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Home arrow The World of Oil arrow Olive Growing


by RICCARDO GUCCI, Professor at Department of Cultivation and Protection of Woody Species, University of Pisa

In Italy, olive trees are grown in 72 provinces on over 1 million hectares under different environmental conditions. The social and economic aspects of growing olive trees are just as relevant as the agricultural ones, since olive groves often play an important role in preserving Mediterranean soils, landscapes and bio-diversity. Part-time olive growing is widespread, representing the main system in marginal areas, and is sometimes so extensive as to involve entire regions, like in Liguria. The average land area per farm dedicated to olive growing is about 1 ha, therefore production costs are high and investments involved hard to recuperate.

The quality of extra-virgin olive oil originates in the field, from healthy, damage-free olive fruits at the right stage of ripening. Fruit storage, processing fruits into oil, and subsequent oil storage can only preserve the initial potential quality, but can never restore properties undermined by fruits of poor quality.


Soil and climate. Olive growing is inherently bound to the Mediterranean climate, with its mild winters, hot and dry summers, and rainfalls that mainly occur in autumn and spring. In relatively cold climates the low temperatures of autumn slow down the olives' ripening process, which results in oils that are highly valued by consumers because of the high content of oleic acid and minor components. Olive trees do not have strict requirements for pH, structure or fertility of the soil; they can adapt even to calcareous and relatively poor soils, as long as soils are well drained, not subject to waterlogging and not too compacted.


Cultivars. The number of Italian olive cultivars is simply extraordinary. A study commissioned by FAO, published in 1998, indicates that there are 538 Italian native cultivars, out of a total number of 1275 cultivars worldwide. Excluding rare cultivars, it is estimated that "only" 148 of these Italian cultivars are widespread. To illustrate Italy's genetic abundance, Spain, which is the largest olive oil producer in the world, has 183 native cultivars, followed by Greece with 52. During the past 15 years, Italian cultivars of the different olive growing regions have been characterized and described in detail in catalogues and books.

The cultivar has a strong influence on tree growth and development, its resistance to disease, pests and environmental stresses, onset of fruit production, speed and time of ripening, yield, oil accumulation process and oil properties. Changing cultivars in an existing olive grove is extremely expensive. In new plantations, the reproductive self-incompatibility of many cultivars as well as the desired olive quality should be taken into consideration. Although high quality extra-virgin olive oils can be obtained from virtually any olive cultivar, the differences in minor components and sensory profiles between oils obtained from different cultivars can be vast. The cultivar has a profound influence on fatty acids composition, the ratio between unsaturated and saturated fatty acids, and the content of phenolic and volatile compounds with sensorial impact that develop during fruit processing via the lipoxygenase pathway.


Protection against parasites and diseases. Protecting olive groves does not require many pesticide applications and, in cool climates, healthy olives can be obtained with no more than two applications per year. The key pest in all Italian regions is the olive fruit fly (Bactrocera oleae Gmelin), whose larvae dig tunnels through the fruit's pulp while feeding on it. The olive fruit fly decreases yield, fastens colour change and ripening, undermines quality because of microbial contamination and oxidations and, in the worst cases, causes premature fruit drop.

Other parasites, which can still be very harmful under certain conditions, are the black scale (Saissetia oleae Olivier), olive moth (Prays oleae Bernard), jasmine moth (Palpita unionalis Hubner), leopard moth (Zeuzera pyrina L.) and the olive weevil (Otiorrhynchus cribricollis Gyllenhal). Other insects can occasionally cause serious harm to an olive grove, but they are controlled naturally by the environmental conditions and useful insects under most circumstances.

Amongst the most common fungal diseases are Spilocea oleagina, Mycocentrospora cladosporioides and Verticillium dahliae; the latter mostly strikes irrigated and newly-planted olive groves. The most widespread bacterial infection is the olive knot (Pseudomonas savastanoi).

Planting density. Planting density can vary widely according to soil and climate conditions. In the past olive trees were grown at low planting densities mixed with other crops, but even specialized olive groves rarely exceeded 200 trees per hectare. The density of modern, intensive olive orchards usually ranges between 300 and 600 trees per hectare. Increasing planting density increases yield, especially during the first few years of production, but the initial planting and management costs are high. Recently, and especially abroad, very high-density olive orchards with over 1000 trees/ha are becoming more and more popular. These orchards are designed for continuous harvesting with straddle harvesting machines. Further testing is required to determine whether these olive orchards maintain high yields in the long run and whether they are suitable for Italian cultivars.


Pruning. Pruning is an expensive but necessary practice that must be performed taking into account both the biology of the olive tree and orchard performance. The growing habit of the cultivar, the natural tendency for high vegetative activity, the type of buds and branches, the predominant formation of fruit on side inflorescences of mixed branches, and alternate bearing are all important biological features of the olive tree. Furthermore, pruning is adjusted depending on tree age, productivity, health, and balance between reproductive and vegetative activities. Pruning does not affect the qualitative properties (analytical and sensory attributes) of the oil, as long as the fruit-bearing part of the canopy is well exposed to light and easily accessible for pest control.

Pruning is the second most expensive practice, after harvesting, therefore saving on pruning implies cutting down oil production costs. Lately, the concept of "minimum pruning" is gaining ground, and it involves reducing and simplifying pruning operations without negatively affecting yield, oil quality, or orchard sustainability. Using pneumatic or electrical equipment for pruning is another way to keep costs low.

From a technical standpoint, pruning is done by eliminating entire shoots or branches or by heading back cuts. The optimal period is from the end of winter through late spring, when inflorescences are fully elongated. Pruning intensity increases with the age of the tree, therefore pruning is light on young trees and severe on old or deprived ones. Pruning young trees drastically is wrong, because this stimulates the growth of strong new shoots and postpones the onset of fruit production. Traditional pruning criteria for adult trees involve eliminating a certain number of fruiting shoots and branches to decrease competition between them and to maintain the fruit-bearing part close to the centre of the canopy. To save time, it is best to perform heading back cuts only on a few branches each year, so that within a 3-4 year timeframe the whole fruit-bearing surface will be renewed. Pruning old trees trees or trees weakened by neglect or trauma requires drastic cuts, so as to stimulate their growth and renew fruit-bearing shoots and branches. Pruning mature trees or rejuvenating old trees includes the removal of suckers at least once a year.


Training system. The training system is the result of the growing habit of the tree and pruning practices performed over the years. The training system must be chosen before the olive trees are planted and must be based mainly on the harvesting method. For mechanical harvests with vibrating-shaking machines, trees must have a single trunk with no lateral branches for at least 1 m above ground (1.3 m if shakers with catching frames are to be used). For hand picking or mechanically-aided hand picking the above is not required.

The most common training system for olive is the vase with several different variants. Trees that are trained to a vase usually have a single trunk, which can vary in height (if there is no trunk or it is very short then the system is known as "bushy vase"), from which three to six branches shoot off in different directions so as to intercept as much sunlight as possible. The centre of the canopy is pruned to allow enough sunlight to come through. The modern interpretation of the vase system ("free vase") has primary and secondary branches amongst those developed naturally, whereas in traditional vase systems the distribution of lateral scaffolds is more regular. If harvesting is done mechanically, primary branches should not exceed a certain length and hanging shoots should be shortened by heading back cuts or eliminated.

Other training systems are the "single-trunk" and the "bush". With the "single-trunk" systems the main axis of the tree is maintained, with a few thinning cuts during the training phase. The primary branches are chosen amongst those that grow vigorously and are uniformly distributed around the trunk for maximum light interception. The primary branches are periodically pruned by heading back cuts or eliminated to renew the fruit-bearing surface. The "bush" system requires very little pruning during the training phase, but is unsuitable for mechanical harvests with shakers due to lateral branches shooting from the proximal part of the trunk.

Regardless of the training system, in order to reduce pruning costs, the hazard of using ladders, and manage (pruning, pesticide applications and picking) canopies directly from the ground, it is important that the height of the tree does not exceed 5 m.


Soil management. Soil management affects the physical properties, chemical and microbiological equilibria of the soil, but does not seem to have an impact on oil quality. Soil management includes soil tillage, weed or grass control and fertilizer distribution. Proper management improves water infiltration and reduces surface runoff, which is very important since 73% of Italian olive groves are located on hills or mountains.

The most widespread technique is periodical tilling, which eliminates weeds, restores the surface porosity of the soil, allows rainwater to infiltrate deeply and helps to distribute fertilizers. Tilling is done superficially within 0.2 m depth, so as to avoid damaging the roots of the olive trees. An efficient and cost-effective method of controlling weeds is using herbicides for weeding, but this has a negative effect on soil fertility. Today, more conservative and more environment-friendly methods are recommended so as to reduce erosion, soil compaction, and preserve soil structure.

Permanent or temporary grass cover is recommended in olive orchards to increase soil resistance to the passage of machinery, decrease soil erosion, and supply organic matter to the soil through root deposits and their exudates. Since weeds and grass cover compete for water and nutrients against the roots of the olive trees permanent grass cover can be maintained when there is a certain amount of rainfall or in irrigated orchards. Usually, grass is allowed to grow between the rows of trees and is cut 2-3 times per year, while weed control on the row is achieved by a herbicide strip or tillage.


Fertilization. Olive trees are not considered particularly greedy for most nutrients but, in order to produce large yields, they need ample availability of nitrogen, potassium, phosphorus and boron. For example, olive fruits accumulate large amounts of potassium, which can become insufficient for high oil yields in sandy soils or soils with limited cationic exchange capacities.

Fertilization helps to restore minerals depleted due to removals (fruit picked, wood pruned, leaves abscised) or used to sustain growth. Fertilization plans ca be developed based on soil analysis, leaf diagnosis and estimates of removals. Nutrient requirements vary with tree age and phenology. Young trees before the onset of production mainly require nitrogen, that should be distributed in batches along the rows of trees during the growing season. Mature trees in full production require 2-3 nitrogen fertilizations starting from budbreak so as to ensure proper availability for shoot growth, flowering, and fruit development, and at least two fertilizations with phosphorus and potassium. Micro-elements are supplied whenever there are signs of deficiencies.

Fertilizers can be applied to the soil, to the foliage, or can be added to irrigation water. The most widespread method is soil fertilization. Fertilizing through irrigation (fertigation) is still rarely used by olive growers, despite its advantages, which include ready uptake by the roots, availability of nutrients in soluble form at high soil humidity levels, use of low doses quantities of fertilizer. Foliage fertilization can be used solely or jointly with soil fertilization. Foliage fertilizations provide nutrients quickly, use low amounts of fertilizer, can be combined with pesticide applications and are well suited to rain-fed olive trees during periods when soil fertilizations would be useless due to a lack of soil humidity.

Finally, the importance of organic matter in the soil should be stressed, since it improves soil structure, increases water retention capacity, cationic exchange capacity and mineral availability. Organic matter and microflora are necessary for soil humification processes. Currently, most soils devoted to olive growing have a low organic matter content, less than 1%, and high rates of mineralization also due to excessive, periodic tillage.


Irrigation. Even though olive trees are the most resistant among fruit crops to water deficit, they do benefit greatly from irrigation. Advantages are greater in relation to how dry the climate is and how shallow or sandy the soil is. Therefore, most irrigated orchards are located in Southern Italy, Sicily and Sardinia, where long dry summers can severely constrain olive yield.

High water availability in the soil during the growing season increases yield, fruit size, pulp-to-pit ratio and the oil content expressed on a dry weight basis. The oil yield at the mill, expressed on a fresh weight basis, of irrigated olive trees, can be less than that of rain-fed ones, due to technological extraction problems or due to a higher degree of fruit hydration. Although extra-virgin olive oils can be produced even under rain-fed conditions, irrigation often helps to improve and diversify oil quality. The irrigation regime does not affect the free acidity, the peroxide number and the fatty acid composition of the oil, but low availability of water in the soil does increase the phenolic concentration (especially ortho-diphenols) and stability to oxidation. From an organoleptic standpoint, the oil's bitterness and pungency are decreased by increasing the volume of water supplied to the trees during the fruit development period. Water availability in the soil also affects the concentration of compounds that give the oil its fruitiness, such as trans-2-exenal, trans-2-exen-1-ol and 1-penten-3-ol.

Critical stages for water shortage are flowering, fruit set, the initial stage of rapid fruit growth and the oil accumulation stage. In Central Italy it is rare to have a water shortage during flowering and fruit set, but the same cannot be said about Southern Italy and the islands. An adequate water supply during the initial stage of fruit development helps reduce the effects of high temperatures and the lack of rainfalls. Irrigation promotes rapid fruit growth and the oil accumulation process during the final stage of fruit development, which usually starts from mid-August and continues until the end of October.

Since water is scarce in Italian olive growing regions, this resource should be used efficiently using localized methods of distribution (e.g. dripping) that reduce water losses due to evaporation. Regulated deficit irrigation protocols, whereby volumes of water lower than the trees' daily needs are supplied at certain phenological stages, are recommended.


Harvesting. Olives can be picked by hand right off the tree (stripping or milking), or with the aid of hand-held equipment. Harvesting can also be done with vibro-shakers or straddle-harvesting machines. Olive fruits that have fallen to the ground naturally or because of parasites can still be picked, either by hand or using machines that sweep them off the ground, but this usually makes it impossible to obtain extra-virgin olive oils, because the oxidation processes of overripe or damaged fruits affect the flavour, shelf-life, and analytical parameters of the oil. Picking olives that have fallen on nets also affects oil quality. In unfavourable areas (e.g. terraced olive groves), where there is no alternative to using nets, olive fruits must be harvested frequently and taken to the oil mill for processing within 24 hours.

Stripping preserves fruit integrity and does not limit the harvesting period, but it is also the most expensive method. An excellent alternative to stripping is using hand-held equipment (pneumatic or electric), which can even double the hourly picking capacity, suits all kinds of training systems and does not require large financial investments. Vibro-shakers of the tree trunk are reliable machines that allow to harvest large amounts of fruits per unit of time. The percentage of yield harvested by trunk shaking machines depends on the cultivar, production and size of the tree, the type of pruning, ripening stage and the fruit retention force. Shaking is a very fast operation, but moving the nets and gathering the olives from them does require some time; vibro-shakers equipped with catching frames drastically improve harvesting efficiencies, but require a distance of at least 4.5 m between trees along the row. Straddle-harvesters are currently the fastest machines and allow large areas to be harvested in very short times. Very high density orchards require steep financial investments at the beginning and have to be planned specifically using the few available cultivars.

Harvesting with trunk vibro-shakers does not alter oil quality when compared to manual harvesting, as long as olive fruits are at the same ripening stage. The analytical parameters and sensory attributes of the oil can vary due to the different ripening stage, as mechanical harvesting is usually done later than hand picking to take advantage of the lower fruit retention force on the tree, which improves harvesting efficiency. On the other hand, mechanical harvesting is quicker and usually ends sooner than hand picking. Mechanical harvesting with pneumatic or electric rakes can be detrimental to quality, not because of damage caused to olive fruits by the rakes as to the fruits often being squashed on the nets by careless operators.