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Monoconical Olive Tree Pruning

A pruning option currently being tested 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 tested 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 500 trees per acre. 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 mechanized. 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 grove 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 grove trials are not proven at present, a summary by Fontanazza on how the monoconical shape is achieved follows. (Summarized from IOOC publication 'Olive Pruning')

First Year - A stake protruding 6 ft 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 12-14 inches 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 2 inches 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 3 - 3.5 inches 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 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 most olive varieties 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 (primarily in California) 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 (10 - 13 ft off the ground) as ladders cannot be well positioned on a younger monoconical tree with a thin leader towards the top. Another method being used in the Italian trials is to set up 10 ft 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 grove can start to cause problems as the trees continue to gain height (or if the trellises 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 and similar 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, favoring 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 groves can lead to increased olive scale and sooty mould problems in mature densely planted monoconical groves. 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 6 ft 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.