Crop Guide: Growing Olives

 

Olives can be and are grown in a wide range of climates in many different countries. The crop is mainly distributed between 25° – 40° North and South latitudes. The crop requires:

It is, therefore, best suited to a Mediterranean-type climate.

 

Some olive varieties, such as those grown in Egypt, Tunisia or Israel, bloom and fruit with very little winter chilling, whilst other varieties require more chilling for a normal flower differentiation.

 

Areas receiving a mean annual rainfall of 400 to 700 mm are most suitable for olive growing. Supplemental irrigation during summer increases fruit yields by 30% – 50%. A long, sunny, warm summer results in a fruit with high oil content. Olives perform well, with humidity varying between 40%–65 %. High humidity, above 80%, at flowering causes flower drop and infestation by sooty-mold producing insects. The olive is a long-day plant and benefits from prolonged sunlight (2,400 – 2,700 sunshine hours annually) and a warm environment.

 

Being an evergreen tree, the olive is sensitive to hard freezing temperatures. Buds and fruiting shoots are usually damaged by temperatures below -5ºC. Large branches and whole trees can be killed if temperatures fall below -10ºC.

 

Main source: Netafim.

 

Olives grow well on almost any well-drained and aerated soil with pH values of 6.5 – 8.5. Therefore, sites where water stands during rainy periods or where ground water is shallower than 1.2 meters deep should be avoided. Olive trees are tolerant to mild saline conditions, but extremely salty or sodic soils should be avoided. Olives have a relatively shallow root system (Figure 2.1) and consequently they only require a 1.0 – 1.5 meter deep soil profile without any serious physical limitations. Olives prefer moderately fine textured soils ranging from sandy to silty clay, loamy soils.

 

Figure 2.1: Olives do well on shallow soils with good drainage.

(Source: Connell, UC Davis)

 

Soils that contain excessive amounts of exchangeable sodium in proportion to calcium and magnesium are termed sodic or alkali soils. They are characterized by a dispersion of soil particles that reduces the soil permeability to water and air. By definition, a sodic soil has an exchangeable sodium percentage (ESP) of greater than 15. This means that 15% of the soil exchange capacity is associated with sodium, and the rest with calcium, magnesium and other cations. Olive trees are affected when ESP levels reach 20 – 40.

 

Source: https://www.oliveoilsource.com/page/fertilizers-and-amendments

 

None of the cultivated varieties can be propagated by seeds. Seed-propagated trees revert to the original small-fruited, wild variety. The seedlings can, however, be grafted or chip-budded with material from desired cultivars. The variety of an olive tree can also be changed by bark grafting or top working. Another method of propagation is transplanting suckers that grow at the base of mature trees. However, these would have to be grafted if the suckers grew from the seedling rootstock. A commonly practiced method is propagation from cuttings. Twelve- to fourteen-inch long, one-inch thick cuttings from the two-year-old wood of a mature tree are treated with a rooting hormone, planted in a light rooting medium and kept moist. Trees grown from such cuttings can be further grafted with stems from another cultivar. Cutting-grown trees bear fruit in about four years.

 

Before analyzing the concept of tree spacing, it is important to introduce the concept of optimal volume of canopy. Many scientific works have proven that there is a single optimal volume of canopy per hectare that depends exclusively on environmental conditions and is independent of the chosen spacing within certain limits.

 

 The optimal volume will be determined by a combination of climate, soil, irrigation and other management practices, and with this volume the grove will produce consistently high yields of high quality fruit.

 

It is possible to achieve the same optimal volume of canopy per hectare, and consequently yields, with various spacing variations. The main advantage of higher densities (Figure 2.2) is the possibility of achieving the optimal volume in a shorter period of time.

 

 

In practice, olives are cultivated in three main production systems according to tree density (see Table 2.1).

 

 

Table 2.1: Main tree densities of olive orchards

 

Pollination and ovule fertilization are essential steps to guarantee a normal fruit-set and crop. Successful pollination requires the pollen grain to germinate and its pollen tube to grow fast enough to reach a still viable embryo sac to fertilize the ovule.

 

Olive flowers are largely wind pollinated with most olive varieties being self-pollinating, although fruit-set is usually improved by cross pollination with other varieties. When environmental or management conditions are not optimal, the presence of pollen from another variety will normally guarantee a better fertilization and fruit-set. Since many cultivars are self-sterile, or nearly so, they are generally planted with a single primary cultivar and 1 – 3 additional cultivars for cross-pollination to optimize yield.

 

 

Proper pruning is important for the olive tree. Pruning both regulates production and shapes the tree for easier harvest. The trees can withstand radical pruning, so it is relatively easy to keep them at a desired height. The problem of alternate bearing can also be partially amended with careful annual pruning. For a single trunk, prune suckers and any branches growing below the point where branching is desired. Prune flowering branches in early summer to prevent olives from forming. Olive trees can also be pruned to espaliers.

Pruning between mid-February and the ripening of fruit in the fall, except for the lightest tipping of new shoots, can result in a reduced crop.

 

Pruning is necessary to adjust the trees to the climatic conditions of the area and increase the productivity of a plantation. The aims of pruning are: (1) to balance vegetation with fruit yield, (2) to minimize the non-productive period, (3) to prolong the productivity of the trees, (4) to delay senescence, and (5) to save soil water, a critical factor in non-irrigated orchards.

 

There are three main types of pruning:

For intensive cultivation where trees are densely planted, short pruning shapes are desired, namely the "short cup" and the "bush". In the former shape, branching takes place very close to the ground, at a height of 30 – 40 cm, while in the latter no pruning is done in the first 5 – 6 years. Afterwards, only weak shoots and top branches exceeding 3 m are removed. The bush shape has certain advantages for intensive cultivation systems, such as:

However, both shapes present a major disadvantage because they obstruct mechanical cultivation of the soil. In addition, harvest is difficult particularly for fruits that have fallen on the ground. An improved short shape without the latter disadvantages is shaping the tree with one central trunk into a Christmas-tree type shape.

 

The main pruning shapes applied in the wider Mediterranean area are the following:

Figure 2.3: Different pruning systems, as explained above

(Main source: https://oliveoilsindia.com/Olive-Tree-Cultivation.htm)

 

The olive tree is, genetically, highly alternating in fruit production. In non-irrigated olive groves, the yield may vary between 7 – 8 tons / ha and a few hundred kg. The occurrence and development of alternate bearing is also potent in intensive orchards with controlled irrigation, nutrition and training techniques, although the level of fruit production is higher and better controlled. Without specific intervention, the gap between 'off' and 'on' years may vary between 5 and 30 tons / ha. Therefore, alternate bearing is of high economical severity.

 

 

Figure 2.4: Leaf nitrogen and potassium contents in different phases of the production cycle

 

The possible involvement of nutrition, irrigation and fertigation in controlling alternate bearing was checked in numerous studies. Under normal, balanced growing conditions all aspects of intensification have little influence on alternate bearing. Intensive olive cultivation increases production but does not significantly affect the alternate fruiting habit of the tree. Nutritional deficiencies and / or water stress might enhance alternate bearing. In such cases nutritional or irrigation intervention would affect the level of biannual bearing as well. Spot-wise use of nutritional and water application are useful to avoid or correct alternative bearing, when induced by an acute nutritional deficiency or water stress, particularly during the early induction period.

 

However, alternate bearing can be balanced by supplying the trees with more water, nitrogen and potassium fertilizers during the "on" years so that they can produce ample shoot growth despite the heavy crop; and reduce the inputs during an "off" year so that they don’t over-grow in the absence of fruit.

 

Horticultural intervention via pruning, thinning, girdling, etc. can reduce and even eliminate alternate bearing in regions with favorable and stable climatic conditions. Under more marginal and unstable environmental conditions, alternate bearing is most difficult to control and even drastic horticultural means have to be reinitiated anew after each of the various climatic events.

Sources: Lavee, 2007, and Vossen & Devarenne, 2007.

 

Wherever table olives enjoy special premium for greater fruit sizes, it is advisable to thin the fruits, when bloom is plentiful only.

 

Spray the foliage 10 days after full bloom with an NAA (Naphthalene-acetic-acid) product at a concentration of 100 ppm, and spray volume of ~2,000 L / ha. If spraying is carried out at a later time, the concentration should be increased by 10 ppm for every day of delay. A surfactant such as Triton X-100 should be added to the final spraying solution at 0.025%.

Spraying must not be done if the following days are expected to be very hot and dry.

 

Because of its small leaves with their protective cuticle and hairy underside that slow transpiration, the olive tree survives even extended dry periods. However, this defense system is at the expense of growth and productivity of the tree. Once established, olive trees are among the most drought-resistant trees in the world. But the olive tree is not a desert plant. It needs regular watering to thrive. Insufficient water will cause the plantation to suffer and even die if left too dry for too long. Thus, olive yield is greatly increased by applying small amounts of water. Moreover, if commercial yields are desired greater amounts of water will be needed, provided that soil humidity does not become excessive.

 

There are 2 – 3 extremely critical periods, during which soil moisture must be kept optimal, for maximum bearing, as follows:

For oil production, the most critical periods to avoid water stress are fruit-set and oil accumulation.

For fresh fruit production, fruit-set and fruit growth, stages # 1 & 3 are the most critical.

 

Table 2.2: The major growth stages of the olive tree and the impact of water stress on tree growth and fruit development

 

 

Table 2.3: Options for irrigation and their implications

 

Olive trees are very sensitive to over irrigation and will not perform well in waterlogged soils. Waterlogged soil, often a result of poor drainage, causes poor soil aeration and root deterioration and can lead to the death of the trees. Trees cultivated in saturated soils are more susceptible to varying weather conditions and soil borne pathogens such as phytophthora and verticillium.

 

Drip irrigation in many diverse agro-ecological conditions brought about considerably higher olive oil yield (30% – 50%), water savings (30% – 45%), and improvement in oil qualitative characteristics, in comparison to rain-fed and surface flood irrigation methods. Subsurface drip irrigation (SDI) proved even better than on-surface drip irrigation, as shown in Table 2.3.

 

Figure 2.5: The olive tree has a shallow, spreading root system,especially when irrigated by a dripping system

 

Table 2.4: The effect of subsurface drip irrigation in comparison to rain fed olive orchard

(Source: Netafim)

Following are some examples for successful irrigation scheduling:

 

As no rain is expected in the summer, the first irrigation should be done 3 weeks after the last effective rain. It should be at a rate of at least 100 m3 / ha to saturate the entire root zone. As mentioned above, water stress during bloom and fruit-set is specifically harmful; effective irrigation should therefore take place during that period. Water consumption increases during flower-bud development and the daily evapo-transpiration (the loss of water to the atmosphere by the combined processes of evaporation from soil and transpiration from plant tissues). It is therefore advisable to adopt the following regime during the spring, summer and fall seasons.

 

A. Young orchards

The irrigation rate of mature olive orchards should be calculated based on the Class 'A' Pan evaporation data, multiplied by the season-specific crop coefficient (CC) (see Table 2.6).

Seasonal crop water requirement: Intensive 600 – 800 mm / ha / year; traditional350 – 600 mm / ha / year, under drip irrigation for range of environments.

 

Table 2.6: Crop coefficients for evaporation rates (mm / day) for olive orchards yielding over10 tons / hectare

 

 

Table 2.7: Irrigation schedule for mature table- and oil-olive plantations in a Mediterranean climate

 

Water utilization efficiency varies between 0.15 to 0.5 kg of oil per m3 of water.

The recommended irrigation system by dripping is surface drippers during the first two years, followed by subsurface drip irrigation for the rest of life span of the entire plantation, combined with Nutrigation. Nutrigation (= fertigation) is the application of plant nutrients through an irrigation system.

 

Two laterals per row in traditional and intensively- cultivated plantations, and one lateral per row in super-intensive orchards.

Effective dripline spacing: Traditional (5 – 10 m), Intensive (3 – 4 m) and Super-intensive (3 – 4 m).

Emitter spacing: 0.50m – 0.75m, depending on soil texture.

Emitter flow rate: 1.0 to 1.6 Liter / h, depending on soil texture.

Dripline installation depth in SDI: 0.3 m.

Source: Netafim.

 

Recent studies suggest that olives can be irrigated with water containing up to 3,200 mg / l of salt (ECw of 5 dS/m) with an SAR value of 18, producing new growth at leaf Na levels of 0.4 – 0.5% d.w.

 

Table 2.8: Irrigation water quality guidelines for olives

Source: Chartzoulakis, 2005

 

Intensively grown olive trees will greatly benefit from a good nutrition regime. During the first years of the plantation a premium slow-release fertilizer, such as Multicote-Agri 17-9-16+2MgO augmented with trace elements, is recommended.

Alternatively, soluble fertilizers may be used with high efficiency, especially by Nutrigation. If the fertilizers are broadcasted separately, be sure to water very well after application.

 

Many growers in Mediterranean countries apply organic fertilizers every other year. Organically derived fertilizers are available, but they are often markedly more expensive per nutritive element unit. Top dressing with organic material such as composted manure or kitchen compost can be done, but the grower should consult carefully before using it because it is difficult to achieve a good balance of nutritional elements by this method. It is environmentally responsible but requires more study and understanding by the grower. Always avoid placing compost or any fertilizer next to the trunk of the tree.

Whatever type of fertilizer is used, it is best to feed lightly and often during the growing season. Avoid heavy applications of soluble fertilizers that could damage plants and leach or run-off into groundwater.

Application rate of mineral fertilizers should be based on the target yield, nutrient uptake, soil nutrient analysis, leaf nutrient analysis, leaf deficiency symptoms, results of fertilizer experiment, and nutrient recycling.

Mineral nutrition of olive plantations is presented in the next chapters of this brochure in greater depth.

 

Fruits that are to be processed as green olives are picked while they are still green but have reached full size. They can also be picked for processing at any later stage, through full ripeness. Ripe olives bruise easily and should be handled with care. Mold is also a problem for the fruit between picking and curing. There are several classical ways of curing olives. A common method is the lye-cure process in which green or near-ripe olives are soaked in a series of lye solutions for a period of time to remove the bitter ingredients, then transferred to water and finally to a mild saline solution. Other processing methods include water curing, salt curing and Greek-style curing. Both green-cured and ripe-cured olives are popular as a relish or snack. The black color of black olives is obtained by exposure to air after lye extraction and has nothing to do with ripeness.

 

For olive oil production, irrigation should be stopped for the weeks leading up to harvest to avoid accumulation of high moisture content in the fruit and difficulties during oil extraction. Optimum moisture content is 50%. Use freshly picked olives (no longer than 24 – 48 hours from picking to processor) for producing extra virgin olive oil. Harvest olive fruits at the correct time. Immature fruit will give less oil. Use the IOOC olive maturation index guide to ascertain the stages of olive ripening. A maturity index enables growers to evaluate varieties in order to specify the oil quality the producer wants to obtain and repeat in successive years. Methods of harvesting include manual, such as small hand rakes, picking bags with harness, pneumatic olive harvesters and limb shakers. Additional methods include mechanical harvesting such as trunk shakers, limb shakers, straddle harvesters and oscillating combs (singular or dual models).

 

The traditional method of beating olives off the tree is not recommended.

 

Figure 2.6: Mechanical olive harvesting in Israel

 

High costs and low labor availability at olive harvest time are the main economic constraints in oil and table olive production, reaching 40% – 60% of the total income. This subject is the main reason for the adoption of mechanical harvesting solutions, using various types of tree shakers / vibrators.

 

Mechanical harvest has proven particularly difficult because (1) fruit require considerable force to be removed from the tree. (2) Trunks of olive trees become stout, fluted and knobby with age, which complicates the use of mechanical shakers that attach to the trunk, often resulting in tree trunk damage, bruised fruit, and poor removal efficiency. Hence there is limited acceptance of mechanical harvesters. Mechanical harvest equipment is not efficient enough and at optimal conditions (variety, tree shape, temperature, ripening, etc.) produces up to 85% fruit release, resulting in the necessity to follow up with manual picking.

 

The ratio between fruit mass and pedicel strength is relatively small as compared with other fruits. As a result, a huge amount of force is required to shake off the fruit from olive trees. Fruit damage is an industry concern because bruising may compromise the quality of the final product. Chemicals of various kinds were tested to promote pedicel loosening. Chemical harvest aids, such as pre-harvest treatment, facilitates fruit removal by lowering the mechanical force required to harvest the fruit, thus minimizing fruit damage. Fruit-specific abscission agents for loosening fruits are used in order to improve mechanical harvesting efficiency.

 

Numerous chemical products have been developed in order to ease the abscission of the fruits from the tree, thereby increasing the picking efficiency of hand-held, as well as mechanical devices. Most of these products are based on Ethrel® (2-Chloroethylphosphonic acid), which is converted in the leaves into the natural plant growth substance ethylene, which promotes natural fruit abscission in olives. Other compounds have been developed that are not based on interference with the hormonal balance of the plant, but have a specific and different mode of action. An outstanding example of this kind of product is Haifa Chemicals' product OliveDrop™, which not only acts as an abscission agent, but also supplies plant nutrients. OliveDrop™ also has practically no adverse defoliation effect, unlike other abscission agents. For more details, including test results, please refer to Chapter 5, paragraph 5.9.

 

 

 

 

 

 

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