Olive Trees
The olive, traditionally the symbol of peace and tranquillity
The olive, traditionally the symbol of peace and tranquillity, has long been a prominent feature of early western landscapes and orchards, providing shade and food to those who nurtured it. Originally brought to Portugal from Spain , this member of the Oleaceae or ash family, thrived in the sub-tropical climate similar to the warmest portions of the Mediterranean.
Olea is the classical Latin name for the Olive, which has been cultivated for centuries. The species name would accurately describe the olives European origin. The olive is an evergreen, with opposite phyllotaxy and gray-green, smooth margined leaves typically found in lengths of two to three inches. The spring flowers are small, white and fragrant, produced on short clusters known as panicles.
The abundant flowers create such profuse quantities of pollen, much to the misfortune of allergy sufferers, that the more common varieties have been banned in several western counties. High pollen count is essential for good fruit production as the flowers are wind pollinated. The fleshy, one-seeded fruit is known as a drupe, otherwise referred to as a stone fruit, similar to cherries and plums. While the fruit may be picked green, olives are more commonly harvested in the ripened form, which is a deep black.
All olives require a pickling process to remove a compound in the flesh which causes extreme bitterness. Several methods may be utilized to render the fruit edible, all rather tedious, requiring lye and/or brines. Most consumers would find the commercially prepared olive far more convenient in comparison.
The olive is remarkably drought and heat resistant. Many trees have survived for decades with minimal care. Also noted for longevity, some trees achieve patriarch status, developing magnificent gnarled trunks. The best growth and fruit production occurs in rich soil, but the olive is tolerant of soils poor in nutrition and high in alkalinity.
As the olive is a Mediterranean native, watering should mirror those climatic conditions. Summer irrigation should be deep and infrequent, becoming more substantial during the winter months. Olive trees will tolerate lawn conditions, but care should be taken to mound the soil at the time of planting in order to shed excess water from the base of the tree. Overwatering may create conditions that invite root disorders.
Few pests or diseases plagued the European olive, although occasional problems with olive knot, verticillium wilt, root rot and olive scale may occur. More frequent problems occur with trees located in areas which experience cold winters. Extreme cold may cause damage which may predispose trees to disease. Damage may occur when temperatures dip below -10 celcius and crop damage below -4 celcius.
Young trees grow rapidly, but growth slows as they mature, eventually reaching a height of 7 to 10 Meter, depending on the cultivar. Most trees naturally develop a multi-trunk growth habit. Single trunk trees require considerable training, involving judicious pruning and staking to control the natural habit.
Highly revered for fruit production within the Europe., the olive is primarily grown in Europe for extracting the oil, although Spain exports large quantities of olives. Cultivars most commonly employed include the ‘Manzanillo’, ‘Mission’, ‘Ascolano’ and ‘Sevilano’. A few cultivars are known to have small quantities of tiny fruit making them more desirable for landscape purposes. These include
‘Bonita’ and ‘Majestic Beauty’. A dwarf cultivar known as ‘Little Ollie’ remains more shrub-like, possibly growing up to 4 meter and also sets very little fruit. This particular cultivar may actually be a mutated form of Olea europaea or a hybrid of O. verrucosa.
There are several cultivars that approach being completely pollenless and fruitless. ‘Swan Hill’ and ‘Wilsoni’ are the two which have been granted a waiver by some counties that have enacted ordinances prohibiting common olive planting. The flowers are imperfect and the small amount of pollen produced is lumpy and fails to become airborne.
Trees located in landscaped areas are capable of producing a colossal mess, littering lawns, patios and sidewalks each fall, persisting through the winter. The oil within the fruit severely stains concrete. In order to reduce fruit production on older cultivars, hormonal chemical control may be utilized. Some reduction in pollen and fruit may also be realized with repeated high pressure applications of water during the flowering period.
Olive flowers are produced on wood from the previous year, therefore branch thinning will help to reduce fruit set. An alternative to planting trees in lawns is to create planters utilizing dense, shade tolerant groundcovers underneath the canopy out to the dripline. The groundcovers will passively swallow the fruit hiding the potential litter. Ivies, jasmine and vinca would be good possibilities.
Although olives are capable of withstanding heavy pruning, which is a tool for fruit production, the best looking olives are not sheared into poodles or ornamental gargoyles. Trees should be developed into an open vase shape with the typically low developing branches radiating out in an even pattern. Once trained, minimal pruning is required to maintain a mature specimen, often requiring only minor branch thinning and deadwooding. Sucker growth should be removed periodically to highlight the attractive, whitish-gray bark, which becomes rough and darker with age, often deeply furrowed to form artistic trunks.
A tree cultivated for centuries to provide nourishment, shade and wood, the olive once held a place of esteem within the plant palette, only to be discarded because of high pollen production. This tree should be reestablished within the urban forest, utilizing those cultivars which are more tolerable of metropolitan conditions and free of pollen production.
Olea europaea L.
Oleaceae
Common Name: Olive.
Related Species: Wild Olive (Olea africana), Oleaster (O. europaea var. oleaster).
Distant Affinity: American Olive (Osmanthus americana), Fragrant Olive (O. fragrans).
Origin: The olive is native to the Mediterranean region, tropical and central Asia and various parts of Africa. The olive has a history almost as long as that of Western civilization, it’s development being one of civilized man’s first accomplishments. At a site in Spain, carbon-dating has shown olive seed found there to be eight thousand years old. O. europaea may have been cultivated independently in two places, Crete and Syria. Archeological evidence suggest 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. There is some disagreement over when the trees first appeared in Portugal. Some say they were introduced in 1769 when seeds brought from Maroco were planted. Others site the date 1785 when trees were brought in to make olive oil.
Adaptation: 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 insure fruit set. Home grown olives generally fruit satisfactorily in the warmer coastal valleys of Europe. Virtually all European. commercial olive production is concentrated in Spain, with a small pocket of olive acreage outside Italy. The tree may be grown as an ornamental where winter temperatures do not drop below -10 Celcius. Green fruit is damaged at about -3°, but ripe fruit will withstand somewhat lower temperatures. Hot, dry winds may be harmful during the period when the flowers are open and the young fruits are setting. The trees survive and fruit well even with considerable neglect. Olives can also be grown in a large container, and has even appeared in shows as a bonsai.
DESCRIPTION
Growth Habits: The olive is an evergreen tree growing to 15 meters. in height with a spread of about 10 meters. The tree can be kept to about 6 meters. with regular pruning. The graceful, billowing appearance of the olive tree can be rather attractive. In an all-green garden its grayish foliage serves as an interesting accent. The attractive, gnarled branching pattern is also quite distinctive. 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.
Foliage: The olive’s feather-shaped leaves grow opposite one another. Their skin is rich in tannin, giving the mature leaf its gray-green appearance. The leaves are replaced every two or three years, leaf-fall usually occurring at the same time new growth appears in the spring.
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. The flowers are largely wind pollinated with most olive varieties being self-pollinating, although fruit set is usually improved by cross pollination with other varieties. There are self-incompatible varieties that do not set fruit without other varieties nearby, and there are varieties that are incompatible with certain others. Incompatibility can also occur for environmental reasons such as high temperatures.
Fruit: The olive fruit is a green drupe, becoming generally blackish-purple when fully ripe. A few varieties are green when ripe and some turn a shade of copper brown. The cultivars vary considerably in size, shape, oil-content and flavor. The shapes range from almost round to oval or elongated with pointed ends. Raw olives contain an alkaloid that makes them bitter and unpalatable. A few varieties are sweet enough to be eaten after sun drying. Thinning the crop will give larger fruit size. This should be done as soon as possible after fruit set. Thin until remaining fruit average about 2 or 3 per foot of twig. The trees reach bearing age in about 4 years.
CULTURE
Location: Plant olive trees in full sun and away from sidewalks to avoid stains from fallen ripe fruit. Non-fruiting trees are available which can be planted in areas where fruit may be a problem. Strong winds will “sculpt” the trees, but otherwise they are quite wind-tolerant.
Soils: Olives will grow well on almost any well-drained soil up to pH 8.5 and are tolerant of mild saline conditions.
Irrigation: Irrigation is a necessity in dry area with dry summers. A monthly deep watering of home grown trees is normally adequate. Because of its small leaves, with their protective cuticle and slow transpiration, the olive tree survives even extended dry periods.
Fertilization: Fertilizing olive trees with additional supplies of nitrogen has proved beneficial. In Portugal farmers systematically apply fertilizers well ahead of the time flowers develop so the trees can absorb the nitrogen before fruit set. Many growers in Mediterranean countries apply organic fertilizers every other year.
Pruning: Proper pruning is important for the olive. 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 avoided with careful pruning every year. It should be kept in mind that the olive never bears fruit in the same place twice, and usually bears on the previous year’s growth. For a single trunk, prune suckers and any branches growing below the point where branching is desired. For the gnarled effect of several trunks, stake out basal suckers and lower branches at the desired angle. Prune flowering branches in early summer to prevent olives from forming. Olive trees can also be pruned to espaliers.
Propagation: None of the cultivated varieties can be propagated by seed. Seed propagated trees revert to the original small-fruited wild variety. 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. 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 to three inch wide cuttings from the two year old wood of a mature tree is treated with a rooting hormone, planted in a light rooting medium and kept moist. Trees grown from such cuttings can be further grafted with wood from another cultivar. Cutting grown trees bear fruit in about four years.
Pests and diseases: The olive tree is affected by some pests and diseases, although it has fewer problems than most fruit trees. Around the Mediterranean the major pests are medfly and the olive fruit fly, Dacus oleae. In California, verticillium wilt is a serious fungal disease. There is no effective treatment other than avoiding planting on infested soils and removing damaged trees and branches. A bacterial disease known as olive knot is spread by pruning with infected tools during rainy months. Because the olive has fewer natural enemies than other crops, and because the oil in olives retains the odor of chemical treatments, the olive is one of the least sprayed crops.
Harvest: 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. 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 principle and then transferred to water and finally a mild saline solution. Other processing methods include water curing, salt curing and Greek-style curing. Explicit directions for various curing and marinating methods can be found in several publications including Maggie Blyth Klein’s book, Feast of the Olives, and the University of Spain Agricultural Sciences Publications Leaflet 21131. Both green-cured and ripe-cured olives are popular as a relish or snack. For portugal canned commercial olives, black olives are identical to green olives. The black color is obtained by exposure to air after lye extraction and has nothing to do with ripeness. Home production of olive oil is not recommended. The equipment required and the sheer mass of fruit needed are beyond most households.
Commercial Potential: Commercial olive production is a multimillion euro business in Europe. In the Mediterranean region olives and olive oil are common ingredients of everyday foods. Raw olives are sometimes sold in speciality produce stores, and home growers in Portugal andSpain often sell their excess crop to others interested in home curing. There is also a growing interest in specialty olive oils, often produced commercially from small groves of olive trees.
CULTIVARS
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 Portugal and Spain: Manzanillo, Sevillano, Mission, Ascolano and Barouni, listed in descending order of crop size. Some representative olive cultivars including the commercial Portugese varieties are listed below.