Olive trees

The olive tree is an evergreen, perennial tree of the Oleaceae family and is one of the most important crops in the Mediterranean. In Greece, farmers cultivate it extensively for both olive oil production and table use. It thrives due to its longevity and ability to grow in poor, dry soils. Fruiting begins in the 4th to 5th year and stabilizes after the 7th to 8th year. Some of the most widespread varieties are Koroneiki, Chalkidiki, Kalamon, and Amfissa. Successful cultivation depends on proper pruning, balanced fertilization, timely plant protection, and, of course, rational irrigation. As a crop, it is a staple of the rural economy, with high export potential and added value.

Botanical characteristics

The olive tree (Olea europaea L.) is an evergreen tree of the Oleaceae family and the only widely cultivated species of the genus Olea. It is a dicotyledonous, long-lived plant with a woody trunk and a strong adaptability to the dry and hot conditions of the Mediterranean. Its crown is spherical or open, and its shoots have a characteristic tendency to become woody over time.
The leaves are simple, lanceolate, leathery, and dark green on the upper surface, with a silvery-white lower surface due to the presence of numerous trichomes. The differentiation of the flower buds occurs in autumn, and flowering takes place in spring. The flowers are hermaphroditic or only female, small and white, and arranged in axillary clusters.
The fruit is a drupe, with a fleshy pericarp and a woody core. The composition of the olive fruit (oil, sugars, and polyphenols) varies greatly depending on the variety, cultivation practices, and ripening conditions. Rooting is superficial, but it can reach greater depths if the soil is deeply tilled and well-drained. The olive tree can regrow vigorously from the roots after pruning or destruction of the above-ground part.

Physiological characteristics

The olive tree grows slowly but steadily and is particularly adaptable to the dry, hot conditions of the Mediterranean. Its root system is extensive and shallow, allowing it to draw moisture and nutrients from the upper layers of the soil. Vegetation begins in spring when the temperature exceeds 12–15°C and consists of two distinct periods: the vegetative period (spring) and the productive period (summer to autumn). Photosynthetic activity remains active throughout the year, peaking in spring and autumn. Olive trees have low water requirements, but sufficient moisture significantly improves fruit production and quality, especially from flowering to kernel hardening.

Morphological characteristics

The olive tree is a tree with a spherical or free-form crown, reaching a height of 5–10 meters. The trunk is sturdy and often branches at a low height, characterized by its rough bark. The leaves are opposite, leathery, and lanceolate, with a silvery underside and green upper side. Their surface limits moisture loss, allowing them to adapt to dry environments. The flowers are small, white, partially self-pollinating, and form clusters. The fruit is a drupe with a fleshy pericarp and a woody pit, with size, color, and oil content depending on the variety and cultivation practices.

Reproductive characteristics

The olive tree is primarily propagated vegetatively through cuttings or grafted plants to maintain the characteristics of the variety. Flowering begins in spring, with the flowers being mostly perfect or female. Pollination is wind-driven (anemophilous), and its success depends on weather conditions during the flowering period. Fruit set is sensitive to low temperatures, heat stress, and boron deficiency. Olive trees exhibit marked biennial bearing, with years of high production often followed by lower yields, which is related to the balance between vegetative growth, fruiting, and fertilization management.

Climate requirements

Olive tree cultivation requires specific climatic conditions to achieve high yields and quality fruit. You must meet the following climatic requirements:

Temperature

The olive tree is a characteristic plant of the Mediterranean’s dry and hot regions, with excellent adaptability to areas characterized by long, dry, and sunny summers and mild, relatively wet winters. The ideal temperature range for growth is between 15 and 30°C, while the most critical phases, such as flower bud differentiation, flowering, and fruit set, are significantly influenced by spring weather conditions.

Olive trees exhibit satisfactory resistance to cold, with most varieties tolerating temperatures as low as –7°C. However, frosts below –10°C, especially when they are prolonged or occur during the emergence of new shoots, cause necrosis of buds, flowers, and annual shoots, which results in yield losses. During flowering and fruit set, temperatures above 35°C, especially in conditions of drought or low relative humidity, significantly reduce fruit set due to stigma drying and flower abortion.

Rainfall

Annual rainfall for satisfactory production ranges from 500 to 700 mm, with most of it required in winter and spring to ensure vegetative growth and bud differentiation. Summer drought is not a significant problem for mature trees, but it can impact the final size and oil content of the fruit, particularly in dual-purpose or table varieties.

Sunlight

Abundant sunshine is crucial for photosynthetic activity and olive fruit quality (oil content and phenolics). In contrast, areas with dense cloud cover or prolonged fog experience increased pathogen attacks and delayed ripening.

Wind

Winds play a dual role: mild winds contribute to natural pollination, especially in dry-seeded varieties, but strong winds (greater than 40–50 km/h) can cause branch breakage, flower drop, or even fruit drop, particularly when combined with drought stress.

Moisture

High humidity (>70%) and rainfall during this period create favorable conditions for fungal attacks (mainly by Spilocaea oleagina), as well as difficulty in pollination.

Soil requirements

Olive trees are one of the most soil-tolerant crops, thriving even in poor, stony, or calcareous soils, provided there is adequate drainage. They prefer medium-textured soils, at least 70–80 cm deep and with a pH between 6 and 8. In heavy, clayey soils with poor drainage, growers often observe root rot problems and reduced production. The presence of organic matter helps maintain water balance and improves soil structure. In soils with high active calcium content, it is necessary to select resistant varieties and monitor for possible iron and zinc deficiencies.

Soil preparation

Soil preparation before planting olive trees is crucial for the long-term success of the crop. You should carry it out at least 3–6 months before planting, particularly in soils that have not undergone previous cultivation.

First, growers perform deep plowing (40–60 cm) to break up the compact soil horizon, enhance drainage, and facilitate root penetration. They follow this phase with scarification and leveling, depending on the slope of the soil. On sloping land, they recommend drawing planting lines parallel to the contours or constructing small terraces to create a more manageable landscape.

They conduct a comprehensive soil analysis to determine the basic fertilization requirements and assess the potential need for pH correction or improvement of organic matter.

If the soil is poor, they incorporate manure or compost (2–4 tons per acre), along with phosphorus- and potassium-based fertilizers determined by analysis.

The planting holes (50 cm × 50 cm × 50 cm) are dug a few weeks before planting. We prefer raised planting or half-underground terraces in heavy soils. Planting occurs from November to March, depending on the local climatic conditions.

Basic Fertilization

Growers typically carry out basic fertilization on olive trees, mainly in the autumn or winter, before the new growing season begins. They usually use phosphate fertilizers (e.g., 0-20-20 or 0-30-20) to strengthen the root system and flowering. The dosages depend on the tree’s age, the previous year’s production, and the results of soil analysis. They recommend approximately 1–2 kg of P₂O₅ and 2–3 kg of K₂O per adult tree. Growers apply nitrogen separately in spring, at doses of 500–800 g N per tree, preferably in two doses, as it is essential for germination. They also recommend manure or compost to improve organic matter.

  • Nitrogen (Ν)

    Nitrogen is essential for vegetative growth and fruiting. Its deficiency causes low production and yellow leaves.

  • Phosphorus (P)

    Phosphorus enhances rooting, flowering, and fruit sets while also promoting the production of healthy, robust flowers.

  • Potassium (Κ)

    Potassium is essential for both the quality of olive oil and water regulation in the plant. It also helps prevent fruit droppings.

  • Calcium (Ca)

    Calcium strengthens cell walls and facilitates the transport of other essential elements. It is vital in fertile soils.

  • Magnesium (Mg)

    Magnesium is a component of chlorophyll, playing a role in photosynthesis and plant stress resistance.

  • Sulfur (S)

    Sulfur participates in protein synthesis and strengthens the olive tree’s natural defenses.

  • Iron (Fe)

    Iron is essential for chlorophyll formation. Its deficiency, especially in alkaline soils, leads to chlorosis.

  • Zinc (Zn)

    Zinc regulates growth and fruiting. Its deficiency causes microphylla and delayed germination.

  • Boron (Β)

    Boron is crucial for fruit sets and fruit development. A deficiency causes reduced production and fruit cracking.

  • Manganese (Mn)

    Manganese plays a crucial role in photosynthesis and metabolic processes in plants.

  • Copper (Cu)

    Copper is essential for the natural protection of plants against pathogens. Many fungicides also contain copper.

  • Molybdenum (Mo)

    Molybdenum is essential for nitrogen metabolism. Its deficiency is rare and mainly observed in acidic soils.

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Foliar applications

Foliar fertilization is a crucial nutritional tool in olive cultivation, especially in conditions of reduced soil nutrient availability or when immediate plant support is required at critical stages.

This practice applies nutrients directly to the leaves, where the foliage quickly absorbs them and transports them to the plant tissue.

We primarily apply the following foliar fertilizers to olive trees:

  • Boron (B) should be applied before flowering to enhance fruit setting and reduce alternate bearing.
  • Zinc (Zn) is essential during the emergence of new shoots to ensure proper bud differentiation and promote excellent vegetative growth.
  • Iron (Fe) is important when symptoms of chlorosis are present, particularly in alkaline soils.
  • Calcium (Ca) is used to strengthen the structure of cells and enhance the stability of fruit.
  • Generally, during times of stress or high production, growers should apply amino acids and water-soluble trace elements.

You should apply in spring, before and after flowering, and in autumn, after harvest, when the foliage is growing. Application conditions are critical; it is best to avoid intense sunlight, high temperatures, and rain immediately after application. Instead of replacing basic fertilization, foliar fertilization complements it and often combines with plant protection sprays to enhance results and reduce costs. In olive groves with a history of deficiencies or areas with alkaline soils, foliar application of trace elements is an essential means of maintaining productivity.

Drip irrigation

Drip irrigation is applied in irrigated olive groves, enabling the flexible and targeted delivery of nutrients through the irrigation water. It is an ideal method for low-fertility soils or during periods of increased demand, such as flowering, kernel hardening, and fruit development. The most common fertilizers are water-soluble N-P-K and micronutrients. However, one should avoid excessive use of nitrogen, as it can increase vegetation at the expense of production. Growers require a well-regulated system for fertilization based on an annual nutrition program adapted to the climatic and production conditions of each year.

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The doses and growth stages mentioned are indicative and are always adjusted according to the recommendations of the agricultural consultant. Because rational fertilization is always assessed in relation to:

  • Expected yield
  • Soil characteristics
  • Planting density
  • Environmental conditions
  • Growth stage
  • Plant vigor
  • Cultivation duration
  • Hybrid

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