The Olive Tree: From Flower to Harvest

In this article

• The olive tree and its botanical family
• How its leaves and roots work
• Spring flowering and pollination
• Fruit development and oil formation
• The olive tree year and harvest timing
• Why pruning, water and cultivation matter
• Frequently asked questions

Introduction

This article aims to give you an insight into the workings of an olive tree. It is largely drawn from the technical publication L’extravergine, from the Cucina & Vini library, translated into English from its original Italian.

As such, it contains a little biological terminology. Where necessary, this is explained in simpler terms to help us understand and appreciate the technicalities.

Producing good oil and high quality olives for the table requires an understanding of the complex processes that allow the tree to bloom, set fruit, bring that fruit to maturity and prepare for the next season.

The olive tree

In botanical classification, the olive tree belongs to the Oleaceae family, which includes other plants such as jasmine, ash, privet and lilac. All are relatives of the olive.

Originally associated with the territories that encircle the Mediterranean, the olive tree has distinguished itself through its ability to withstand difficult growing conditions, acquiring over time the characteristics of a rustic and long lived tree.

The olive tree, Olea europaea, demonstrates remarkable resilience. However, its ability to survive drought and temperature extremes does not mean that its crop is unaffected by them. Flowering, fruit development and oil production all depend on the conditions it encounters throughout the year.

The shape of the tree

In its natural state, the olive can develop into a large bush, with several stems and growth around its base. The familiar visible trunk and shaped canopy of a cultivated olive tree owe much to the intervention of the grower.

If left to develop naturally, the foliage can take on a hanging appearance, with branches growing towards the ground, lightly bent by the weight of the fruit.

In olive groves, the size of the trees and the shape of their foliage vary according to the variety and the pruning and training they undergo.

The main branches stem from the trunk, whose bark becomes increasingly textured with age. The root system provides anchorage and ensures that the tree receives water and nutrients. Its spread and depth depend on the soil and growing conditions.

How olive leaves work

The leaves of the olive tree are generally oval or elliptical, with short stalks. Their upper surface is grey green, while the underside is paler and has a fine, felt-like covering.

The upper surface faces the sun to absorb light for photosynthesis, the process through which the tree uses sunlight to make sugars from carbon dioxide and water. A waxy covering helps protect the leaf and limit water loss.

The underside contains most of the leaf’s stomata, the tiny pores that regulate gas exchange and moisture release. These pores open and close to let in carbon dioxide and release oxygen, while helping the tree manage water loss.

This release of water vapour is called transpiration. Water is absorbed from the soil through the roots, moves through the tree and is eventually released into the atmosphere through its leaves.

Olive leaves are commonly around five to seven centimetres long, although their dimensions vary with the variety, the vigour of the branch and environmental conditions.

Biology and phenology: the tree’s seasonal calendar

Phenology is the study of recurring biological events in plant and animal life and how they are influenced by seasonal variations and climate.

Often described as “nature’s calendar”, it tracks events such as the first blooming of flowers, the arrival of migratory birds and the shedding of leaves.

For the olive tree, this calendar includes the development of flower buds, flowering, fruit set, stone hardening, oil accumulation and ripening.

These stages are closely connected. The success of the harvest depends on much more than the weather in the weeks immediately before picking.

Spring bud development and flowering

A period of cooler winter temperatures helps prepare the olive tree for successful flowering in spring.

Growers often refer to the accumulation of suitable winter temperatures as the tree’s chilling requirement. This varies between olive varieties, so there is no single number of cold hours that applies to every tree.

As conditions become warmer, flower clusters develop. The olive tree produces these mainly on shoots grown during the previous year, often described as “flowering on last year’s wood”.

Its flowers are small and creamy white. Some contain both male and female reproductive organs, while others function mainly as male flowers.

The flowering phase is sensitive to the weather. Spring frosts, excessive heat and other unfavourable conditions can disrupt the processes that lead to fruit set.

How olive trees are pollinated

Wind is the primary means of olive pollination.

The botanical term is anemophilous, meaning wind pollinated. Rather than relying principally on bees or other insects, the tree produces pollen that can be carried on the breeze.

Wind pollinated plants commonly produce large amounts of small, light pollen grains. Their flowers do not need the bright colours and abundant nectar used by many insect pollinated plants to attract visitors.

Successful fertilisation also depends on pollen compatibility. Many olive varieties benefit from a compatible variety growing nearby.

Fruit set and early growth

After successful fertilisation, a tiny green fruit begins to develop.

The tree naturally sheds many flowers and small fruits. Only a small proportion of its original flowers need to become retained fruit to produce a worthwhile crop.

As the olive develops, its stone begins to harden, forming a protective barrier around the seed while the surrounding flesh continues to grow.

The olive itself can be elongated, oval or almost spherical. It is a drupe, or stone fruit, consisting of three main layers:

• The epicarp, or skin, is the thin outer layer that protects the fruit.

• The mesocarp, or flesh, makes up most of the fruit and is where most of its oil accumulates.

• The endocarp, or stone, is the hard inner shell enclosing and protecting the seed.

The stone and the seed are often referred to as though they were the same thing, but the seed is contained within the stone.

Maturation and oil production

As the fruit develops, cells in its flesh grow and expand.

Within this soft tissue, the tree synthesises and accumulates lipids, or fats. This phase is called lipogenesis, which simply means the formation of fat. In the olive, it is the process through which oil builds up in the fruit.

The timing and extent of oil accumulation depend on the variety, growing conditions and stage of development.

Light is fundamental to this process. Through photosynthesis, the leaves produce the sugars that support the tree’s growth and the development of its crop.

Water availability matters too. Although olive trees are well adapted to dry conditions, prolonged drought can affect fruit growth and production.

Colour change and harvest timing

As olives ripen, their skins change colour. Depending on the variety, they move from green through reddish or violet shades towards deep purple or almost black.

The beginning of this colour change is often called veraison, a term also used in grape growing.

Harvest timing directly affects the characteristics of the finished olives or oil. However, there is no single stage of colour change that guarantees the best result for every variety and every grove.

For table olives, the grower considers the required colour, size, firmness and style of the finished product.

For oil production, the decision involves balancing oil yield, flavour, fruit condition and other quality characteristics. Earlier and later harvests can produce quite different oils.

The olive tree year

In Mediterranean growing regions, the tree generally flowers in spring, sets fruit in late spring or early summer and carries its developing crop through summer towards an autumn or winter harvest.

The precise calendar varies according to the variety, location and weather.

This maturing cycle is affected by environmental events, including drought and rain, as well as cultivation practices such as fertilisation, irrigation and pruning. All play a role in the development of the olives and the resulting crop.

The tree’s evergreen leaves remain active when conditions allow, while growth slows during the cooler months and the next reproductive cycle develops.

Why cultivation matters

Skilled pruning is required to maintain the balance between the tree’s vegetative growth and its fruit production.

Good pruning also increases the light reaching productive branches. Excessive growth that crowds the inner part of the tree limits the available light and competes for water and resources, affecting the development of the fruit.

Where irrigation is used, it can support fruit growth and help produce larger, more consistently sized olives. Its value depends on the conditions and the way it is managed.

Nutrition matters as well. Nitrogen, phosphorus and potassium all contribute to the tree’s growth and development, but their use needs to reflect the requirements of the soil and the tree.

The relationship between appropriate cultivation techniques and the quality of the resulting olive oil or table olives is hugely important.

From the tree to the table

Producing good oil and high quality olives requires an understanding and appreciation of the processes that take the tree from flowering to fruit set, maturation and a successful harvest.

Climate, cultivation and the tree itself all play their part. These processes must be understood and worked with to achieve a good end product, whether that is Extra Virgin Olive Oil or olives for the table.

Harvesting is also the beginning of another transformation. Table olives need preparation to reduce their natural bitterness, as we explain in Why Are Olives Fermented?.

For the finished oil, our guide to How to Taste and Store Extra Virgin Olive Oil explains how to recognise its character and keep it tasting its best.

Frequently asked questions

When do olive trees flower?

Olive trees generally flower in spring. The precise timing varies with the variety, location and weather.

Do olive trees need bees for pollination?

Olive trees are primarily wind pollinated. Wind carries pollen between flowers, and many varieties benefit from a compatible olive variety growing nearby.

Why do olive trees produce so many flowers but fewer olives?

Only a small proportion of flowers develop into fruit that remains on the tree until harvest. The tree naturally sheds many flowers and young fruits during the early stages of development.

Are green and black olives from different trees?

They can be, but colour alone does not identify the variety. Olives begin green and darken as they ripen, with the final colour depending partly on the variety.

Where does the oil form inside an olive?

Most of the oil forms and accumulates in the flesh surrounding the stone.

When are olives harvested?

In Mediterranean growing regions, harvesting generally takes place from autumn into winter. The timing depends on the variety, growing conditions and whether the fruit is intended for table olives or oil.

Can olives be eaten straight from the tree?

Most fresh olives are intensely bitter and need curing or other preparation before eating. Read more in Why Are Olives Fermented?.

Giles Henschel
Founder, Olives Et Al

Source acknowledgement: This article draws largely on L’extravergine, from the Cucina & Vini library, translated from the original Italian.