:%* ,,F N Division of Agricultural Sciences UNIVERSITY OF CALIFORNIA *'''*h;^Q inn Olive Trees in California Karl W. Opitz Hudson T. Hartmann CALIFORNIA AGRICULTURAL Experiment Station Extension Service CIRCULAR 537 PRUNING OLIVE TREES IN CALIFORNIA THIS CIRCULAR contains detailed instructions to olive growers on • How to prune and train young olive trees • How to prune bearing trees for maximum production • How to prune trees under special situations, including trees that are newly grafted, old, subject to alternate bearing, diseased, and frost damaged. Also included are some words on machine pruning. Pruning and training your olive trees will provide you with a number of advantages. • Proper pruning of your young trees will develop a good struc- ture without unnecessary delay in production. • Correct pruning provides a mechanically strong trunk and scaffold framework which can sustain heavy crops and can resist strong winds without limb breakage throughout the life- time of the tree. • A good pruning system will help you obtain satisfactory yields of good quality fruit from your trees over the years. • Judicious pruning will make spraying and harvesting operations easier, and be an aid in insect and disease control. JANUARY, 1966 THE AUTHORS: Hudson T. Hartmann is Professor of Pomology and Pomologist in the Experiment Station, Davis. Karl W. Opitz is Farm Advisor, Agricultural Extension Service, Visalia, Tulare County. Fig. 1. Young olive trees not pruned properly grow as a bush (top) and eventually develop into a mass of scaffold branches arising at the ground level (bot- tom). These push against each other as they grow and break off. , - ... ,.,?»"•> Pruning and training young trees ' Experiments show that severe pruning of young olive trees markedly reduces their total vegetative growth and delays the onset of bearing. The more severe is the pruning the greater is this dwarfing effect and delay in bearing age. Therefore, prune young olive trees with caution and as lightly as possible. Remove unwanted branches when they are still quite small. Although pruning of young, nonbear- ing olive trees retards vegetative growth, some pruning should be done during the first several years after planting to form a strong trunk and primary scaffold sys- tem. An unpruned olive develops as a bush rather than a tree; the suckers arising at ground level produce a multiple trunk system. These suckers grow bigger, push against each other, and break off in strong winds or under a heavy crop (fig. 1 ) . To develop a structure strong enough > to withstand winds and to support heavy crops, train the young tree to a single trunk with three primary scaffold branches. Space these branches well up and down and around the tree, within 12 to 24 inches from the ground. This trunk and scaffold formation is most easily de- veloped during the first one or two sea- sons of growth. Much can be done during the first summer of growth after the trees are planted. First, select a single trunk, then cut or rub off all suckers growing around the trunk at ground level and keep eliminating them all through the summer as they appear. In addition, you may use an impervious white paper trunk wrap to protect against sunburn and to sup- press sucker growth from the trunk. After selecting the three primary scaf- folds remove all other shoots during the- summer as they appear. During the sec- ond summer, too, keep removing excess; shoots and suckers arising from the [3] »4W| Fig. 2. A well-trained, three-year-old olive tree having a single trunk and three primary scaffold branches. ground, and also the strong upright-grow- ing watersprouts that will grow near the base of the trunk. By removing the un- wanted branches several times during the summer while they are still small, the dwarfing effect of the pruning is mini- mized and the tree's growth can be di- rected into permanent branches. This will provide, rapidly and inexpensively, a good basic framework for the tree. If you cannot do this basic pruning during the first summer of growth then do it during the following winter if olive knot is not present. When this disease is present in even slight amounts, prune only during the dry season. Starting with the available growth, select a trunk and the required three primary scaffold branches. Remove all other growth along the trunk or competitive with the desired limbs. With either type of pruning — summer or winter — remove little or no wood from the upper portion of the tree during the first several years. The aim at this time is to develop a trunk and primary scaf- fold system, as illustrated in figure 2. This training is done with the minimum amount of top reduction to avoid delay in the on- set of fruiting. During the first few years suppress all suckers arising from the base of the tree. This is particularly important where the trees are grafted on rootstocks. Suckers from vigorous stocks quickly take over the entire tree and retard the growth of the grafted scion variety. After the trees come into bearing, at 3 to 5 years of age (fig. 3) another phase [4] Fig. 3. The same tree as in figure 2 two years later. A strong trunk and a low-headed scaffold system has been developed that is not likely to break as the tree grows older. sists of about three permanent branches arising from each of the three primary scaffolds. Space these secondary scaffolds around the tree so as to form a strong supporting framework for the fruiting top and side branches. Figure 5 shows a good tree formation and figures 6 and 7 are ex- amples of poor tree formation commonly seen. Olive limbs are very sensitive to sun- burn. Drastic pruning exposes the primary or secondary scaffolds to the direct rays of the sun. To avoid sunburn, maintain a good foliage cover. If pruning or break- age causes limbs to become exposed to direct sun cover them with whitewash as soon as possible. of pruning is required to develop a sec- ondary scaffold system. Because of the increasing density, some branch thinning is needed at this time. It is best to do this type of pruning over a period of several years rather than all at once. Pruning the trees lightly and systematically will keep them fruitful. A very heavy thinning out of unwanted branches all in one year causes the trees to become so strongly vegetative that they stop bearing until fruiting wood again develops. Drastic cut- ting out of many branches as shown in fig- ures 4 and 9 removes so much fruiting wood that production is greatly reduced. The gradual formation of the secondary scaffold system over a period of several years, together with the continual main- tenance of a large leaf and fruiting area, accomplishes the desired training and still permits adequate, continual fruiting. A good secondary scaffold system con- Fig. 4. A drastic pruning given young olive trees eight years old and just coming into good production. Most of the fruit-bearing wood is now on the ground. There is no evi- dence to justify such heavy pruning of young trees and it is not recommended. Fig. 5. A ten-year-old olive tree with a well- , formed trunk and scaffold system. [5 Fig. 6. A poorly-formed tree subject to limb breakage at the ground level. Fig. 7. Very poor branch formation. As the limbs become larger they will push against each other and break off. Pruning mature olive trees As the trees become older and larger, they produce more fruit. Pruning systematic- ally helps maintain fruiting surfaces where the olives may be harvested or sprayed economically. The natural fruit- ing area of the olive tree is a hemispheri- cal "shell" reaching from the ground up and over the top of the tree (figs. 8 and 10). To get the highest production keep this shell in a healthy, fruitful condition on the sides and over the top. Cutting out portions of this shell decreases produc- tivity by removing potential fruit-produc- ing wood. Do not remove the fruiting top of the tree to get fruit production on the inside of the tree. By attempting to obtain "inside" fruit, more fruiting wood and fruit are removed from the top of the tree than can possibly be gained inside the tree. Generally, too, any fruit produced in the lower, inside portions of the tree, tends to be small, of poor quality, and late maturing while the largest and best quality fruit grows in the top. Opening up the top of the tree may expose the main scaffold branches to sunburn dam- age. Pruning experiments with bearing olive trees in California show that yields Fig. 8. The natural fruit-bearing area of the olive is a shell around the periphery of the tree. Highest yields can be expected if this area is maintained as large as possible, reaching close to the ground where the fruit can be harvested easily. [6] Fig. 9. Heavy pruning of olives, such as this, is usually followed by very vigorous vegetative growth and lack of fruiting for several years. Sunburn of the exposed limbs can also occur. Moderate annual pruning is much preferred to a single drastic pruning. are reduced in proportion to the severity of pruning — a fact you should always keep in mind when pruning bearing trees. As the tree ages, the fruit-producing Fig. 10. A well-formed ten-year-old Man- zanillo olive tree. The fruiting surface has been allowed to develop close to the ground where it can be harvested easily. shell tends to accumulate dead wood and weakened twigs. Increasing top density requires the removal of dead growth and some thinning out of live branches. An- nual pruning provides an opportunity to keep the trees in a healthy, productive condition. This is largely accomplished by "undercutting" the dense areas and "heading back" the tall shoots. Removal of useless, dead, brushy growth also facilitates picking and spray- ing and keeps the tree more "open" which tends to prevent the development of pea- cock spot (Cycloconium oleaginum) and black scale {Saissetia oleae). High harvesting costs require that the trees be kept relatively low for ease of picking. After the tree's fruiting top has reached a height of about 15 to 18 feet, judicious pruning can hold it at that level. During the annual pruning make a special effort to remove or cut back upright-grow- ing branches in the center of the tree which tend to raise the height of the top fruiting surface. Even with adequate spacing, on fertile, irrigated soils where growth is vigorous, [7] . ,- * •'♦'"* ,;W Fig. 11. A dense, closely-planted orchard. The trees are set 20 feet apart. Little can be done by pruning to remedy such a situation. The best solution is to remove a portion of the trees in the orchard. the tree finally reaches a size which util- izes all the space that can be allotted to it and still maintain the fruiting shell all around the tree. When the tree grows further, it shades out the productive, easily-harvested lateral portions of the fruiting shell, and fruiting will finally exist only in the tops of the tree (fig. 11). A good system of pruning does not allow this condition to develop. As the trees reach the maximum volume that can be given to them, prune to keep them in this space and to allow sunlight to penetrate between the trees. As with other tree fruits, olive trees will be most fruitful when they are main- tained in a moderate state of vigor, neither extremely rank nor weak growing. This desirable moderate state of vigor is main- tained by such practices as irrigation, nitrogen fertilization, and pruning. Trees receiving ample amounts of nitrogen and water, if pruned excessively, may produce only strong-growing, extremely vigorous shoot growth which is not likely to be fruitful for several years or until the strong vegetative growth decreases. Pruning olive trees in special situations Pruning following top-grafting The fruiting variety in bearing olive or- chards in California is sometimes changed by top-grafting the trees. Olives are easily grafted during the spring months using the bark graft method. 1 Usually three or four primary scaffold branches in the central portion of the tree are grafted, leaving two or three lateral growing branches as nurse limbs (fig. 12). These should be kept from one to three years while the new growth from the grafted scions develops. Retaining several nurse limbs also reduces the amount of sucker growth which otherwise may become ex- cessive. In addition, the nurse branches generally bear heavily and produce crops while the varietal change is being made. 1 See Propagation of Temperate-Zone Fruit Plants, California Agr. Exp. Sta. Circ. 471 (rev.), and Grafting Olive Trees, California Agr. Ext. Serv. OAS #65. Fig. 12. Nurse branche top-grafting of olives are ; for two to three years and s retained illowed to are then re during remain moved. [8 Fig. 13. Growth from scions following top- grafting often becomes too long and may cause the graft union to break. Heading this growth back, as shown here, may prevent such breakage. Whitewash the lower portions of the nurse branches, as well as the grafted limbs and scions, as soon as the grafting is com- pleted; otherwise the exposed areas sun- burn. In two or three years, after the scions are well-established and have made con- siderable growth, the nurse limbs can be removed completely. If an insufficient number of grafts "take" the nurse limbs themselves may subsequently be grafted. Top-grafted olive trees tend to sucker heavily and it is necessary to keep these suckers removed or severely cut back with hedge shears. You may have to go over the trees several times each summer for the first two or three years. After the new top from the scions becomes sufficiently large the suckering will be considerably reduced. If several shoots start from each in- serted scion it may be advisable to rub off all but the best-placed shoot, otherwise Fig. 14. Three scions have been allowed to grow for several years on this branch follow- ing topgrafting. Later, as they become larger, one and possibly two should be removed. the new growth from the point of the graft union becomes too dense. In windy areas, however, it is best to let all growth develop to the point of crowding. The numerous branchlets help avoid breakage. The shoots from the inserted scions on grafted trees often become extremely vig- orous and may grow so tall that they tend to fall over and may break the scions off at the graft union. This breakage can be prevented by nailing strips of wood onto the grafted branch and by tying the new vigorously growing shoots onto the strips. A more inexpensive method, as shown in figure 13, is to cut off approximately the terminal one-third of these shoots when they show evidence of becoming top- heavy. This prevents them from falling over and causes additional branching lower down on the scion. Depending upon the size of the branch grafted, several scions can be retained and permitted to develop to bearing age (fig. 14). This helps heal over the grafted [9] stub more quickly than if just one scion is retained. During the following few years observe the grafts and, as they en- large and push against each other, remove those that are poorly placed, and leave only one, or perhaps, two grafts growing permanently. However, if a very large branch has been grafted it may be satis- factory to retain more scion branches. After the grafts of the new variety be- come well-established, prune the tops as described previously for other bearing trees. It is always necessary, however, to watch for and remove shoots which may arise from below the graft union; they may be overlooked and then become es- tablished again as undesirable fruiting branches. Rejuvenation pruning of old trees In some olive orchards, as the trees be- come older, the annual vegetative growth is reduced and fruitfulness decreases even with ample irrigation and adequate amounts of annual nitrogen fertilization. Frequently, the trees have developed so much in width that the lower fruiting wood becomes shaded out from adjacent trees and no longer produces fruit, the only bearing surface is then in the tops of the trees. Yields are reduced and the fruit is difficult to harvest. Under such a situation a fairly heavy "rejuvenation" pruning is advisable. Cut the upright and laterally growing main scaffold branches back to about half of their length, 8 or 10 feet from the ground, preferably to a smaller lateral branch if possible. Where an excessive number of primary scaffold branches arise from the trunk, remove the undesirable ones completely. White- wash all branches which have their foliage cover removed and become exposed to the direct sun, to prevent sunburn. Such heavy cutting back forces out considerable new growth from the older Fig. 15. Example of an old olive orchard which has not been pruned properly. A rejuvena- tion-type of pruning, including heavy cutting back of the scaffold branches, can bring this orchard back into good production. [10] branches. This new growth needs to be judiciously selected in the following few years to space the branches properly. It is necessary to remove entirely some shoots where an excessively large number arise. This new vegetative growth is un- fruitful for two or three years but after that, when the growth rate is reduced, it again produces fruit. Figure 15 shows an orchard in which a rejuvenation type of pruning would be desirable — to lower the height of the trees, and to reduce the number of scaffold branches to stimulate the production of new vegetative growth. Some olive trees — particularly the tall- growing Mission variety planted on fer- tile, deep soil with ample irrigation — grow to such heights that it is almost impossible to harvest the fruit in the tops of the trees. In such cases a drastic heading back of the trees is required, even though this greatly reduces or stops bearing until the new vegetative growth slows down and again becomes fruitful. Figure 16 illus- trates a group of Mission olive trees where this type of heading back was done. A better solution to such a problem is to not permit the trees to reach these heights. After the trees attain the maximum height at which economical harvesting can be done, all upright-growing branches above that height should be removed annually. Some olive orchards in California were originally planted with the trees set 20x20 feet apart or even closer. Up to about 10 years of age, such trees produced crops very well but as the trees grew older the lower parts became shaded out by the adjacent trees, and the only fruitful areas were in the tops of the trees. Yields dropped markedly and harvesting became very difficult. With trees planted this close together little could be done by pruning. The only solution was to remove some of the trees in the orchard. Some growers removed every other tree in each row, alternating the trees removed in adjacent rows to leave a planting distance of about 28 feet between trees. This wider spacing permitted the development of more easily- harvested fruiting wood around the sides of the trees and increased yield over that obtained in the original close planting. Pruning in relation to alternate bearing Alternate bearing in olives is apparently associated with exhaustion of nutrients and lack of production of fruiting wood in the tree during the heavy crop or "on" year. The food reserves and growth be- come insufficient to set fruit during the following or "off" year. Fruit thinning early in the season, either by hand or by sprays of naphthaleneacetic acid, is the best way to reduce excessive fruit set dur- ing the "on" year and to prevent over- bearing. It is desirable to retain a maxi- mum leaf area so the tree can manufac- ture carbohydrates while reducing the number of fruits which will utilize such carbohydrates. Attempts to accomplish this by pruning are not entirely success- ful since leaves are removed as well as fruit and the leaf-fruit ratio is not mate- rially changed as it is in fruit-thinning operations. In orchards where alternate-bearing re- curs, some benefits may be obtained by doing a portion of the pruning — thinning out of the fruiting area — just after fruit set of the "on" year. When it becomes ap- parent that the tree is going to bear a far too heavy set of fruit, some of the small branches and twigs carrying large num- bers of fruit can be cut off, using care to avoid removing any branches bearing only a few fruits. The goal here is to re- move as many fruits and as few leaves as possible. Pruning in relation to diseases Olive knot (Bacterium savastanoi E.F.S.) is a bacterial disease transmitted mechan- ically from tree to tree in various ways, mostly by pruning tools. If the pruning is done in the dry summer season, the bacteria are unlikely to become estab- lished in their new location. On the other hand, in the rainy season, when the trees are wet much of the time, bacteria moved about by pruning tools quickly resume growth in the newly cut surfaces and produce new galls there. Limbs and brush full of bacteria, pulled or dropped on healthy limbs, carry infection into the small breaks in the tissue. To prevent [in ^ppw V Fig. 16. Top. Mission olive trees about 80 years old which have grown to a height of about 50 feet. Hand-picking of fruits in the tops of such trees ob- viously would be imprac- tical. Bottom. The same trees after a drastic head- ing back. Fruit produc- tion will be curtailed for several years until new fruiting wood has de- veloped. spread of the bacteria, prune olive or- chards infected with olive knot only dur- ing the summer season. If pruning must be done in damp weather in an orchard infected with olive knot, disinfect the pruning shears when moving from one tree to the next. A sat- isfactory disinfectant is Clorox, prepared by adding one part of the concentrate to ten parts of water. Add a small amount of household detergent to facilitate wet- ting the surfaces of the pruning tools. Dipping the tools into this solution kills the olive knot bacteria and prevents its spread from tree to tree. Peacock spot {Cycloconium oleaginum Cast.) is a fungus disease controlled pri- marily by spraying the trees with 10-10- 100 Bordeaux mixture or fixed copper sprays in the fall after harvest. The or- [12] Fig. 17. Hedging and topping olive trees with the use of gang circular ganism is not spread by pruning tools. The disease thrives in heavily-shaded por- tions of the trees which are not exposed to sunlight. Peacock spot is likely to be worse in unpruned, dense, closely-planted orchards than where the trees are widely spaced and adequately pruned. Verticillium {V erticillium albo-atrum R. and B.) is a soil fungus which attacks olive trees and may kill large branches in older trees. Cut out such dead branches in the summer after you know the extent of the infection and dispose of them in the same manner as any other prunings. Branches killed by verticillium are not considered to be a source of reinfection. What to do with brush from pruning Shred or haul it out of the orchard. Do not burn the brush in the orchard. It injures the trees even at a consider- able distance from the fire. Pruning frost-damaged olive trees Following periods of cold winter weather (15° to 17° F.), excessive leaf fall, bark cracking, and occasionally death of limbs occur. Heavily pruned trees are more sus- ceptible to such cold damage than light or unpruned trees. The effect of foliage loss from freezing injury is similar to that of severe pruning. New growth appears in abundance during late spring from un- injured buds. If the trees are cut back too soon following a damaging freeze, it is likely that some limbs will be removed that may have recovered and become fruitful the following season. Experience and experimental evidence show that the best treatment for frost- damaged olive trees is to leave them un- pruned until the following June or July. By that time you can see what corrective cuts need to be made. In addition, trees infected with olive knot may be pruned with safety. The chief pruning to be done [13] ■■■/■■■■■ ■■■■:■;< - : • •:.•/ : ; rmmwm Fig. 18. L^/r. Removing brush from trees following topping and hedging by a mechanical pruner. Right. Appearance of Manzanillo olive trees one year after mechanical topping. at that time is the cutting out of dead limbs. Young olive trees are more sensitive to cold than older trees. In areas where temperatures are likely to drop to 23 °F or below, trunk damage may be prevented by wrapping them with an insulative cov- ering for protection. Machine pruning of olives Trees that have grown too large or are overcrowded, can be brought down to a satisfactory size by machine pruning, (figs. 17 and 18). Hedging and topping with gang circular saws, however, is dras- tic and unselective, and unless wisely em- ployed, may fail to produce desired re- sults. To maintain grove production, pruning alternate rows is preferable to solid block treatment. Do not cut back the unpruned trees until the pruned trees again become fruitful. Machine-pruned trees require follow- up work to remove snags and broken limbs. Use hand saws or hand-operated circular or chain saws to cut roughly-cut stubs or broken limbs smoothly to lateral limbs or across the smallest diameter. Re- move by hand dead or weak, unproduc- tive wood remaining on the inside of the tree. Following hedging and topping, a large number of watersprouts develop. If these shoots remain, many weaken and die be- cause of crowding and shading. A light thinning out after the first season encour- ages the best shoots to develop into pro- ductive wood. It is, however, a waste of time to thin them until enough leafy sur- face is produced to shade exposed limbs. [14] For more information on olive pruning: BlOLETTI, F. T. 1922. Pruning young olive trees. California Agr. Exp. Sta. Bui. 348:1-110. Hartmann, Hudson T. and Karl W. Opitz 1966. Olive production in California. California Agr. Exp. Sta. Ext. Serv. Man. 7:1-60. Hartmann, Hudson T., Karl Opitz, and R. M. Hoffman 1960. Pruning olives in California. California Agr. Exp. Sta. Bui. 771:1-30. Jacob, H. E. 1934. The effect of pruning in the training of young olive trees. California Agr. Exp. Sta. Bui. 568: 1-26. Pansiot, F. P. and Henri Rebour 1961. Improvement in olive cultivation. F.A.O. Agricultural Studies No. 50:1-249. Chap. IX. Pruning. Food and Agricultural Organization of the United Na- tions, Rome. To simplify this information, it is sometimes necessary to use trade names of products or equipment. No endorsement of named products is intended nor is criticism implied of similar products not mentioned. Co-operative Extension work in Agriculture ond Home Economics, College of Agriculture, University of California, and United States Department of Agriculture co-operating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. George B. Alcorn, Director, California Agricultural Extension Service. 15m-l,'66(F9255)J.F. ME! A FARM product; Well, not exactly — you can't grow auto- mobiles on farms, but farm products are essential in manufacturing them. Consider the annual agricultural needs of just one major automobile company. 900,000 bushels of corn 736,000 bushels of flax- seed 74,000 bales of cotton | or, in terms of approximate acreage: 15,000 acres of corn 80,000 acres of flax 78,000 acres of cotton During the same period this company used products derived from 364,000 sheep and 36,000 cattle— plus many other items such as hog bristles and beeswax. In all, produce equivalent to the output of 1,000 good-sized farms is needed yearly. No wonder a top executive in the automotive industry has said: "Our plants, here and throughout the world, would have to close their doors in a few days if their flow of agricultural materials were to stop." Supplying Americas countless industries— and feeding the nation bountifully— makes agriculture America's biggest and perhaps most important business. That is one reason why anything which affects agriculture affects everybody.