The olive tree, scientifically known as Olea europaea, has a rich history and is deeply intertwined with human civilization. Originating in the Eastern Mediterranean basin, most likely Crete, the olive tree has been cultivated for over 7,000 years, making it the oldest cultivated tree in history.
An ancient olive tree in Kalamata, Greece
Origin and Spread
The olive is native to the Mediterranean region, tropical and central Asia, and various parts of Africa. O. europaea may have been cultivated independently in two places: Crete and Syria. Archeological evidence suggests that olives were being grown in Crete as long ago as 2,500 B.C. From Crete and Syria, olives spread to Greece, Rome, and other parts of the Mediterranean area. Olives are also grown commercially in California, Australia, and South Africa.
Adaptation and Growth Habits
The olive requires a long, hot growing season to properly ripen the fruit, no late spring frosts to kill the blossoms, and sufficient winter chill to ensure fruit set. Home-grown olives generally fruit satisfactorily in the warmer coastal valleys. The trees survive and fruit well even with considerable neglect.
The olive is an evergreen tree growing to 50 ft. in height with a spread of about 30 ft. The tree can be kept to about 20 ft. with regular pruning. Olives are long-lived with a life expectancy of 500 years. The trees are also tenacious, easily sprouting back even when chopped to the ground.
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Characteristics of the Olive Tree
The olive tree exhibits several distinctive characteristics:
- Leaves: The olive's feather-shaped leaves grow opposite one another. Their skin is rich in tannin, giving the mature leaf its gray-green appearance. Leaves are opposite and elliptical with a narrow apex that is sharply pointed, the leaf color differs, gray green on the upper surface and lighter on the lower surface, almost appearing white to silver.
- Flowers: The small, fragrant, cream-colored olive flowers are largely hidden by the evergreen leaves and grow on a long stem arising from the leaf axils. The olive produces two kinds of flowers: a perfect flower containing both male and female parts, and a staminate flower with stamens only. Olive flowers have four green sepals, four white petals, two stamens, and a pistil containing a single carpel which will eventually become an olive if the flower is successfully pollinated.
- Fruit: The olive fruit is a green drupe, becoming generally blackish-purple when fully ripe. The cultivars vary considerably in size, shape, oil-content and flavor. Raw olives contain an alkaloid that makes them bitter and unpalatable.
Soil and Environmental Preferences
Olives grow well on almost any well-drained soil up to a pH of 8.5 and are tolerant of mild saline conditions. Sand-dominated soils are best as olives are susceptible to root rot diseases in moist or wet soils. Nutrient requirements for olive are low, and the trees do not produce well on highly fertile soils. Avoid planting olives in low areas where standing water accumulates after rainfalls.
Once established, Olive trees tolerate infertile and dry soils. They prefer deep, well-drained, fertile soils and require a sunny position. Olive trees are drought-resistant and can tolerate salty air, making them ideal for coastal regions.
Cultivation and Propagation
None of the cultivated varieties can be propagated by seed. The seedlings can, of course, 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. A commonly practiced method is propagation from cuttings.
Pests and Diseases
The olive tree is affected by some pests and diseases, although it has fewer problems than most fruit trees. In California, verticillium wilt is a serious fungal disease. A bacterial disease known as olive knot is spread by pruning with infected tools during rainy months.
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Harvesting and Processing
Olive 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 up through full ripeness. Ripe olives bruise easily and should be handled with care.
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 principle and then transferred to water and finally a mild saline solution.
Olive harvesting in progress
Commercial Varieties
Over the centuries mankind has produced and propagated a myriad of olive varieties. Today several dozen varieties are grown commercially around the world. Five commercially important varieties are grown in California: Manzanillo, Sevillano, Mission, Ascolano and Barouni, listed in descending order of crop size.
Here's a summary of some key varieties:
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| Variety | Description | Uses |
|---|---|---|
| Manzanillo | Large, rounded-oval fruit. Skin brilliant purple, changing to deep blue-black when mature. Resists bruising. Ripens early. | Excellent for oil and pickles. |
| Mission | Medium-sized, oval fruit. Skin deep purple changing to jet-black when ripe. Flesh very bitter but firm, freestone. Ripens rather late. | Good for pickling and oil, specially ripe pickles. Most widely used for cold-pressed olive oil in California. |
| Sevillano | Very large fruit, bluish-black when ripe. The largest California commercial variety. Stone large, clinging. Ripens early. Low oil content. | Only useful in pickling. Used for making Sicilian style salt brine cured olives, also the leading canning cultivar. |
| Ascolano | Very large, ellipsoidal fruit. Skin color very light even when ripe, pit very small. Fruit is tender. | Excellent for pickles, but needs proper aeration during pickling to develop ripe color. |
| Barouni | Large fruit, almost as large as Sevillano. Trees spreading and easy to harvest. Withstands extremely high temperatures. | The variety usually shipped to the East Coast for making home-cured olives. |
Subspecies and Related Species
Related Species: Wild Olive (Olea africana), Oleaster (O. europaea var. Distant Affinity: American Olive (Osmanthus americana), Fragrant Olive (O.
Two olive subspecies have been spread by humans: Olea europaea subsp. europaea (the Mediterranean olive) and O. e. subsp. cuspidata (Wall. ex G. Don) Cif. (the African olive).
Uses of the Olive Tree
The olive tree has a multitude of uses:
- Food: Olive fruits are widely used, especially in the Mediterranean, as a relish and flavouring for foods. The fruit is pickled or cured with water, brine, oil, salt or lye.
- Oil: The seed is rich in edible non-drying oil used in salads and cooking and is considered a condiment because of its distinct flavour.
- Medicinal: The leaves are antiseptic, astringent, febrifuge and sedative. A decoction is used in treating obstinate fevers, they also have a tranquillising effect on nervous tension and hypertension.
- Other Uses: The non-drying oil obtained from the seed is also used for soap making, lighting and as a lubricant. The wood is very hard, heavy, beautifully grained, takes a fine polish and is slightly fragrant. It is used in turnery and cabinet making, being much valued by woodworkers.
Olive trees are excellent for agroforestry as they provide shade, prevent soil erosion, and improve soil quality.
Ecological Impact
Humans have introduced plants and animals into new continents and islands with negative effects on local species. This has been the case of the olive that was introduced in Australia, New Zealand and Pacific islands where it became invasive. After dispersal and establishment, the olives outcompete the native vegetation (such as eucalypts) by preventing regeneration. Olea europaea forms a crown under which olive seedlings can grow, but most native flora cannot.
In addition, the establishment of African olive can affect the local fauna by changing the vegetation structure and fruit availability. The speckled warbler has been shown to be negatively affected by the African olive invasion, while nonindigenous bird species such as the common starling and Eurasian blackbird are attracted by the presence of the African olives. This further encourages the displacement of the native fauna.
Symbolism and Cultural Significance
The olive tree has a lot of mythical associations from the Greek Gods to Roman Legends and to the Bible. An olive branch is a symbol of peace. Olympic wreaths were made from them, representing the ultimate symbol of peace and fairness. Van Gogh loved them, and frequently painted them.
Olive Branch Symbol of Peace
Olive trees are native to Africa, Temperate Asia and Europe. They have long represented abundance, peace, power and wealth. Trees are long-lived, and there are claims of 1,600-year-old trees still producing fruit.
The herbarium at Kew Gardens, UK, contains a wreath of olive leaves found in the sarcophagus of King Tutankhamun and is over 3,300 years old.
