All about olive trees
Olives are widely planted in gardens but are not always grown for their fruit. This productive, long-lived evergreen tree from the Mediterranean is also valued in gardens for its good looks and heat and drought tolerance. Young olive trees have slender trunks and silver green leaves. Reaching 5 to 7 metres high they are used as shade trees or as hedges or espalier and can be grown as potted plants or as topiary specimens.
To produce a worthwhile harvest of olives, space trees about 6 metres apart to form a grove. This makes maintenance and harvesting manageable and also improves yield. Higher crops are obtained when trees are grown together as olives are wind pollinated so benefit from having other olives and a mix of varieties nearby.
To calculate how many trees to grow to meet your needs, estimate that a well-grown mature tree of nine years of age produces around 40 kilogram of fruit. Seven kilograms of olives yields a litre of oil so a single tree produces about six litres. Crops may vary considerably from year to year depending on growing conditions and tree management.
Varieties and harvest
The most widely grown varieties for gardens are ‘Kalamata’, a dual purpose black olive suited to either pickling or pressing, ‘Manzanillo’ a large green fruiting olive, ‘Verdale’ which is used either for pickling or pressing, and ‘Nevadillo Blanco,’ a heavy cropping black olive that’s the widest grown oil-producing olive in Spain.
Autumn to early winter - when the fruit is plump and either dark green or purple-black - is the time to harvest. Ripe olives are bitter and must be processed in brine before they can be preserved for eating. Olives used for oil are pressed or put through a centrifugal extraction process to extract their oil.
Olives come from the Mediterranean region and this is the best climate zone for olives. They are versatile however and grow in cool zones (although severe frosts can damage olives) and also in parts of the subtropics, although in humid areas they are more prone to pest and disease problems. Most olives require around 200 hours of chilling before flowering and fruiting occurs.
Olives grow best with a slightly alkaline (pH 7-8) well-draining soil. Olives can be grown in large containers but pot-grown plants are not as productive as those grown and managed in the ground.
Olives flower in early spring. Trees take three to five years of growth until they produce their first harvest and most only become fully productive after eight or nine years.
Young plants benefit from early pruning and training to form a single trunk and a framework of three to four main branches. Protect young trees from frost.
To maintain trees, prune them in winter after harvest to remove dead branches and to keep the tree structure open. Olives fruit on the wood from the previous year’s growth i.e. one-year-old wood, so heavy pruning can reduce the next year’s crop.
Olives do not need large amounts of fertiliser but benefit from light applications of organic fertiliser in late winter or spring (prior to flowering) and again in autumn after harvesting. They may also need trace elements.
Major pests include olive lace bug and scale, which attack the leaves, and olive moth caterpillars, which attack leaves and flowers and can reduce fruit production. Pests can be controlled organically with soap sprays or, in the case of caterpillars, the use of Dipel or other organic insecticide.
Diseases include leaf spots such as peacock leaf spot, anthracnose and root rots. These can be controlled with applications of organic fungicides, pruning for good air circulation and growing the trees in well-draining soil.
A note about the African olive plant: In New South Wales, the African olive plant is classified as an aggressive woody weed that invades native bushland. Although related to the edible European olive, African olive plant fruit is not edible.