Although common olive trees (Olea europaea) are officially hardy in U.S. Department of Agriculture plant hardiness zones 8 through 10, microclimates within those areas may prohibit some gardeners from have success with the trees. In order to bear fruit, the trees require a period of cold temperatures in winter to initiate flowering in spring. They also thrive best when summer days are warm and dry. If you live in the appropriate climate and provide for these Mediterranean native trees' specific needs, they will bear all the olives you can use.
An olive tree's shallow root system resides very close to the surface of the soil. During periods of hot weather, therefore, olive trees require substantial amounts of water to remain healthy and to bear fruit. During your olive trees' first four years of life, water them when the soil surface appears dry. The best way to provide the moisture is with a soaker hose placed at least 2 feet from the base of each olive tree and set to run for one hour each day during summer. Mature olive trees need water when you stick your finger 1 inch into their soil and it comes out dry.
Olive trees don't require a lot of fertilizer and may not even require fertilizer every year. Nitrogen is the most important nutrient and should be applied in late winter or early spring to assure delivery throughout the trees in time for fruit set. Apply 1.3 to 3.2 pounds of ammonium sulfate by sprinkling it on the soil 6 inches from each olive tree's base and spreading it outward to the drip line, which is below the tips of the outermost branches. As a general rule of thumb, 2.1 cups of ammonium sulfate weigh 1 pound. After the fertilizer application, water the olive trees to a soil depth of at least 6 inches.
The most persistent and destructive pest you'll encounter on your olive trees is the fruit fly. Its larvae take up residence inside the fruit, where they feed. Manage olive fruit flies by spraying the trees – two to three weeks before fruit ripening -- with an insecticide containing spinosad. Choose a time to apply the product when no rain is in the forecast and there will be no wind. Wear a long-sleeved shirt, long pants, socks, closed-toe shoes, waterproof gloves and a hat. Mix the insecticide with water in a 1:1.5 ratio, or 1 portion of insecticide to 1 1/2 portions of water, and spray 1 ounce of the solution to a different part of the underside of each tree every week. Aim for the undersides of the foliage and foliage toward the interior of the canopy. This insecticide acts as a bait to the flies; they're attracted to it, eat it and die. Do not allow people or pets into the treated area for four hours after the application.
Olive trees are quite rugged and resist many diseases naturally. Fungal foliar infections may crop up now and again and can be treated with a fixed copper soap spray. This substance treats two of the most common diseases: peacock spot, which produces brown spots on the tops of leaves, and Cercospora leaf spot, which causes gray fungus on leaves' undersides. Fixed copper fungicides are available to homeowners in a ready-to-use formula that requires no mixing. Prevent these diseases by spraying each entire tree -- leaves, branches and trunk -- right after harvest. Spray until the surfaces are wet but not dripping. If it rains a lot in your area, treat the trees again in the middle of winter. If the disease is already present and the trees aren't in fruit, then spray them thoroughly to cover all surfaces, and spray again weekly until the symptoms disappear. Wear protective clothing during the application process.
During their first six years of life, olive trees require no pruning and just the removal of suckers – small branches that grow from the lower part of the trunk. Suckers can be removed by rubbing them away with your hand while they're small. In fact, removing all growth below 3 feet during the first three to four years of an olive tree's life helps build a strong structure. Perform all pruning chores in early spring.
Although olive trees require a chill period every year, sustained temperatures below 17 degrees Fahrenheit may damage them, and the trees may die if the temperature dips to 10 degrees Fahrenheit. If you expect a particularly brutal winter, then protect each olive tree by piling 1 foot of soil around the tree's base. Do this in November, and remove the soil in March. You also can cover each tree with a floating row cover to protect it during cold snaps.