Olives

Related to jasmine, lilacs, and Forsythia, olive trees have been cultivated since the Bronze Age, some 6,000 years ago, for oil, fruit, and wood.

Ripe olives (Hans Braxmeier) Pixabay

Walking through rural northern Spain in 2016, I was delighted to see groves of ancient, gnarled olive trees. Surprisingly small, most of the olive trees were less than 10’ high and only 12-15’ in diameter.

Local cafe bars frequently maintained ceramic urns behind the bar filled with green olives in brine, which were scooped into small bowls and handed out the way American bars offer peanuts or pretzels. There are so many olives in Spain, that you can buy sealed bags of green pickled olives from vending machines!

Containerized olive tree in Paris

Growing olives

Unpruned trees can reach a height of 26-49 feet. The trunk tends to twist and becomes beautifully gnarled as it ages. Olive trees are evergreen, retaining their oblong, silvery leaves year round. Small white flowers occur on the previous year’s growth, which then produce the olive fruit. Some people are allergic to the pollen of the olive tree. Botanically, the fruit of an olive tree is known as a drupe, as are plums, cherries, and almonds.

Olive trees (Olea europaea) prefer soil that is less than ideal. Like many herbs, olives thrive in rocky, poor quality soil. They can perform well in clay soil as long as it has good drainage. Soggy soil will kill an olive tree. Olive trees grown in nutrient rich soil are more disease prone and they produce poorer quality oil. Olive trees are very drought tolerant and they love lots of direct sunshine. They also seem to have a preference for coastlines.

When selecting a site for olive trees, keep in mind that they may live for centuries. Most olive cultivars are self-sterile, which means they cannot pollinate themselves, so you may need to plant two trees. Olive cultivars can be primarily for table olives, olive oil, or both. The Santa Clara County Master Gardeners website recommends Ascolano, Manzanillo, Mission and Sevillano cultivars for the Bay area. You can contact your local County Extension office to see which cultivars are best for your area.

Olive trees can be propagated by cuttings and layers, from suckers, or grown from seed. To ensure that you are planting a specific cultivar, it is a good idea to buy root stock from a reputable nursery. Cuttings and suckers can be started by covering with aged manure and watered regularly. Seeds are more likely to germinate if the oily outer flesh (pericarp) is encouraged to rot slightly by placing it in hot water or in an alkaline solution, but olives grown from seed often do not produce fruit.

Once trees are established, fertilizer should be applied annually, just before flowering. Olive tree roots tend to be shallow, so avoid digging nearby. This is also what makes them excellent container plants.
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Pruning olive trees

Olives grow very slowly, but pruning is necessary for good health and fruit production. Since fruit is borne on the previous year’s growth, two-year old wood should be removed, to maintain a manageable size. In many commercial orchards, fruit laden branches are simply cut off the tree before the olives are removed.

Pests & diseases of olive trees

The olive fruit fly is a common pest that lays its eggs in the fruit. Once the eggs are laid, the fruit becomes unfit for consumption. Dusting with kaolin clay is an effective organic preventive measure. Rabbits can cause considerable damage to young trees by eating the bark. If the bark is removed all the way around the tree, girdling it, the tree will likely die. Verticillium wilt is a serious threat to olive trees in California. Curculio beetles, black scale, some caterpillars, and the Cycloconium oleaginum fungus can also cause problems. Once an olive tree is established, its extensive root system can recover from the complete destruction of the above ground portion of the tree.

Harvesting olives

Olives are harvested in September through February, depending on the cultivar and the use for the fruit. Green (unripe) olives are picked September to November, blonde olives are picked October to November, and black olives are picked mid-November through early February. Generally, olives are picked by hand to preserve the fruits’ shape and texture. Oil quality from fallen fruit is not as good as handpicked fruit because of bruising. Semi-dwarf varieties may produce 30-40 pounds of fruit, each year, while full sized trees may produce 100 pounds each year.

Making olives edible

Like acorns, uncured olives are quite bitter. This bitterness is caused by oleuropein and other phenolic compounds. Olives must be cured and fermented before they are eaten. The only exception is the Greek Throubes olive, which is allowed to dry on the tree and can be eaten as is. There are several commercial curing methods:

• Spanish or Sevillian - Green olives are soaked in lye and then fermented in salt water that is changed several times to remove the bitterness.
• Sicillian or Greek - Green, blonde or black olives are placed directly in brine and allowed to ferment (lactic, acetic or citric acid may be added).
• Picholine or directly-brined - Similar to the Spanish method, except that green, semi-ripe and ripe olives are soaked in lye longer.
• Water-cured - Olives are soaked in water that is changed daily for 2 weeks, then brined.
• Salt-cured - Ripe olives are washed and then packed in alternating layers with salt.
• California - This is the method most of us are accustomed to and it involves no fermentation. Green and semi-ripe olives are soaked in lye, then repeatedly washed in water injected with compressed air, which oxidizes the fruit, turning it black. Afterward, the fruit is brined.

These slow-growing trees can produce fruit for the next several hundred years or more. Some olive trees are over 2,000 years old and still produce fruit!