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The pruning of the olive tree: instructions for use

The pruning represents a difficult and expensive agronomic practice. It can weigh up to the 30% on production costs. Still, it is an unrenounceable operation. The advice of Prof. Gucci, University of Pisa

The pruning of the olive tree responds to specific agronomic needs, with precise purposes:
- confer and maintain the chosen form of nursery
- balance the aerial and root systems to exploit the potential for the development of the plant and obtain the highest production
- allow the air to circulate and a good enlightenment of the foliage
Each of these goals either directly or indirectly affects the physiology of the plant. In particular, the hormonal balance are altered by intervening on the foliage, cutting branches and gems. It is just the hormonal status of the tree, that is to say the relationship occurring between auxines, cytokines and giberellins, that affects the single phenologic steps, like the induction and differentiation of flowers, the growth of sprouts and roots.
Nevertheless, exogenous factors, not intrinsic in the metabolism of the tree, can have a heavy influence too. A precarious phytosanitary status, with consequential weakening, reduces the vigor and the vegetative and productive potentials as well. Keeping a good ventilation and lighting through adequate cutting interventions means prevention of the insurgence of pathogenic agents, such as fungi.
A good lighting of the foliage directly affects the metabolism, favoring the maximum photosynthetic efficiency and the formation of enzymes which promote the carbohydrates synthesis and their transportation and utilize for growth and production.
Hence, the first rule for a correct pruning is being aware of the metabolic and physiologic effects following planned interventions. It is obviously necessary not only consider the localization of the cuts, in function of the aforementioned purposes, but also the intensity of the pruning. Indications regarding the total mass of foliage that has to be removed are to be determined from the global status of the tree and its vegetative and productive conditions. Conventionally, cutting less than 20% of the overall branching is considered a mild intervention, 20% to 35% medium, over 35% severe or heavy intervention.
A mild pruning enables an abundant fructification and a limited foliage bloom. It is especially recommended during the nursery phase and the discharge years. Conversely, severe pruning induces sprouting and it is to be practiced in case of reconstruction and rejuvenating.

And what about esthetics?
Up to this point, I only expressed observations and recommendations for olive trees cultivated for the highest productivity, in which the plant esthetics is totally marginal. Not always have the beauty and harmony to be considered secondary, of scarce importance, though. As a matter of fact, olive groves assume more and more often a non negligible relevance for the landscape and the environment, representing an added value for extra-agricultural activities, such as rural tourism. In this case, the pruning is therefore supposed to model the foliage, in order for it to be pleasant for the eyes, a sort of green sculpture. Working time and production become in these cases secondary parameters.

The role of the pruner
The pruner is a highly specialized professional, both for competence and expertise, although these two elements cannot always be found in the same person. Many elderly people practice pruning relying only on their lifelong expertise, following instructions dating back to the fifties and sixties.
The pruning, just as it was conceived in those years, was a marginal practice. The olive was seldom the main cultivation, which was instead entitled to all the care and light it needed. Thus, severe pruning, which implied drastic cuts and made the tree bare to the free trunk, was deemed necessary. Such a technique presented many disadvantages, though, the most relevant being a scarce or non existent production during the trimming year and a larger importance of the rotation phenomena.
The main skills of today’s pruners are malleability, sensibility, receptiveness of the indications of the committing olive farmer, or his trusted technicians, which the pruners should adapt to circumstances, because each tree has a story of its own, especially in terms of vegetative-productive status.
Unfortunately such a set of working skills, albeit highly sought after, like other artisanal professional figures, is hard to find.

In order for our readers to have a deeper insight and get precious suggestions, we have consulted Prof. Riccardo Gucci, teacher of olive culture at the Faculty of Agrarian at the University of Pisa. Prof. Gucci also co-authored with Dr. Claudio Cantini (Ivalsa-CNR) a technical textbook which was expressly dedicated to these issues, titled “Pruning and nursery of the olive tree”.

- Based on the diverse models of olive culture (hobby, landscape-environment oriented, productive,…), different pruning interventions are possible in function of the approach to the tree and the desired result. Are there any principles and rules valid in all cases, though? If so, which ones?
The general rules require, in order to successfully realize the pruning, to take into account the age of the tree (a lighter pruning is recommended on younger plants, a more severe on older ones), to prune the foliage from the top to the bottom and to cut large branches first.
- After frost or calamities the foliage may show juvenile or “wild” features (large number of “male shots”, small and tough leaves, extraordinarily hard wood) following a reconstructive trimming. What to do in these case?
First, it is necessary to tell whether the wild features are related to the sprouting and the growth of branches under the graft point (this is not the case with plants nursed by scion) or to the branches of the cultivated species. In the first case, the new branches have the same genetic features of the natural crossbred which originated the seed that gave birth to the graft carrying plant and it is necessary to cut them. In the second case, it is instead necessary to limit the thinning cuts (especially if they have esthetical purposes) during the first and second year, so that a new foliage can build up again and find a new balance with the roots; later on, a more marked thinning of the branches can be carried out since the third year after the reconstruction cuts.
- In some single-coned plants, the mechanical pruning by means of mowing bars is commonly being used, in Italy too, every three years. It looks like a convenient method from the economical standpoint, but it presents some cons as well. This type of mechanical pruning only satisfies the goal of reducing the foliage of the tree. On the other hand, it alters its vegetative-reproductive balance, because it only performs top cutting cuts instead of removal cuts and could break branches and tear the bark. This technique can be utilized in vast olive groves as part of appropriate cycles of pruning, combined with manual pruning and non pruning interventions; that is to say, the mechanical pruning should be part of the pruning strategy followed by the farm.

by Alberto Grimelli
02 february 2009, Technical Area > Olive & Oil