“Climate Conditions for Growing Olive Trees”
The olive tree is a type of evergreen tree that can grow to heights of fifty feet and has a remarkable life expectancy of 500 years. The tree bears the olive fruit, a green drupe that gradually turns blackish-purple as it ripens. Cultivated since antiquity, the olive remains one of the most sought-after agricultural commodities in the world. Olives are the source of olive oil, a common ingredient used in cooking, as well as in the production of certain cosmetics and pharmaceuticals. Demand for olives has increased in recent years due to studies confirming that consumption of olive oil can help prevent heart disease and reduce the risk of some cancers. In response to the growing market for olives, “its cultivation area has tripled in the past 44 years, passing from 2.6 to 8.5 million of hectares” (Bartolini, 2005). Besides the valuable fruit the olive tree produces, the tree appeals to growers because it can withstand periods of drought, requires little maintenance, and is extremely resilient. Despite their tough exterior, however, olive trees require specific climate conditions to yield high-quality fruits and oils. Only a few locations in the world – the Mediterranean basin, Southern California, central Chile, southwestern South Africa, and southwestern Australia – possess the Mediterranean climate ideal for olive cultivation. The Mediterranean climate regions of the world typically experience cool, wet winters and mild-to-hot, dry summers. Mediterranean climates prevail “only in the mid-latitudes and then only on the western side of continents because of the earth’s atmospheric circulation of high pressure cells that control the temperature and precipitation patterns of the climate” (Rosenvall, 2009). During the summer, sinking air from the subtropical highs produces generally fair weather, diverting any storm systems to the north. In the winter, the subtropical highs shift southward toward the equator, exposing Mediterranean climate regions to storms that bring desperately needed rainfall. Average annual precipitation amounts vary between 11 and 35 inches, most of which falls during the winter months. Situated near large bodies of water, locations with Mediterranean climates experience a moderate temperature range throughout the year. Prevailing winds blowing in from the water prevent locations along the coast from registering bitterly cold temperatures in the winter and very hot temperatures in the summer. Locations further inland encounter larger, but still moderate, annual temperature ranges. Generally, temperatures during the warmest months in Mediterranean climates average between 70 and 80 degrees Fahrenheit, although some inland locations can experience triple-digit temperatures during the summer. Temperatures during the winter months rarely fall below freezing.
The relatively mild year-round temperatures allow for a long growing season, making Mediterranean climate regions especially attractive to olive growers. The olive growing season begins in the spring as temperatures warm to around 70 degrees Fahrenheit, causing trees to blossom after a dormant winter. Frost can damage the flower buds, but the mild spring temperatures in Mediterranean climates provide hospitable conditions for the budding to occur. Once the actual olive fruit emerges, it requires plenty of sunlight and relatively high temperatures to continue growing and ripen properly, conditions Mediterranean climates amply provide in the summer. To survive the dry summer months, an olive tree has certain features that allow it to retain sufficient moisture. Olive trees have relatively shallow, but extensive root systems that soak up any water in the soil. Complementing the roots are leaves that have “a protective coating and hairy undersides that slow transpiration” (“Olive Tree,” 2001). By autumn, the olive harvest begins as growers pick the green olives from their plants. The remaining olives will continue to ripen through autumn and even into the winter in some Mediterranean climate regions. Ultimately, the timing of a harvest depends on how the growers intend on using the olives. For instance, olives used in the production of the finest olive oils may require a ripening period that culminates in late November (in the Northern Hemisphere). Any unseasonably cold temperatures, frost, or excessive rainfall this late in the year could destroy the entire year’s crop. Mediterranean climates, however, provide the stable, generally mild conditions needed to yield a successful olive crop.
Mediterranean climates also offer appropriate conditions for vernalization to occur in olive trees. Like most fruit tree species, olive trees require exposure to low temperatures in the winter to induce flowering the spring. Although it can tolerate cold temperatures, the olive tree “will sustain damage to leaves and small stems at 17 degrees Fahrenheit and more severe damage at 12 degrees Fahrenheit” (McEachern & Stein, 1997). Winter temperatures in Mediterranean climates are cold enough to initiate bud development in the spring, but not cold enough to inflict any damage on the plant. The Mediterranean climate is the only climate that can consistently provide these ideal temperatures each winter. While the Mediterranean climate regions supply the best olive yields in terms of quality and quantity, fruit-bearing trees will grow outside traditional olive-producing regions. Unlike other fruit trees, olives can tolerate and recover from long periods of drought, below freezing temperatures, and even physical damage to fruits, shoots, and branches. These characteristics allow olive trees to grow in the temperate climates of the southeastern and southwestern United States, New Zealand, and even parts of the United Kingdom where plenty of sunlight and warm temperatures prevail during the summer. Olive trees are also scattered along the Nile River basin, as they have adapted to grow in the sandy soil that covers the region. Tropical areas, however, possess unsuitable conditions for olive cultivation, since the humid climate and incessant rainfall expose the trees to too much moisture. The large annual temperature ranges and extreme cold of the continental climates also do not support the cultivation of olives. Thus, anyone wishing to cultivate olives outside the Mediterranean climate regions should research local climate data and determine if the area is prone to extreme cold or any other conditions that could adversely affect their crop.