is also some similar work being done with young trees where a
main leader trunk is allowed to grow up to two metres tall. This
leader acts as a 'pump' which draws nutrients up for all of the
branches and allows them to grow more vigorously. Such a
main leader can be left as in the monoconical trials above or
it can be removed in about the third or fourth year. If you plan
to use this leader method only in the early years, don't
forget to keep your vase shape growing around the central
leader as you will need it to produce the fruit when your
central leader is removed.
