U NIVERSITY OF CALIFOBNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA PRUNING YOUNG OLIVE TREES BY FREDERIC T. BIOLETTI BULLETIN No. 348 September, 1922 UNIVERSITY OF CALIFORNIA PRESS BERKELEY 1922 David P. Barrows, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Dean. Edward J. Wickson, Horticulture (Emeritus). , Director of Resident Instruction. C. M. Haring, Veterinary Science, Director of Agricultural Experiment Station. B. H. Crocheron, Director of Agricultural Extension. C. B. Hutchison, Plant Breeding, Director of the Branch of the College of Agriculture at Davis. H. J. Webber, Sub-tropical Horticulture, Director of Citrus Experiment Station. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Ralph E. Smith, Plant Pathology. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Fruit Products. Warren T. Clarke, Agricultural Extension. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. Walter Mulford, Forestry. James T. Barrett, Plant Pathology. Fritz W. Woll, Animal Nutrition. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. L. D. Batchelor, Orchard Management. W. L. Howard, Pomology. •Frank Adams, Irrigation Investigations. C. L. Roadhouse, Dairy Industry. R. L. Adams, Farm Management. W. B. Herms, Entomology and Parasitology. John E. Dougherty, Poultry Husbandry. D. R. Hoagland, Plant Nutrition. G. H. Hart, Veterinary Science. L. J. Fletcher, Agricultural Engineering. Edwin C. Voorhies, Assistant to the Dean. DIVISION OF VITICULTURE AND FRUIT PRODUCTS F. T. Bioletti L. O. Bonnet W. V. Cruess G. Barovetto A. W. Christik A. J. Winkler J. H. Irish H. E. Jacob * In cooperation with Division of Agricultural Engineering, Bureau of Public Roads, U. S. Department of Agriculture. PRUNING YOUNG OLIVE TREES By FREDERIC T. BIOLETTI CONTENTS I. INTRODUCTION page Objects of pruning 87 Pruning and rate of development 89 II. INVESTIGATIONS The experiments 91 1. The trees 92 2. Pruning at planting 92 3. Pruning the first year 92 4. Pruning the second year 94 5. Pruning the third year 94 6. Pruning the fourth year 94 7. Pruning the fifth year 94 The Results 96 1. Increase of circumference of trunk 98 2. Increase of height of tree 101 3. Increase of bulk of tree 101 4. Attainment of desired form 102 5. Bearing 103 III. DEDUCTIONS Pruning a deterrent of growth 105 Pruning a deterrent of bearing 105 Minimization of pruning the ideal 105 VI. APPLICATION Development of young trees 106 Maintenance of bearing trees 106 V. SUMMARY AND CONCLUSIONS I. INTRODUCTION OBJECTS OF PRUNING The pruning of an olive tree has two purposes : one, to give it the form that is deemed best for cultivation, harvesting, and other orchard operations ; the other, to regulate the size and the quality of the crop. In pruning a young olive tree before it commences to bear, only the first of these purposes is considered. The object should be to give the tree the desired form in the shortest time, and to do this as econom- ically as possible. 88 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The form desired must be clearly in the primer's mind and before he can hope to accomplish his object he must understand the nature of the tree and how it will react to each operation. The form almost universally accepted as the best is the open vase form. This consists of a single trunk sufficiently high (2 to 3 feet) to permit of close cultivation, with 3 to 5 main branches rising from near the top of this trunk at an angle of about 60° to a height of 6 or 7 feet and then gradually curving outward until they become hori- zontal or even slightly pendant. These main branches divide repeat- edly into secondarjr and tertiary branches and so on until they reach the final division called twigs, which bear the leaves and the 'fruit. A fully formed tree of this kind will be a hollow inverted cone com- posed of branches of various sizes upon which the bearing twigs form an open network. The interior of the cone, while free from large branches, should be well supplied with small branches and bearing twigs — in fact, all parts of the tree should be well supplied with healthy bearing twigs. This can be accomplished only by a tree of this general, open vase form, which allows the sunlight to reach all parts. While there is not much difference of opinion as to the best form of tree, there is much difference of both opinion and practice regard- ing the best method of obtaining this form. European writers describe and advise very painstaking and detailed methods of pruning the young tree several times during the 3 r ear, with the object of forcing it to take the desired shape. Much of this work is done in the nursery and the trees are usually 4 or 5, or even 8 or 10 years old, before they are planted in the orchard. Calif ornian writers and many growers adopt similar methods though there is usually little pruning in the nursery and the trees are usually only 2 to 3 years old when planted out. The method commonly adopted by careful growers in California is, each year, to head back, more or less, every branch of young olives, with the object of forming a solid, stocky framework which will hold a heavy crop and resist wind damage. In heading back, the cut is made just above a side branchlet. The branchlet chosen is one which points in the direction in which it is desired to have the branch grow. On upright growers, like Mission, an outer branchlet is left ; on spread- ing growers like Manzanillo, an inner branchlet. Unfortunately the tree seems to resent this interference with its natural inclinations. In both cases the strongest branches grow in 1 1 io direction the tree prefers, so that it is usually a weak branch which the pruner must choose to leave. The removal of the strong inner > 48 ] PRUNING YOUNG OLIVE TREES 89 branch, in the case of Mission, or of the strong outer branch, in the case of Manzanillo, creates the open space well supplied with sunlight that the pruner desires. But during the following growing season, this space, which it is desired to keep vacant, is filled up again by a new vigorous shoot. This new shoot grows more strongly than the desired shoot which was left, because it is following its preferred direction. At the next pruning a similar attempt is made by the pruner. He removes the new vigorous shoot and retains the older and weaker one. In time, by very severe and constant pruning, this method often results in a fairly well shaped tree. PRUNING AND RATE OF DEVELOPMENT There is some difference of opinion among horticulturists, both practical and theoretical, as to whether pruning increases vigor or, as it is sometimes expressed, ' ' devitalizes the tree. ' ' And it seems worth while to inquire whether this severe pruning does not stunt or at least delay the development of the young olive. L. H. Bailey 1 states that pruning, even when somewhat heroic, is not devitalizing and supports this opinion with various a priori and observational arguments. Babo 2 states that investigations at the Geisenheim School of Horti- culture have shown that unpruned vines much surpass those pruned annually, in circumference of trunk and especially in size of root sys- tem. He also gives many examples of pruning practices in various parts of Europe to show that vines which are pruned lightly and allowed to grow somewhat in accord with their natural habit are more vigorous and more resistant to disease and injuries than vines pruned heavily in the usual way. P. Pacottet 3 states that a pruned vine has certainly a shorter dura- tion of life than one not pruned. Recently much convincing evidence has been supplied by the researches of J. C. Whitten and others to show that pruning is essen- tially a "devitalizing" process, if by this term we mean that it cur- tails the activities of the plant as manifested by rapidity of growth and early bearing. Most or all of this evidence is based on work done with deciduous trees. It remains to be seen whether the pruning of trees whose leaves last more than a year and which are therefore never completely bare has similar effects. i The Pruning Boo};, ed. 2, 1899, pp. 5-12; ed. 18, 1916, pp. 6-14. 2 Babo and Mach, Weiiibau und Kellerwirschaft, ed. 3, 1910, pp. 625. 3 P. Pacottet, Viticulture, 1905, p. 193. 90 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION G. Foex 4 states that the activity of a plant or of a branch, other conditions being equal, is the greater the larger the number of leaves it bears. This statement seems to be based on a correct view of the principles of plant physiology involved. To be more exact, perhaps it should include also the time during which the leaves function. If we accept this modification of the statement of Foex we should expect that the weakening or retarding effect of pruning would be greater on evergreen than on deciduous trees. When we prune a deciduous tree we simply remove a certain part of the buds, of which Bailey estimates that only 20 per cent would grow if no pruning were done. The actual number of buds which grow in each case may be expected to be approximately the same, also the resulting shoots and leaf surface, and consequent^ the growth and vigor of the tree during the ensuing season, 5 if no other factor is concerned than the number of buds which develop. When we prune an evergreen tree, on the other hand, we remove leaves which are actually adding to the vigor and bulk of the tree or which are ready to function as soon as the temperature is sufficiently high. This places a great handicap on the pruned trees which, however many new shoots they may produce, will require weeks or months to develop a leaf surface equal to that with which the unpruned trees commence. These theoretical considerations are supported by observations on the growth of young olive trees under the various systems of pruning. Nursery trees given the elaborate and continuous pruning practiced in Europe are several years longer in reaching the size attained by trees grown in California in nurseries where they receive little or no pruning. Olive trees planted as wind breaks on the border of orchards of other trees are often left without pruning and their quick develop- ment has been noted. In spite of all the evidence tending to show that pruning depresses the vigor of plants of all kinds, the belief in an invigorating effect of pruning is very general among both growers and horticultural writers and is, in its turn, supported by much evidence. The renovation of old orchards by severe cutting back ; the removal of all buds but one on a young vine to insure a long vigorous shoot; the severe pruning of weak vines and the light pruning of strong vines are all common and accepted practices. On the other hand, the weakening and even the death of fruit trees and vines following failure to prune are fre- quently observed. * Cours complet de viticulture, ed. 4, 1895, p. 379. 5 That dormant pruning of decidous trees has a retarding effect has been shown by J. C. Whitten and others, but this does not affect the argument. Any other factors which enter into the problem may be expected to occur in the case of ever- green trees, also. Bulletin 348] PRUNING YOUNG OLIVE TREES 91 This paradox is easily explained by two considerations. One is that the activity and the amount of growth of individual shoots does not necessarily indicate a corresponding vigor of the whole plant. A small number of vigorous shoots may represent less total activity in the plant than a larger number of less vigorous shoots. The other is that pruning when sufficient^ severe not only decreases the number of vegetative, leafy shoots produced, but decreases the amount of fruit. The production of a large crop may absorb so much of the energies of the plant that the amount of leaf surface produced where the pruning is light may be less than where it is heavy. Moreover, the development of the fruit may also absorb so much of the nutritive material elabor- ated by the leaves that the plant is starved. During the formative or pre-fruiting period of the life of a tree the complication of crop production does not enter and the problem is much simplified. In order to devise a suitable method of pruning young olive trees, the specific effect of the severity of pruning on vigor, shape, and rate of development uncomplicated by fruit-bearing or other disturbing causes must be known. To determine this an investi- gation was commenced at Davis in 1917 and has yielded interesting results which point the way to a method of pruning which seems more effective and more economical than any now in general use. II. INVESTIGATIONS THE EXPERIMENTS In April, 1917, three rows of olive trees were planted at the Uni- versity Farm, Davis, on a piece of deep, level loam soil from which an old Muscat vineyard had been removed one or two years before. The rows are 36 feet apart and the trees 12 feet apart in the rows. (See Fig. 1.) A row of very large Black Walnut trees is growing 48 feet from the north end of the rows and has had a depressing effect on the first two or three trees of each row. ** UDPING _ % I II l O 1/ I O ll I Q II | II I O ll I |i I j| I II I II / O i\ I O It I _j MISSION ^ 3 • o a i o ii i o it i o ii i a ii i o ii i o ii i o H » o it i o if i o ii i o \i i ° 5 MIHION I • L^ I O l| I O II I O II I O II I O II I O II I G II I O ll l O II ( O II I O II 1 II - "■ Ml S S I N *~ -* S __.f- ** ASCOLANO Fig. 1. — The Experiment Plot. 92 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION The previous preparation and the annual tillage of the plot have been fair. During the first year an irrigated intercrop of corn was grown. In the spring of the second year two rows of vines were planted between the rows of olive trees. Since the second year the rainfall has been deficient and the trees have received very little irrigation. The trees on the whole have done well though the combined effects of close planting, intercrops, and deficient moisture have resulted in less growth than the favorable conditions of soil, temperature, and location are capable of producing. Difficulties of cultivation have also resulted in mechanical injuries to about 15 per cent of the trees which have more or less limited their growth. This probably accounts for the abnormally small growth of a few trees in each lot. The trees were subjected to a heavy frost in the nursery during the winter before they were transplanted. This may explain the dying of 8 of the 108 trees planted. 1. The Trees. — The trees used were Mission, grafted on Redding seedlings. The seedlings were grown in a greenhouse at Berkeley in 1913, planted in the nursery at Davis in 1914, and grafted in the nursery in 1915. The grafted trees when planted in the experimental plot were 2 years old from the graft and 4 years old from the seed. 2. Pruning at planting. — The trees were dug out of the nursery by hand and the roots shortened to a few inches. The tops were then cut back to from 20 to 24 inches, according to their size, and all leaves removed. 3. Pruning the first year. — In the winter after the first growing season (1917), before new growth had started in the spring, the trees were pruned. Every third tree commencing with tree 1 (see Fig. 1) in each row was pruned heavily. The pruning consisted in removing all twigs and branches on the trunk except 2, 3, or 4 of the largest near the top. The branches reserved were then cut back to within 3 to 6 inches of the trunk. (See three trees at top of Fig. 2.) This is the method commonly used by most careful primers. Such heavy cutting is, how- ever, becoming less common, though severe shortening of the reserved branches is still generally practiced. On the remaining trees all new growth to within a few inches of the top of the trunk was also removed entirely ; but all the remaining branches and twigs were reserved and not pruned in any way. (See two trees at bottom of Fig. 2.) Bulletin 348] PRUNING YOUNG OLIVE TREES 93 Fig. 2. — First winter. Typical trees, January, 1918, nine months after plant- ing. Top row heavily pruned; bottom no pruning. 94: UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 4. Pruning the second year. — During the second growing season (1918) every third tree commencing with tree 3 in each row was lightly summer-pruned. This pruning consisted in topping the more vigorous shoots (Fig. 3, 2-12, 2-18, 3-15). No other summer pruning was done except to remove *all shoots which started on the trunk below the branches on all trees. In this way the trunks were kept clear of growth. Typical trees were photographed on May 29, four months after the winter pruning. Fig. 3 shows that a depressing effect of heavy prun- ing is already evident after active growth for a few weeks. The pruned trees have not only a smaller number of growing tips, but the growth of the individual twigs is much shorter than on the unpruned trees. This can be seen plainly by comparing the heavily pruned trees, 2-10, 2-16, and 3-13, with the unpruned, 2-12 and 2-18, which appear in both figures 2 and 3. At the end of the second growing season the same treatment was continued. The series commencing with tree 1 was heavily winter pruned and no pruning given to the others. (See Fig. 4.) 5. Pruning the third year. — The treatment during the third year was the same as during the second, except that in winter an occasional vigorous spreading branch low down was removed on the trees which were to be pruned as little as possible. The extent of this pruning is shown by the fact that from 24 heavily pruned trees the branches removed weighed 13.25 pounds, while from 24 unpruned trees only .5 pound was removed in all. On May 12, 1920, 25 months after plant- ing, the circumference of each tree at about 25 cm. from the ground was measured. The circumference of the heavily winter-pruned trees was only 63 per cent of that of the unpruned trees and of the summer- pruned trees only 87 per cent. 6. Pruning the fourth year. — Since nothing seemed to have been gained by summer pruning, it was decided at the end of the third year to omit it in the future and to prune heavily the trees of the series beginning with No. 3 in the same way as the series beginning with tree 1. All other forms of treatment were identical for all trees and the same as during previous years. 7. Fifth year. — During the fifth year the methods of the fourth year were continued. The trees were measured on August 15, 1921, after the growth of the fifth year was almost completed. These meas- urements show that the circumference of trees heavily winter-pruned at the end of the first, second, third, and fourth growing seasons was only 49 per cent of that of the trees which were not pruned at all; Bulletin 348] PRUNING YOUNG OLIVE TREES 95 Fig. 3. — Second growing season. Typical trees, May 29, 1918, at 13 months. Left column heavily winter pruned. Middle and right columns, no pruning. 96 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION and that the circumference of trees summer-pruned during the first and second growing seasons and heavily pruned at the end of the third and fourth seasons was only 70 per cent. The depressing effect of heavy pruning is well shown by Fig. 6. The difference in size, i.e., total bulk, of the two trees shown is esti- mated to be about as 100 :10. Fig. 4. — Second winter. Typical trees, January 4, 1919, at 21 months. Tree to left never pruned. Tree to right shows second pruning. THE RESULTS The experiments included three series of 36 trees each, represent- ing different degrees of pruning or amounts of removal of annual growth. Series 1. No pruning. (N.) Series 2. Summer pruning in the first and second growing sea- sons and heavy winter pruning at the end of the third and fourth growing seasons. (P.) Series 3. Heavy winter pruning at the end of the first, second, third, and fourth growing seasons. (PP.) Bulletin 348] PRUNING YOUNG OLIVE TREES 97 Fig. 5. — Second, third and fourth winters. Trees to right: top before third, bottom before fourth pruning. Trees to left: top second, middle third, bottom fourth winter pruning. 98 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The measurements taken were : The trunk circumference of four typical trees at the end of the first growing season, when all trees had received the same treatment. (Table 1.) The trunk circumference of all trees at the end of 25 months and 40 months after planting. (Table 2.) The height of all trees at 40 months. (Table 4.) 1. Increase of circumference of trunk. — At the end of the first growing season the average size of the trees was estimated from the measurements shown in Table 1. Fig. 6. — Result of four winter prunings of young trees. A. Never pruned. B. Pruned yearly. TABLE 1 Size of Trunk at Thirteen Months after Planting (Circumference in mm. at 25 cm. from ground.) Row 2, tree 10 Row 2, tree. 16 Row ?,, tree 13 Row 4, tree 16 PP N 52 Row 2, tree 16 45 38 Row 2, tree 17 38 49 Row 3, tree 14 38 19 Row 4, tree 11 31 Row 2, tree 12 41 Row 2, tree 18 45 Row 3, tree 15 66 Row 4, tree 13 29 Mean 39.5 38.0 Mean circumference of all trees a1 13 months, 41 mm. 45.3 Bulletin 318] PRUNING YOUNG OLIVE TREES 99 TABLE 2 Size of Trunk at Twenty-five Months and at Forty Months Circumference of trunk in mm. at 25 cm. from the ground. Tree Twenty-five months ' PP N P ' Row 2 — 1, 2, 3 85 106 77 4, 5, 6 90 103 7, 8, 9 73 118 100 10, 11, 12 97 159 133 13, 11, 15 81 155 116 16, 17, 18 61 112 86 19, 20, 21 94 70 149 22. 23, 24 88 87 144 25. 23, 27 83 156 99 28, 29, 30 68 173 176 31, 32, 33 89 123 85 34. 35. 36 94 130 146 Row 3 — 1, 2, 3 80 101 105 4. 5, 6 89 120 105 7, 8, 9 94 172 117 10. n. 12 121 189 13. 14, 15 105 203 161 13, 17, 18 94 132 110 19. 20, 21 117 187 127 22. 23, 24 109 110 182 25. 26, 27 124 180 190 28, 29, 30 118 145 139 31, 32, 33 84 177 136 34, 35, 3d 106 136 109 Row 4 — 1, 2, 3 "j 96 85 4, 5, 6 80 120 36 7, 8. 9 50 146 59 10, 11, 12 56 172 129 13, 14, 15 93 140 16, 17, 18 68 87 19, 20, 21 32 71 22, 23, 24. 44 71 120 25, 26, 27 94 136 153 28, 29, 30 .... 72 31, 32, 33 58 144 126 34, 35, 36 56 70 I i'orty months A PP N P 267 368 158 267 394 241 495 343 318 749 508 318 749 406 251 495 267 368 343 546 394 368 495 318 648 381 229 851 635 254 533 330 318 648 572 216 343 368 279 495 205 267 749 406 368 838 394 953 609 368 699 406 409 813 406 394 406 648 483 635 559 381 559 508 292 826 521 381 648 343 178 381 279 241 508 190 165 737 203 216 851 495 394 685 205 381 124 216 178 470 483 368 584 205 584 229 660 483 216 368 Mean of all trees 84 134 117 295 596 417 PP =: Trees heavily pruned each year. P = Trees lightly pruned tAvo years, heavily two years. N = Trees not pruned. 3 4 (40mo.)t 40 mo.* (437) 1429 (357) 1017 (351) 720 100 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION The extreme variation between trees at the beginning was there- fore about as 100 :29. At the end of the experiment the extreme vari- ation was as 100:13. (See table 2.) The extreme variation shown here between series at the beginning is as 100:84. At the end of the experiment it was as 100:49. (See Table 2.) The differences found between the series can therefore be ascribed to the difference in pruning. TABLE 3 Increase of Trunk Calculated from Tables 1 and 2 1 2 13 mo. 25 mo.* N— Trees not pruned 100 327 P— Trees lightly pruned 100 285 PP— Trees heavily pruned 100 205 * Average circumference, 41 mm. at 13 months = 100. t Average circumference at 25 months = 100. Table 3 shows that heavy winter pruning diminished the increase in circumference of trunk 37 per cent during the first two years (PP, col. 2) and 20 per cent during the second two years (PP, col. 3), or a total retardation of 50 per cent for the four years (PP, col. 4). Light summer pruning diminished the increase of circumference 13 per cent during the first two years (P, col. 2). This does not indi- cate that summer pruning is less depressing than winter pruning. The summer pruning was very light and the winter pruning heavy. The difference therefore indicates that the retardation bears a direct rela- tion to the amount of foliage removed. Heavy winter pruning during the second two years of trees not winter pruned during the first two (P, col. 3) diminished the increase in circumference 18 per cent. This corroborates the result with the series heavily pruned every winter, which was 20 per cent. The retardation due to pruning during the first two years was greater than during the last two. As a greater proportion of the leaves was removed during the earlier years (see Figs. 2 and 4), a direct relation of retardation to the amount of leaves removed is again indicated. The large amount of foliage removed by a heavy winter pruning is shown by Fig. 5. In pruning the trees shown in this figure the part of the foliage left was 8.6 per cent on the two-year-old trees, 16 per cent on the three-year-old, and 10 per cent on the four-year-old trees. Bulletin 348] pruning YOUNG OLIVE TREES 101 2. Increase of height of tree. — The height of all the trees of the three series was measured at 40 months. The height was taken from the ground to the top of the highest branches omitting single abnor- mally long twigs. TABLE 4 Height of Trees at Forty Months Average height in cm. and compared to that of unpruned trees taken as 100. N — Trees unpruned 433 cm. = 100.0 per cent P — Trees lightly pruned 206 cm. = 47.6 per cent PP — Trees heavily pruned 154 cm. = 35.6 per cent Heavy pruning every year retarded increase of height 64.4 per cent (PP) and during the third and fourth years,* 52.4 per cent (P). The retardation of height was therefore about twice the retardation in circumference of trunk. This is opposed to the opinion of many pruners who believe that pruning (cutting back) of the tops of olive trees has a tendency to increase growth in the tops and thus increase the height. The explanation is probably that we are here dealing with non-bearing trees. With bearing trees the fruit borne on the top of the tree may depress growth more than the removal of leaves. 3. Increase of hulk of tree. — A comparison of the height of the trees or of the circumference of the trunk gives an inadequate idea of the difference in size of the trees. The areas of cross-sections of the trunks vary as the squares of the circumferences, and these areas multiplied by the corresponding heights should give a fair approximation of the relative sizes or bulks of the trees. This is the method by which the relative bulk of the trees of the three series has been calculated for Table 5. TABLE 5 Comparative Bulk of Trees at Forty Months (Circumference squared, multiplied by height; average of unpruned trees taken as 100) Max. Min. Mean N— Trees not pruned 209 4 100 P — Trees lightly pruned 70 2 24 PP — Trees heavily pruned 311 9 *Some of this retardation was probably due to the summer pruning of the first two years. 102 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The retarding effect of severe winter pruning shows very clearly in this table. Four primings reduced the average growth to one- tenth of that of an unpruned tree, and two primings to one-quarter. The trees which made the smallest growth — middle column — were undoubtedly dwarfed by mechanical injuries. The trees which were unpruned made a more even growth than the pruned trees. The difference between the largest and the average, is with the unpruned trees as 100 : 48, with the trees pruned twice, as 100 : 34, and with the trees primed four times as 100 : 29. 4. Attainment of desired form. — The evidence shows that heavy pruning restricts the growth of olive trees very greatly during the first five years. It indicates also that all degrees of pruning are restrictive in proportion to their severity. It remains to be seen whether the object of pruning young olive trees — the attainment of a desired form — can be accomplished without pruning or to what degree pruning can be reduced without preventing the attainment of this object. An inspection of the trees at 4% years showed that nearly all in all three series had main branches growing at the desired angle of 60°. Those of the unpruned trees, however, were straighter and more regu- lar. The cause is indicated in figures 3, 4, and 5. In figure 3 the unpruned trees commencing their second year's growth are developing main branches at angles varying from 40° to 60°. In figure 4 the unpruned tree at the end of its second year's growth has developed four or five main branches at almost exactly 60°. In figure 6 an unpruned tree after its fifth year's growth has also its complement of main branches at 60°. Unpruned Mission olives, therefore, under the conditions of the experiment, naturally develop main branches at the desired angle. The branches tend to be too upright at first (Fig. 2), but gradually spread (Fig. 3) until they reach close to the desired 60° (Fig. 4). The cause of this spreading is probably the interference of the central growth and the weight of the branches themselves. As the trees become older and larger, the lower part of these main branches becomes thick and rigid, and they retain the position reached by the second or third year. In the pruned trees the usual procedure was adopted of cutting out the central twigs and branches to make an open center, and leaving the branches or twigs which were growing most nearly at the required anerle of 60°. Bulletin 348] PRUNING YOUNG OLIVE TREES 103 The defects of this method are shown in figures 2 and 5. At the first pruning (Fig. 2) the two to four branches left were all at just about the required angle. During the following summer the dense growth in the middle of the tree forced the shoots from the branches to extend too horizontally. Later in the season the central shoots grew more vigorously than the spreading shoots. At the second pruning (Fig. 5, upper tree), the strong growing shoots in the middle of the tree were removed and the weaker, spread- ing shoots retained and cut back to twigs growing most nearly in the desired direction. In this way the center was made open and the twigs left were approximately at the required angle. This method gave the young tree the required shape, but it sacrificed the most vigor- ous part of the tree, the strong central shoots. The next two summers and the next two primings (Fig. 5, middle and bottom trees) the same course was followed. The result was finally that main branches were built up in approximately the required position. But these branches were crooked, irregular and in every way less perfect than the branches on the unpruned trees. (See Fig. 6 b.) The only apparent defect of form of the unpruned trees was a dense center. (See Fig. 6 a.) This dense growth, however, up to the age of five years has been a great advantage. The unpruned tree (a) of figure 6 has about ten times the amount of foliage that the pruned tree (b) has developed. The result is that it is ten times as vigorous and ten times as capable of bearing crop. The leaves in the center of the tree were still green and healthy. The interior of the unpruned trees was undoubtedly too shady for the best production of fruit but the trees were not yet of bearing size. 5. Bearing. — Olives in central California rarely bear until they are four years old and paying crops are usually not obtained until the sixth or seventh year after planting. The conditions at the experiment plot are not favorable for early bearing. At the end of the fourth season a few olives were seen on an occa- sional tree. At the end of the fifth enough were borne to indicate the most precocious trees. Of the 33 unpruned trees only 5 showed no fruit and the remain- ing 28 had about 70 pounds in all, varying from a few ounces to 6 pounds to the tree. Not one olive was found on any of the pruned trees. 104 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION In May of the sixth year, 61 months after planting, an estimate of the amount of bloom on each tree was made. The trees were classified as follows : = No bloom or only a trace. 1 = Light bloom. 2 = Medium amount of bloom. 3 = Full bloom. 4 = Heavy bloom. Number of Trees with Various Degrees of Bloom Bloom o N — No pruning 2 P — Light pruning 26 PP — Heavy pruning 33 Nearly all the unpruned trees showed bloom irrespective of their size. Most of the pruned trees showed no bloom and none showed more than a small amount. The relative fertility of the three types of trees may be expressed in percentages about as follows : N — No pruning, blossoming 100 P — Light pruning, blossoming 11 PP — Heavy pruning, blossoming 4 The influence of pruning on the fertility (blossoming) of young olive trees is thus seen to be of the same order as its influence on vigor (bulk). 1 2 3 4 7 10 15 1 8 9 3 Bulletin 348] PRUNING YOUNG OLIVE TREES 105 III. DEDUCTIONS From these experiments it is safe to formulate some general prin- ciples. These principles, which indicate that some, if not most, of our pruning practices as applied to young olives are erroneous, are a valuable guide in devising better methods of accomplishing the object* of pruning. PRUNING A DETERRENT OF GROWTH That both summer and winter pruning retarded the development of young olive trees has been thoroughly demonstrated. Whether this retardation is due only to the removal of leaves or also to the removal of woody parts of the tree is not shown. It seems reasonable to conclude that both factors concur though the loss of leaves is prob- ably the most important. PRUNING A DETERRENT OF BEARING That pruning young olive trees also postpones bearing is evident. Pruning, by dwarfing the trees, makes them less capable of supporfing a large crop. That none of the pruned trees produced any fruit at the end of the fifth growing season while nearly all of the unpruned trees pro- duced a little should not be given much weight, because, as the bear- ing wood is produced the previous year, and the heavy pruning re- moved nearly all the fruit buds all possibility of bearing was destroyed. However, that heavy pruning had prevented the production of fruit buds was demonstrated during the sixth year. At the end of the fifth year all pruning was omitted on all the trees, but, in spite of this, practically no blossoms were borne during the sixth spring by most of the trees which had been pruned during the first four winters, while nearly all the trees which had never been pruned were well loaded with blossoms. MINIMIZATION OF PRUNING THE IDEAL Even though we are convinced that the specific effect of pruning is to retard growth and bearing, it does not follow that pruning of all kinds and degrees should be abandoned. There are certain objects of the grower, to compass which no means has been found except prun- ing. We should simply recognize this specific effect and try to accom- plish our object with as little pruning or removal of leaves and other parts of the tree as possible. 106 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION IV. APPLICATION It has been shown that under the conditions of the experiments a young olive will develop rapidly into a tree of bearing size and nearly perfect shape without any pruning during the first four years, except the removal of shoots on the lower part of the trunk during the first two years. The conditions, however, were very favorable. The trees used were of the Mission variety which naturally takes the form desired, and the plot was well protected from wind. Would this method succeed with young trees of other varieties and in other weather conditions f Also what form of pruning should be adopted after the tree has reached the size of the tree shown in Fig. 6a when it can be expected to begin to yield paying crops ? DEVELOPMENT OF YOUNG TREES In situations where the trees are subjected to heavy prevailing winds it would doubtless be necessary to support them with a stout 7 or 8-foot stake during the first two or three growing seasons. It might be advisable also to shorten the branches on the leeward side to encourage them to grow more upright and to throw more growth into the branches on the windward side. With spreading trees, such as the Manzanillo and Ascolano, the main branches tend to grow horizontal. This can be overcome by tying these horizontal branches to each other by a stout cord or band of burlap passing through the center of the tree. A stake would also be necessary in this case. After a branch has been held in the right position in this way for one year, it becomes rigid and will maintain the angle given. This tying should commence at the end of the first growing season and should be continued for one or two years more. If neglected until the branches are over half an inch thick their angle cannot be changed without danger of breaking. MAINTENANCE OF BEARING TREES When a tree reaches the size and form shown in Figures 6 a and 7 a it is ready to be treated as a bearing tree. As a bearing tree it has too dense a mass of foliage and lacks the open center which its desirable. Figure 7 shows how such a tree can be very easily given the required form. Bulletin 348] PRUNING YOUNG OLIVE TREES 107 — £T -: •-J 3 E = p - ca ^ -» P X kd C+" — T3 J^ -i o s C* K i-ts p c+ p 35 < cr P 3 p 75 rs © ■ J> 2 2 ""^ X •d X I o QTQ o X p 3 © P • ►■* X no OQ • p 5 w -< 3 © ID <5 P - Hs >-i ~ C -t © © r*- 3 a ►d ►-J a 3 a 3 © o 3 -i OQ O X 3 3 O r* P O B o Ha © © © B- 3 P 3 h- P a w a 108 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION First, one or more of the central branches are removed entirely at their origin on the trunk or on the base of another large branch, leaving a hollow center and a ring of branches rising at about the desired angle of 60°. Usually this ring of branches will be too dense and some of them should be removed in the same way, taking care to have the reserved branches as equally spaced as possible. All this work is done with a good sharp pruning saw and requires from 5 to 10 minutes. The tree has then the form shown in figure 7 b. On certain parts of the tree after the saw-pruning there will be too dense a mass of twigs and foliage. This must be thinned out with a pair of small one-hand shears. The densest parts of the tree will usually be low down, from near the ground to the height of the shoulder. The thinning out does not require very precise work. It should consist in "cutting out" entirely twigs and small branchlets, not in "topping" or "cutting back." Branchlets which reach the ground should first be removed and then enough of the rest to let daylight through into all parts of the tree. Little or no thinning should be done on the upper part of the tree. The removal of main branches with the saw usually opens up the top sufficiently. This thinning will take two or three times as long as the saw work. When finished the tree should have the appearance shown in figure 7 c. The age at which the first pruning should be done will depend on the rapidity with which the tree develops. Figure 8 shows a border tree, six years old, which has just reached the stage of development at which the first pruning appears advisable. If the pruning is done too early the development of the tree is retarded. If it is done too late the wounds made in opening up the interior will be unnecessarily large and the bearing twigs in the interior will commence to weaken through lack of sunlight. The correct stage seems to be when the tree is large enough to produce a paying crop without danger of overtaxing its energies. This is a matter of judgment based on experience. This stage will be reached in well-handled orchards at the fourth, fifth, or sixth year, according to the soil, climate and variety. Bulletin 348] PRUNING YOUNG OLIVE TREES 109 V. SUMMARY AND CONCLUSIONS 1. Pruning retards the growth of olive trees, especially when they are young. The excessive pruning often practiced on young trees may retard their development 90 per cent. Fig. 8. — First pruning at six years. A. Mission tree six years old in a row of border trees — never pruned. B. Same tree after first pruning at six years. 2. Mission olives which receive no pruning during the first four or five years develop a more perfect form than pruned trees. 3. The method of handling young Mission olives recommended on the basis of five years' experiments hastens the development of the trees, improves their form, promotes early bearing, and saves the whole expense of pruning for four or five years. 4. The method is probably applicable to any varieties of naturally upright habit. 110 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 5. The use of stakes and tying may be required during the first two or three years with varieties of spreading habit like the Manza- nillo and Ascolano. This expense would be offset by the saving in pruning and the improved form of the tree, and the more rapid devel- opment would be a net gain. ACKNOWLEDGMENTS The planting and pruning and much of the note-taking in the experiments were done by Messrs. F. Flossfeder and G. Barovetto, without whose efficient cooperation the work could not have been done. Valuable assistance was also rendered by Messrs. W. F. Oglesby, L. 0. Bonnet and A. J. Winkler. STATION PUBLICATIONS AVAILABLE FOR FREE DISTRIBUTION BULLETINS No. 251. Utilization of the Nitrogen and Organic Matter in Septic and Imhoff Tank Sludges. . 253. Irrigation and Soil Conditions in the Sierra Nevada Foothills, California. 261. Melaxuma of the Walnut, "Juglans regia." 262. Citrus Diseases of Florida and Cuba Compared with Those of California. 263. Size Grades for Ripe Olives. 267. Experiments with Stocks for Citrus. 268. Growing and Grafting Olive Seedlings. 270. A Comparison of Annual Cropping, Bi- ennial Cropping, and Green Manures on the Yield of Wheat. 273. Preliminary Report on Kearney Vine- yard Experimental Drain. 275. The Cultivation of Belladonna in Cali- fornia. 276. The Pomegranate. 278. Grain Sorghums. 279. Irrigation of Rice in California. 280. Irrigation of Alfalfa in the Sacramento Valley. 282. Trials with California Silage Crops for Dairy Cows. 283. The Olive Insects of California. 285. The Milk Goat in California. 286. Commercial Fertilizers. 287. Vinegar from Waste Fruits. 294. Bean Culture in California. 297. The Almond in California. 298. Seedless Raisin Grapes. 299. The Use of Lumber on California Farms. 304. A Study on the Effects of Freezes on Citrus in California. 308. I. Fumigation with Liquid Hydrocyanic Acid. II. Physical and Chemical Prop- erties of Liquid Hydrocyanic Acid. No. 309. 310. 312. 313. 316. 317. 320. 321. 323. 324. 325. 330. 331. 332. 334. 335. 336. 337. 339. 340. 341. 342. 343. 344. 347. 348, I. The Carob in California. II. Nutri- tive Value of the Carob Bean. Plum Pollination. Mariout Barley. Pruning Young Deciduous Fruit Trees. The Kaki or Oriental Persimmon. Selections of Stocks in Citrus Propa- gation. Control of the Coyote in California. Commercial Production of Grape Syrup. Heavy vs. Light Grain Feeding for Dairy Cows. Storage of Perishable Fruit at Freezing Temperatures. Rice Irrigation Measurements and Ex- periments in Sacramento Valley, 1914-1919. Dehydration of Fruits. Phylloxera-Resistant Stocks. Walnut Culture in California. Preliminary Volume Tables for Second- Growth Redwoods. Cocoanut Meal as a Feed for Dairy Cows and Other Livestock. The Preparation of Nicotine Dust as an Insecticide. Some Factors of Dehydrater Efficiency. The Relative Cost of Making Logs from Small and Large Timber. Control of the Pocket Gopher in Cali- fornia. Studies on Irrigation of Citrus Groves. Hog Feeding Experiments. Cheese Pests and Their Control. Cold Storage as an Aid to the Market- ing of Plums. The Control of Red Spiders in Decidu- ous Orchards. Pruning Young Olive Trees. CIRCULARS No. 70. 82. 87. 110. 111. 113. 115. 126. 127. 129. 138. 144. 148. 151. 152. 155. 157. 159. 161. 164. 165. 166 167 169 170 No. Observations on the Status of Corn 172. Growing in California. 173. The Common Ground Squirrels of Cali- fornia. 174. Alfalfa. 175. Green Manuring in California. The Use of Lime and Gypsum on Cali- 178. fornia Soils. 179. Correspondence Courses in Agriculture. Grafting Vinifera Vineyards. 181. Spraying for the Grape Leaf Hopper. House Fumigation. 182. The Control of Citrus Insects. The Silo in California Agriculture. 183. Oidium or Powdery Mildew of the Vine. 184. "Lungworms." 188. Feeding and Management of Hogs. 189. Some Observations on the Bulk Hand- 190. ling of Grain in California. 193. Bovine Tuberculosis. 198. Control of the Pear Scab. 201. Agriculture in the Imperial Valley. 202. Potatoes in California. Small Fruit Culture in California. 203. Fundamentals of Sugar Beet Culture 205. under California Conditions. 206. The County Farm Bureau. 208. Feeding Stuffs of Minor Importance. The 1918 Grain Crop. 209. Fertilizing California Soils for the 1918 210. Crop. 212. Wheat Culture. The Construction of the Wood-Hoop Silo. Farm Drainage Methods. Progress Report on the Marketing and Distribution of Milk. The Packing of Apples in California. Factors of Importance in Producing Milk of Low Bacterial Count. Control of the California Ground Squirrel. Extending the Area of Irrigated Wheat in California for 1918. Infectious Abortion in Cows. A Flock of Sheep on the Farm. Lambing Sheds. Winter Forage Crops. Agriculture Clubs in California. A Study of Farm Labor in California. Syrup from Sweet Sorghum. Helpful Hints to Hog Raisers. County Organizations for Rural Fire Control. Peat as a Manure Substitute. Blackleg. Jack Cheese. Summary of the Annual Reports of the Farm Advisors of California. The Function of the Farm Bureau. Suggestions to the Settler in California. Salvaging Rain-Damaged Prunes. No. No. 214. Seed Treatment for the Prevention of 236. Cereal Smuts. 215. Feeding Dairy Cows in California. 217. Methods for Marketing Vegetables in 237. California. 218. Advanced Registry Testing of Dairy 238. Cows. 239. 219. The Present Status of Alkali. 223. The Pear Thrips. 240. 224. Control of the Brown Apricot Scale and the Italian Pear Scale on Decid- 241. uous Fruit Trees. 225. Propagation of Vines. 242. 227. Plant Diseases and Pest Control. 243. 228. Vineyard Irrigation in Arid Climates. 230. Testing Milk, Cream, and Skim Milk 244. for Butterfat. 245. 231. The Home Vineyard. 246. 232. Harvesting and Handling California Cherries for Eastern Shipment. 247. 233. Artificial Incubation. 248. 234. Winter Injury to Young Walnut Trees during 1921-22. 249. 235. Soil Analysis and Soil and Plant later- 252. relations. 253. The Common Hawks and Owls of Cali- fornia from the Standpoint of the Rancher. Directions for the Tanning and Dress- ing of Furs. The Apricot in California. Harvesting and Handling Apricots and Plums for Eastern Shipment. Harvesting and Handling Pears for , Eastern Shipment. Harvesting and Handling Peaches for Eastern Shipment. Poultry Feeding. Marmalade Juice and Jelly Juice from Citrus Fruits. Central Wire Bracing for Fruit Trees. Vine Pruning Systems. Desirable Qualities of California Bar- ley for Export. Colonization and Rural Development. Some Common Errors in Vine Pruning and Their Remedies. Replacing Missing Vines. Supports for Vines. Vineyard Plans.