Experts tackle readers’ garden questions.
Q: I would appreciate your help with a 3-year-old olive tree that we have had planted in a container in our backyard for the past three years. It has doubled in size over and each spring it starts to develop olives, but the very small light brown olives dry up and never produce like it did the first year. How do I get the Kalamata olives to grow?
Rodger Freeman, Sacramento
Sacramento County Master Gardener Cathryn Rakich: Olive trees (Olea europaea) originated in the Mediterranean and thrive in regions with hot, dry summers and cool winters where temperatures do not fall below 25 degrees. Therefore, they do well in most of California, including the Sacramento Valley.
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Some types are grown for oil production while others – such as the smooth, dark purple Kalamata, named after the city in Greece – are perfect for eating. These slow-growing, drought-resistant evergreens need full sun. When planted in the ground, they can live for more than a hundred years and reach 20 feet or taller. But they also do well in containers under the right conditions. Most olive trees will start to bear fruit after three or four years, so your tree is just getting started.
Poor fruit set can be attributed to a number of different factors, including freezing temperatures, improper irrigation and excessive fertilizing. A second or third olive tree also may be necessary for cross-pollination, which can lead to better fruit yield.
Because olive trees prefer a warmer climate, they can succumb to freeze damage during a severe cold spell. Damage can vary depending on the duration of the cold, as well as the variety and age of the tree. Flowers and fruit, especially on younger trees, can sustain damage at 28 degrees, causing olives to shrivel, turn brown and fall to the ground. Because your olive tree is in a container, it can be moved to shelter or covered with a tarp or sheet for protection when freezing temperatures are expected.
Even though established olive trees are considered drought-tolerant, they require regular watering to thrive and produce better fruit. Potted trees need to be irrigated more often than those in the ground. The first 2 inches of soil should be allowed to dry out in between watering, and the soil should never become water-logged. Good drainage is critical. Also, if your tree is root bound, it is a good idea to repot it in a larger container. When repotting, it is important not to damage or disturb the roots.
With olive trees, proper irrigation is more important than fertilizing. “Olive trees are not big feeders,” said Paul Vossen, farm adviser for the University of California Cooperative Extension. “Olive trees tend to fruit better under conditions of low vigor, including minimal nutrition without being deficient. They respond quite amazingly to adequate irrigation water with good growth, large fruit size and much better yields without making any changes to their nutritional status.”
Olive trees are considered “self-fruitful,” meaning a single plant can self-pollinate to develop fruit. However, research shows that better fruit production will occur when there is more than one olive tree in close proximity. Consider adding at least two compatible varieties close to the current tree that bloom at the same time. This should lead to cross-pollination and better fruit yields. The flowers will gradually develop into olives that mature throughout the summer and ripen in the fall.
Olive trees also are alternate bearing, which means they will produce a heavy crop one year and a light crop the next. Thinning the fruit (removing excess olives during the early growing period) will help achieve a more consistent annual crop, as well as improve the size and quality of the olives. Fruit thinning also will promote better shoot growth, which will result in increased bloom and crop yield the following year.
Young olive trees do not require a lot of pruning. Dead limbs, suckers and branches that cross each other should be removed. As the tree matures, some interior branches can be pruned to open up the center, which will allow more sunlight to reach all sides of the fruit-bearing branches, increasing the crop yield. It is important to remember that olive trees bear fruit on last year’s growth, so be careful to balance the number of branches removed so as to not reduce fruiting potential for the next season.
Olives straight off the tree are bitter; therefore they must be brined, pickled or cured for eating. All olives trees start out with green fruit. For green olives, harvest the fruit before it turns dark. For black olives, wait longer before harvesting. Remember that fallen black olives can stain concrete pavement.
For a list of seasonal tasks for growing olive trees, go to the UC California Backyard Orchard website at homeorchard.ucanr.edu. For general information on olive trees and how to cure the fruit, visit the UC Division of Agriculture and Natural Resources website at anrcatalog.ucanr.edu.
Cathryn Rakich is a UC Cooperative Extension master gardener for Sacramento County.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to firstname.lastname@example.org. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:
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