Geography and distribution
This species is widely distributed across the Mediterranean region, Africa and Asia.
Overview: Olea europaea is an evergreen shrub or tree, which grows up to 15 m tall. It is slow to mature but can live for hundreds of years.
Leaves: The leaves are borne in opposite pairs. The leaves are evergreen, 3 to 9 cm long, elliptic, and silvery in appearance.
Flowers: The flowers are borne in axillary clusters, with a four-lobed calyx, and a four-lobed corolla. The two stamens (male parts) project beyond the mouth of the flower.
Fruits: The fruit has a hard endocarp (the olive stone), which is surrounded by a fleshy, edible mesocarp.
Six genetically distinct subspecies are currently recognised.
- Subspecies europaea is distributed around the Mediterranean as both the cultivated olive (var. europaea) and its feral relative (var. sylvestris).
- Subspecies cerasiformis is restricted to the island of Madeira.
- Subspecies guanchicaoccurs only on the Canary Islands.
- Subspecies maroccana is to be found as a small population at the southern end of the High Atlas Mountains of Morocco.
- The isolated Saharan massifs of the Hoggar, Air and Jebel Marra also have a distinct subspecies, laperrinei.
- The remaining subspecies, cuspidata, occurs widely through eastern and southern Africa, Arabia, and across Asia as far as southern China.
There are over 1,000 cultivars of olive, which have much larger, fleshier fruits (drupes) than their wild ancestor, which grows throughout the Mediterranean.
Grown in the Mediterranean for over 5,000 years, the olive has shaped the landscape and culture of the region: 90% of all olives are produced in the Mediterranean. It is the region's most versatile and valuable crop with the fruit, oil and leaves having been used for food, fuel, medicine and embalming.
The birth of olive-farming is shrouded in the mists of time. Discoveries of olive stones at archaeological sites in Israel show at least 20,000 years of use and by 5,000 years ago olives had been taken into cultivation and spread throughout the Levant. Domestication may have taken place in the eastern Mediterranean region, or in the region of the Nile Delta where the climate of the time would have been more suitable for cultivation. Today, there are thought to be around 1,000 million olive trees in the world.
Olives are harvested in autumn. If they are to become table olives they are soaked in water for five days to extract the bitter phenolic compounds such as oleuropein. The fruit is then cured in brine for around four weeks. Green olives are unripe, whereas black olives are ripe and less bitter. Olives are eaten as snacks or appetisers with a variety of accompaniments, and are a key ingredient of Mediterranean cooking.
Olive oil is obtained from the fruit. Shortly after harvesting, the fruit is cleaned and processed into a paste from which the oil is extracted. Olive oil is classified according to the production method and the oleic acid content. Virgin or cold-pressed olive oil is obtained by pressing alone and contains up to 2% oleic acid. Refined olive oil is obtained with the use of heat or solvent extraction and requires further processing to yield edible oil (it contains up to 3.3% oleic acid). The leftover cake is used as a source of inedible industrial-grade oil (containing more than 3.3% oleic acid), and is also used in livestock feed and compost.
Olive oil is used for food, cooking and for a multitude of therapeutic purposes.
A line of olive trees at Pont du Gard in southern France
The safe dosage for adults is two tablespoons (28 g) of olive oil per day. Evidence suggests that people whose diets include olive oil have a reduced risk of developing breast and colorectal cancers. Likewise, a diet rich in olive oil (and low in saturated fats) is associated with reduced risk of cardiovascular disease, high cholesterol levels and high blood pressure.
The beneficial qualities of olive oil have been attributed to the fatty-acid composition and the presence of phenolic compounds, which seem to have antioxidant, vasodilating, antiplatelet and anti-inflammatory effects. Kew is investigating how the waste products of olive oil production could be used as sources of compounds for medicines to treat cardio-vascular disease.
The fine texture and intricate grain of olive wood is sought after for turnery and furniture, although it is difficult to work with due to the density (it is very hard and heavy). Olive wood and olive stones are excellent for fuel.
Olive trees are planted for ornamental purposes, as firebreaks and to control soil erosion in Mediterranean climates.
Millennium Seed Bank: Seed Storage
Kew's Millennium Seed Bank Partnership aims to save plant life world wide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.
Number of seed collections stored in the Millennium Seed Bank: 11
Seed storage behaviour: Published literature compiled in the Seed Information Database (SID) indicate that this species’ seeds survive drying to low moisture content and cooling to sub-zero temperatures and are thus amenable to long-term storage in facilities such as the Millennium Seed Bank. However, survival in storage may be quite short, and more research is needed.
Germination testing: Collections so far tested at the MSB have failed to pass germination tests. However, seeds remaining at the end of the tests are healthy, indicating dormancy rather than low viability, and work is continuing to establish appropriate dormancy-breaking treatments.
Composition values: Oil content: 1 - 41.9 %
Young olive plants are grown from seed in the Arboretum Nursery. It has been noted that germination is spasmodic, taking from a few weeks to a few months. The compost used as a growing medium is an open, gritty, free-draining mix.
One of the Olea europaea specimens held in the Herbarium at Kew.
The seedlings are pricked out into ‘air pots’. Air pots prevent the plants from becoming pot-bound by encouraging the roots to grow outwards rather than spiralling. Planting out into the required position in the garden can be carried out straight from the air pot. The glasshouse zone in which the seedlings are grown is kept at a minimum temperature of 5˚C. Only natural light is provided. The young plants are well watered and not allowed to dry out.
Many of the trees growing in the Mediterranean Garden are over 100 years old, having survived transplantation whilst mature to provide instant impact. These trees were sourced from a nursery in northern Italy, which purchases them from land-owners when they need to remove or replace them, and hence ensures a longer life for these trees.
This species at Kew
The oldest of Kew's herbarium specimens is a wreath of folded olive leaves, which was found in the sarcophagus of King Tutankhamun. This well-preserved olive wreath is over 3,300 years old. Details of specimens of Olea europaea, including images, are available online in the Herbarium Catalogue.
View details and images of specimens
In the summer of 2006 several mature trees were planted in a newly-landscaped Mediterranean setting. These included specimens of cork oak (Quercus suber), stone pine (Pinus pinea) and an olive (Olea europaea).
Tony Hall, who manages this collection, notes that olives respond well to the sandy, free-draining soil at Kew. In the winter of 2008 they survived snow, frost and temperatures of -7˚C. So far, flowers and small fruits have been observed, but ripening of the fruits has yet to occur at Kew.
In the Temperate House there is a very fine specimen of Olea europaea, which is over 150 years old.