The feathery gray-green foliage and the gnarled and furrowed trunks of the Olive (Olea europa) make them a valued ornamental. The Olive is one of the hardiest of evergreen fruit trees, being able to withstand 12-15° F. with little or no damage. The tree grows vigorously in a wide range of climates, but is best adapted to the hot, dry areas of the Southwest. Some winter chilling is needed to induce flower formation; the 2 coldest winter months should have average temperatures 50° F. or below. At higher temperatures the trees grow well but fail to flower and fruit. The Olive blooms very late, so that it normally escapes all frost danger to the bloom and young fruit. Olives grow well in the Southeast, but have not borne well there; the fruit ripens too late, being subject to fall and winter frosts; fruit is injured below 28° F.
Olives are normally propagated from cuttings, and show a remarkable ability to root from cuttings of any size but tip cuttings. Sub-terminal cuttings, 4-5 in. long, are generally used; remove all but the 2 terminal leaves. Hard-to-root varieties are treated with 4000 ppm (0.4%) indolebutyric acid (IBA) in 50% alcohol for 30 seconds; and planted in sharp sand with bottom heat. Older wood will root even more readily if the bottom is soaked in a 13 ppm IBA solution for 24 hours, then buried in moist sawdust until callused before planting in the cutting bed. Older and larger branches may be planted directly in place and will usually root. Slabs of the trunk placed a few inches below the soil, with the bark side up, will usually send up shoots and root. Suckers are often found at the base of trees; these may be removed and planted.
Trees are sometimes produced by grafting; seeds are planted, usually after clipping the end off; they germinate slowly. They may not be ready to graft for a year. Grafting is by whip-graft or side graft. Trees are top worked by bark-grafting, usually in the spring. Trees of the varieties mentioned here are dwarfed somewhat by grafting to certain clones.
Trees dug from the nursery row are usually bare rooted, in which case all branches are removed, and only a few leaves along a single stem are left; they are whitewashed after planting. Trees in cans need only to have low water-sprout shoots removed.
Except in extremely sandy or shallow soils the planting hole need be only large enough to take the roots. Most varieties attain large size under good conditions, and need a spacing of about 35 ft. for full development; shorter distances may be used if the trees are kept smaller by consistent pruning.
During the first 2 years in the ground, olives are trained to 3-5 scaffold branches, well spaced. Thereafter, pruning is normally limited to removing unwanted growth, as it reduces yield. If the tree becomes too tall, it may be topped severely.
The Olive will grow on a wide variety of soils, including many that appear too poor for other tree species; water-logged soils should be avoided.
Olives grow and produce the best when supplied with about as much water as other evergreen trees. Nevertheless, they are among the most drought-resistant of all trees, but growth and fruiting will be greatly reduced with restricted watering.
Olives are light users of nitrogen, and respond to it only on relatively infertile soils. Even then, a pound of nitrogen per mature tree should be adequate. Potassium and boron deficiencies occasionally are seen. In the arid West an application of 25 lbs. of potassium sulfate should be applied. In the Southeast, complete fertilizer applications will provide the needed potassium; the boron applications will need to be slightly greater, and may need to be repeated every year or two. For both deficiencies, yellowing of the leaves occurs. Potassium deficiency leads to leaf-tip burn; boron deficiency to death of shoot tips, and to death of the blossoms end of the fruit, a condition called Monkey-face.
The Olive flowers very profusely, the bloom developing as several flowered inflorescences in the axils of the alternate leaves. Enough may develop that the Olive bears more flowers than almost any other fruiting tree, but a high percentage of these are normally female sterile. In any case, less than 1% set will result in a heavy crop. Cross-pollination is not usually needed, but may increase set in cold springs.
Excess set results in small fruit, and increases to alternate bearing. Spraying 20 to 28 days after full bloom with naphthalene acetic acid, 125ppm in water containing 11% light summer oil, will thin the fruit adequately in most years, although there is some danger of over-thinning, and occasionally the spray is not effective. Reducing the crop load to 3 or 4 fruits per foot of shoot by hand is also effective, but very tedious.
For home use, olives will be harvested for pickling only. For this purpose, they are picked as they turn from green to straw or pink color. A simple and effective home pickling recipe is: Soak the fruit overnight in water; replace the water with a 2% concentrated lye solution per gal. of water, leaving it until the flesh color change shows that the lye has penetrated to or nearly to the pit. Replace the lye with water, leaving for 3 or 4 days, with daily or more frequent water changes until the lye is all removed. Replace the last wash solution with a solution of 3 oz. of salt per gal. For storage of more than a few days, gradually increase the strength of the salt solution in increments of 3oz. per gal. every other day until a final solution of 12 oz. per gal. of water is used. Change this solution about every 3 weeks until the olives are used. From this strong solution, remove olives as needed and soak in fresh water for a few hours before use.
The varieties of Olive differ primarily in size and oil content. Small varieties are ‘Mission’ and ‘Manzanillo’; large varieties are ‘Ascolano’ and ‘Sevillano’.
Scales of several kinds attack Olives; 1-2% summer oil sprays are recommended for control in general in the summer. Peacock spot, a fungus disease, is controlled with Bordeaux spray. Shoots affected with Olive knot, a bacterial disease, should be cut out, using care not to spread the organism with the pruning implements. Verticillium will attacks olives; do not plant where susceptible annual crops have been growing, as tomatoes, potatoes, cotton and melons. Nematodes are known to attack olive roots.