Extension Horticulturists
Texas A & M University
College Station, Texas 77843-2134
January 27, 1997
The olive was spread from its place of origin on what is today Turkey and Syria to other parts of the Mediterranean basin in a very early period. The olive found conditions for its greatest cultivation in Italy and Spain. It was the Spanish who spread the olive to America. Catholic missionaries spread the olive to Mexico and later to California, as well as to South America.
Olives play a significant role in horticulture today in California, but that state produces less than one percent of the world’s olives. California furnishes only 40 percent of the canned olives consumed in the United States and less than two percent of the oil; the rest comes from the Mediterranean area. If the Spanish introduced the olive to Texas in the 17th and 18th centuries, no record or remnant of that introduction exists today.
The late Earnest Mortensen of the Texas Agricultural Experiment Station brought olive trees to the Winter Garden area in the 1930’s. Studies there showed that the olive trees would produce in Texas conditions. Isolated plantings of olive trees, mostly used as ornamentals, can be found in parts of central and South Texas today. An olive tree found in La Villita in San Antonio produced regular crops of fruit until it was severely damaged in the freeze of December, 1983. This tree was most likely planted in the 1930’s when the Works Progress Administration performed the restoration and landscaping in downtown San Antonio.
Climate is the most important limiting factor in the distribution of the olive in Texas and elsewhere. Temperature controls growth, reproduction, and survival of the olive. Growth begins after mean temperatures warm to 70 degrees F in the spring and continues until temperatures drop below this point in the fall. Unlike the fruit trees that we are familiar with, such as the peach, the olive does not set fruiting buds in the fall. Instead, the olive will only set flower buds after being exposed to cool night and warm day temperatures during the winter. This unique warm day/cool night vernalization is essential for fruit bud development.
The olive must experience vernalization to produce fruit; however, it will freeze from extreme cold. Although the olive is the most cold-hardy of the subtropical fruit trees, it will sustain damage to leaves and small stems at 17 degrees F and more severe damage at 12 degrees F. The tree can be killed to the ground with temperatures below 10 degrees F. Mature trees can regrow from underground parts following a severe freeze.
There are very few sites that meet the climactic requirements of the olive in Texas. Studies by the late Jim Denny at Texas A&M University indicated that the olive could be grown as a fruit tree in large parts of East, Central, and South Texas; however, the trees would freeze to the ground three of ten years. Extreme South Texas does not experience enough cool vernalization weather to set fruit on the olive. Reports from the Rio Grande Valley indicate no fruit production on olive trees there, and reports from Corpus Christi indicate that fruiting is very sporadic. The olive may be grown as an ornamental in these areas.
In North and West Texas and the Hill Country, the frequency of freezing temperature is too great to allow for cultivation of olive. Because very cold, dry air may sometimes invade the entire state during severe winters, damage to the olive is a threat almost anywhere olive trees are planted in the state, with danger increasing the further north you go. Efforts must be taken to protect olive trees, especially young ones, from damage when severe cold takes hold.
Botanically,the olive (Olea europaea L.) is a subtropical evergreen tree or shrub with opposite leaves. The leaves are lanceolate (lance-shaped), waxy green on top and grayish green on the bottom. Young bark is green, but older bark is gray. In the Mediterranean, olive trees are known to live for over a thousand years. If the top is damaged, a new tree will sprout from underground parts.
Under the proper conditions, at about five years of age, the olive will begin to bear the familiar olive fruit. Fruit is borne on panicles, or fruiting branches, arising from buds above the point where the leaves join the stem on the previous season’s growth. The cream-colored flowers are very similar to those of the waxleaf ligustrum (privet), a member of the same botanical family (the Oleaceae) which is widely grown in Texas as an ornamental.
Two types of flowers arise on the tree: perfect and staminate. Staminate flowers contain only male parts; the pistil is aborted. Only perfect flowers can become fruits. Bees and other insects play a minor role in olive pollination; wind moves most of the pollen from tree to tree. Most olive varieties are self-fertile, but increased production often results from cross pollination.
The olive is the only member of the Oleaceae to bear edible fruit. The fruit, a drupe like a peach, cannot be eaten fresh because of the presence of a bitter glucoside. Thus the olive must be processed in order to be served as food; either processed for its oil or processed with lye and salt to produce the canned or preserved table fruit. While fruit processed in California has almost all of the bitterness removed, that processed in the Mediterranean area is often left somewhat bitter.
The olive should not be confused with the Russian-olive (Elaeagnus angustifolia) or the Anacahuita (Cordia boissieri),which is sometimes called the Texas or Mexican Olive. Both of these plants belong to different botanical families. The olive, however, is related to the Desert Olive (Forestiera sp.) and the American Wild-Olive (Osmanthus sp.). The fruits of these two "olives" are not edible.
- Propagation
- Olives can be propagated very easily. There are a number of ways to propagate the plant. Plants may be grown from seed, but a cultivar will not come true from seed. Seedling olives are sometimes used as rootstocks to which are grafted known cultivars; seeds are also used for the selection of new cultivars. Seeds are cracked or treated with sulfuric acid to aid in germination because the pits are very hard.
Most olives are, however, grown on their own roots. Asexual propagation is from leafy cuttings, from larger stem cuttings called truncheons, from knotty growths at the crown of the tree called ovules, or from suckers.
Most modern propagation is from leafy cuttings rooted under mist. Take eight-inch long, pencil-sized cuttings from the tree in August or September for best results. Remove the lower leaves and treat the base of the stem with Indole-butyric acid (IBA) at 4000 ppm in diluted alcohol for five seconds. (level 1/4 teaspoon IBA, 509 ml 95% ethyl alcohol, 50 ml water) or with a commercial rooting compound.
The top two inches of the cutting may be removed or left on. Place the cutting stem-down in a mixture of equal parts peat, perlite or vermiculite, and sand. The media should be pasteurized and treated with a fungicide. Hold the cuttings under intermittent mist.
After six to eight weeks, roots should begin to form. Cuttings may be potted after 10 to 12 weeks. After potting, fertilize the rooted cuttings with a dilute fertilizer, but avoid burning the roots with excessive nitrogen. The cuttings may be transferred to the nursery the following spring.
- Soils
- The olive has a wide adaptability with regard to soils; it will tolerate a variety from sands to clays with a pH of 5.5 to 8.5. Olive trees have shallow root systems so they do not need a deep soil, but the soils must be well-drained.
Water
- The olive is drought tolerant, but grows best when it has sufficient water. Overwatering should be avoided. Chose a sunny, well-drained site with a fertile soil to plant the olive. Water regularly, but do not allow waterlogging to take place.
- Nutrients
- The olive is very efficient at extracting nutrients from the soil, and nitrogen is usually the only element which must be applied. Mature trees need from 1/2 to 2 pounds of actual nitrogen per year, depending on tree size. Deficiencies of potassium and boron are rare but possible. Fertilize in December to aid in fruit bud development and in the spring when new growth begins. Additional fertilizer may be added in the summer months if growth is poor.
- Training
- The tree should be trained to three or four main scaffold branches beginning at about three feet in height. A full canopy should be allowed to develop from the scaffold branches. Fruiting will take place in this shell of foliage.
- Pruning
- Pruning should be delayed until early spring. Because the tree does not go dormant, any increase in temperature after pruning will stimulate growth which might be damaged by freezing temperatures. The olive is pruned by thinning out dead or otherwise unproductive wood. It should not be topped. An exception to this rule is the use of the olive as a hedge. It will form a dense, attractive hedge if topped and trimmed.
Topping causes the formation of numerous lateral branches and suckers so that a bush is produced. Again, all cutting should be delaying until spring or summer.
- Cultural Practices
- To avoid being killed by severe cold, olive trees should be mounded with soil up to about 1-1/2 feet on the trunk until they are about five years old. Mound up the soil in late November and remove it in late March. If possible, cover the foliage when temperatures of 17 degrees F. or below threaten. If the tree is damaged by cold weather, wait until new growth appears in the late spring before removing dead or damaged parts.
Cultivars
- As yet, there is no firm information about what cultivars will do best under Texas conditions. Because it is cold-hardy, Ascolano may be a good choice as an ornamental under Texas conditions. Barouni may be a good choice as a fruit tree because it comes from a country which is warmer than the place of origin of the other cultivars. The last variety for trial plantings is Mission.
Some cultivars may require more cold temperatures than others to set fruit, and in general, recommended Texas sites have less cold weather than most olive-producing sites.
Figure 1. The olive and its fruit.