UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA A SIXTEEN -YEAR EXPERIMENT ON APRICOT PRUNING H.S.REED BULLETIN 574 MAY, 1934 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://www.archive.org/details/nsc68politicalec574card A SIXTEEN-YEAR EXPERIMENT ON APRICOT PRUNING 12 H. S. KEED n Apricot pruning is one of the important orchard operations in southern California. The pruning is usually done during the winter months. The trees, owing to their vigorous habit, grow rapidly in response to a defi- nite system of pruning (fig. 1). TABLE 1 Climatological Data for 1930 at the Citrus Experiment Station* Precipita- tion, inches Mean temper- ature, deg. Fahr. A ctual hours of sunshine Per cent of possible sunshine Relative humidity, per cent Evapora- tion from open pan, inches Mean daily wind move- ment, miles January. February March April May June July August September October November December 5 64 44 3.67 70 3.63 00 00 0.00 0.00 0.79 1.01 47 49.8 56.9 56.5 62.2 60 1 69.5 76.9 72.3 68.8 64 4 59.8 50 3 167 258 235 313 240 224 330 288 260 267 231 225 53 84 78 86 61 71 75 75 72 77 74 73 74.6 69.7 68 5 72.5 65.9 69.0 60.2 72.0 75.0 66.7 66.2 58.5 19 2 7 3.8 5 3 5.6 7.7 9.5 8.5 6.2 4 4.2 2.9 106 87 118 87 107 91 95 87 81 97 122 104 ♦Riverside, California. Latitude, 34° N.; elevation, 1,046 feet. To ascertain the effects of certain types of pruning, the experiments described herein were conducted over a period of 16 years in an orchard at the Citrus Experiment Station at Riverside, California. The orchard was on land classified as Ramona loam which had formerly been used for grazing and for grain production without irrigation. The location is representative of a large area in southern California occupied by apri- cot orchards. According to figures kindly furnished by agricultural commissioners, the apricot orchards of Riverside and San Bernardino counties covered 6,997 acres in 1931. About 14 per cent of the acreage was classed as nonbearing. 1 Beceived for publication September 16, 1933. 2 Paper No. 282, University of California Graduate School of Tropical Agriculture and Citrus Experiment Station, Riverside, California. 3 Professor of Plant Physiology in the Citrus Experiment Station and Plant Physiologist in the Experiment Station. [3] University of California — Experiment Station CLIMATIC CONDITIONS The locality in which this experiment was conducted has a short rainy season in the winter and hot dry summers, during which the evaporation rate is high. A detailed account of the meteorological conditions cannot be presented here; however, table 1, which shows data for 1930, will give the reader an idea of a sample year. The yield of fruit may be lowered Fig. l.- -An apricot tree pruned to form a vase-type tree. Photograph taken 7 years after the tree was planted. by extreme temperatures during or after the blossoming period. Flowers or small fruits are sometimes injured by hot, desiccating winds before the foliage is well developed. The maximum temperature (table 2), obtained from instruments in or near one particular orchard, was above 72° F on 8 days in March, 1925, and on 10 days in 1931. Low temperature caused some damage to the fruit in 1931 but none was observed in other years. Frost injury was probably one of the causes for a high degree of variability in the yield in that year. Bul. 574] Apricot Pruning During May the weather is generally cool and often cloudy, changing rather abruptly early in June to cloudless days on which the maximum temperature may be 95° F. Toward the end of June the temperatures may rise daily above 100° F. This rapid transition is often a detriment to the trees and the fruit crop because the fruit is sunburned where it is exposed to strong sunlight during the long days of June. When ripe, the surface of the sunburned fruits is often badly cracked, decreasing the TABLE 2 Temperatures in the Blossoming Period* Year Date of full bloom Number of days in which maximum tem- perature was above 72° F. Number of nights in which minimum tem- perature was below 32° F. Number of nights in which minimum tem- perature was below 30° F. 1921 1922 March 5 March 25... . 1 5 3 8 5 2 4 5 10 1 2 2 1923 March 28 1924 1925 1926 March 1 March 8. . 1928 1929 1930 March 15 March 10 1931 March 3 1 * The records for 1927 were lost soon after they were taken. value of the crop. Apricots in exposed situations frequently bear a blemish designated locally as "fog mark," which is characterized by irregular russetted areas covering a large proportion of the surface. On account of this "fog mark," fruit must generally be sold at a reduced price. Fruit produced in the interior part of the tree is generally free from this blemish, but that which is produced in the upper part of the tree is seriously blemished. These climatic factors are mentioned because their effects on the trees have a bearing on the pruning practices. Any means of training the trees which produces foliage to shade the fruit is a decided advan- tage to its quality in the climatic conditions prevailing in southern California. An effort was made to maintain the moisture content, as well as to give the orchard good care in other ways. The soil was sampled in definite areas to a depth of 4 feet at frequent intervals, usually preceding and following an irrigation. During the autumn and early winter the rainfall is usually light. Storms giving a total precipitation of less than 0.5 inch do not yield enough water to penetrate to the root zone in dry soil. Unless the rain- 6 University of California — Experiment Station fall in these months is supplemented by irrigation, the apricot trees are in danger of a serious water deficit. The cultural conditions were satisfactory during the course of the experiment; at least they were as uniform as could be maintained under BLOCK A DJ 1 2 3 03 4 5 6 a 7 8 9 K 10 BLOCK B 8 DJ 9 10 i [2] 2 3 4 & 5 6 7 H BLOCK C 0] 5 6 7 H 8 9 10 [3] 1 2 3 E9 4 BLOCK D 2 DJ 3 4 5 6 7 8 [3] 9 10 ■ H Fig. 2. — Plan of orchard used for pruning experiments. Numbers in brackets indicate control plots in which trees were pruned as little as possible. Numbers without brackets indicate plots to which refer- ence is made in the text. field conditions. Fertilizing materials were not added. The trees were sprayed annually after the winter pruning to control insects and para- sitic fungi. SCOPE OF THE INVESTIGATION It is a bit difficult to visualize today the acute condition of the pruning problem in southern California from 1914 to 1919. Many growers did not realize the necessity for regular pruning of apricots. In some dis- tricts nearly all the pruning was done by pruning contractors, each of whom had his own system of tree pruning. These systems often differed Bul. 574] Apricot Pruning 7 radically from each other. Admittedly some were better than others, but the horticulturist was generally at a loss in deciding on their relative merits. Discussions of the pruning problem were frequent (and acri- monius) but satisfactory conclusions were less frequent. Under the cross- fire of opposing arguments the apricot grower was apt to award the contract to the lowest bidder, with certain mental reservations about the entire problem of pruning. The experimental work to be related here was planned during this period of uncertainty and dissension, to obtain some information under conditions that would exclude the more glaring inequalities of soil and cultural conditions which prevail when one attempts to compare growth or yields of different orchards. The experiment was designed to yield information on the relation of various types of pruning to the growth of tree and fruit production. This experiment differs from many others in that the orchard was planted for the purpose of conducting this particular work and con- tinued until after the trees came into full bearing. The orchard consisted of 280 trees of the Royal variety. The trees were planted 20 x 24 feet apart in the orchard. There were 14 plots of 20 trees, each of which consisted of 4 subplots of 5 trees. Every fourth subplot was a control plot in which the trees were not pruned, except as was necessary to allow the passage of implements of cultivation or to remove broken branches. A diagram of the plot arrangement is shown in figure 2. Annual records were taken of : ( 1 ) area of cross section of tree trunk (computed from the circumference measured at a marked height), (2) weight of wood removed by pruning, (3) weight of fruit produced, and (4) average number of fruits per pound. THE DIFFERENTIAL PRUNING TREATMENTS The trees were planted in February, 1916. The differential pruning began in January, 1918, although several more years passed before the types were well established. The first commercial crop was produced in 1922. The types of pruning employed fall into three general classes : Head- ing back in winter and thinning in summer; heading back in winter; and lightly pruned in winter (long pruning) . A detailed description of the types follows. 1. Central-Shaft (Modified Leader) Type, Heavily Pruned in Sum- mer and Winter. — One limb near the center of the tree was allowed in the early years to grow higher than the scaffold branches (fig. 3). A central limb was chosen if possible; otherwise one of the scaffold limbs 8 University of California — Experiment Station (modified leader) was developed higher than the others. Each winter these trees were primed to develop a framework, to head back the shoots produced in the preceding season, and to remove superfluous branches as in the case of other plots. In addition they were pruned lightly in Fig. 3. — Central-shaft type before pruning. Tree D-6-1, January, 1931. In the preceding season this tree produced 319 pounds of fruit which averaged 12 fruits per pound. July each year after the fruit had been harvested. No shoots were headed back at this time, but new growth was removed, especially from the top of the tree, to admit more light. The shoots removed were from 2 to 5 feet long. The weight of the leafy shoots removed was not determined inas- much as it was not comparable to the weight of brush removed at the winter pruning. The trees in this plot were very severely pruned during Bul, 574] Apricot Pruning the first 6 years of the experiment, but subsequently there was less difference between them and the next type. As the trees grew in size it was difficult to maintain the central leaders because the original central leader sometimes failed to maintain a vertical position. In a few in- stances it was necessary to start a new leader and to make the original Fig. 4. — Central-shaft type after pruning. Tree D-6-1, January, 1931. This framework distributes the bearing wood on the larger limbs in a way that avoids sunburned fruit. leader a subordinate branch. The dense top formed by the central leader intercepted light, and was later detrimental to the lateral limbs. 2. Central-Shaft (Modified Leader) Type, Moderately pruned in Sum- mer and Winter. — There was no significant difference between this and the preceding type except in the amount of wood removed. During the first 6 years the pruning was less severe than that of type 1. 3. Vase Type, Heavily Pruned in Summer and Winter. — This type of tree is found in nearly all the apricot orchards in the southern part 10 University of California — Experiment Station of the state (figs. 1 and 5). The young tree was trained to 3, 4, or 5 main scaffold limbs, and these were pruned to develop an open vase form. Above the height of 6 feet secondary branches were left, ext end- Fig. 5. — Vase type before pruning. Tree B-8-1, January, 1931. In the pre- ceding season this tree yielded 309 pounds of fine fruit, averaging 9 fruits per pound. ing the framework of the tree outward in a radial direction as far as was consistent with the mechanical strength of the framework. The heavy pruning done when this type of tree was young had a detrimental effect upon the fruit production of that period. Bul. 574] Apricot Pruning 11 4. Vase Type, Moderately Pruned in Summer and Winter. — The difference between this and type 3 lies in the severity of the prun- ing. As an illustration we may say that the tips of the main scaffold limbs after pruning were between 6 and 8 feet from the ground at the Fig. 6. — Vase type after pruning. Tree B-8-1, January, 1931. The new growth was cut back to stubs 8 to 12 inches long. A large part of the fruit is produced on slender, pendant laterals. A picture of this tree taken 8 years earlier is shown in figure 1. end of the third year. When 14 to 18 inches of the preceding season's wood was left on the heavily pruned trees of plot 3, 28 to 40 inches was left on plot 4. The plots which were pruned twice a year generally re- ceived less pruning in the winter than types 5-8, which were pruned only in the winter (table 3). The total weight of the primings from 12 University of California — Experiment Station o CM _ t- r~~ — ^ H * o co O m i-l CO CD CO O Tfl CO o oo ,_, gq H i-i H Tf< CM ■* iCO co CO ^H CO OS -f! -H -H -H 4 -H -H 4 4 4J 4 •«*< in i-H OS © os © o> CM <-H CM <-H 59.8 53.5 65.7 78.0 00 •e CM CO oc o N ■* ce N O CM (M r- m CO OO O OO CO lO OS OS CM 3! ■<*i CC O © *-h © ©' CM CO CO >0 ,_i ^H o CO OS -H -H -H 41 41 -H -H -H 41 4 4 11.1 13.1 10.8 11.9 37.8 38.8 42.2 52.2 CO OS CM m oo w 00 © CD CM 00 OS Tf< CM >* « N T(l W N * ^ O O o oo H OS li N (N N Ttl ■* Ifl OS m CO o CM OS -H -H 4 4 -H 4 41 4 4 4 1 Ph O CO OO 00 >* OtON m CO in OS CO 00 i< n » m m in o ■^ CO -*1 ■>* 00 O0 OO CM CO CO ft & 00 N N « 115 CN n ra co CO oc p cm m cm <-h CD O "0 l-H C o oo M N N £N| CO ^ CO CD CO ,_i cc Ph CM OS 4 41 -H 4 -H -H -H -H 4 4 4 fe O O co co co o t^- oo o OS 'CD OS CM CO t— o m oo w Tf O CM CO CD CO OO m CM iC o fc p Ph Ph CM CO in CM rt< in m oo CM CO cc O) ih N <* !>. ■^ lO ^h e T3 s o CO m cq CM - CM CM CM CO cc CM CM , CO Eh is P fe t) 0i ta £ c N OO M rt C rt H N M ec u CM or US ccj CO CO W H CO CM CM CM '5 <1 COH iO N CN M Tf C CO o O M ^ n CD cy (3 . CO N M N a 3 A CO O0 OO 00 — i ID V U H OO to <-, a o O CO O N CO CO c© os oo m m T3 OJ o- 3 N » Tf H 9 co ■**! O0 •"* T3 r^ o „, C^' _j ^H _i O) CM CM CM "^ CO 0) CO CM H CM OS 4, -H 44 -H H 4H -H -H -H 4 J 4 4 w V ■>*1 ^H O 00 O CD CO OS t^ •>»< o Ph o> O CM OS in CO OO OS CO o t 8 H "fl K5 f * CO CO N » * Tf CO lO m 01 2 is w >> £ Tf CO CM m co ~h «n © t^ CO CO o cc CM <1 z W CC o W 0> 01 Eh s£ 6 CM OS o c O s CC c ; co - "S 8 o> H * > Fig. 9. — Unpruned type. Tree A-(l)-5, January, 1931. The photograph was taken after a broken limb had been removed. The unchecked growth has formed a dense top which contains much dead wood. The upper branches show the usual tendency to grow toward the prevailing wind. In 1930 this tree produced 441 pounds of fruit, averaging 14 fruits per pound. 7. Vase Type, Heavily Pruned in Winter. — This type was like type 3 except for the time of year at which the trees were pruned. 16 University of California — Experiment Station 8. Vase Type, Moderately Pruned in Winter. — This type was like type 4 except for the time of year at which the trees were pruned (figs. 5 and 6). 9. Vase Type, Girdled, and Moderately Pruned in Winter. — The large limbs were girdled in 1922 and in 1927. The pruning was like type 8 except for the girdling by removal of a very narrow ring of bark in the early summer of the years noted. The girdling was harmful to the trees. Some of the wounds never healed and large limbs died as a result. 10. Lightly Pruned (Long Pruning) Type, Lightly Pruned in Winter. — Trees pruned by this plan were headed back for the first 3 years, and then were trained in the form of a tall vase whose scaffold limbs were thinned but not headed back. The tendency for apricot branches to bend horizontally was counteracted by heading back to a vigorous vertical lateral. This was designated as a type of light prun- ing, but from time to time large limbs broke and had to be removed, re- sulting unintentionally in rather heavy pruning. The trees figs. 7 and 8) were taller than those of any other type and difficult to prune. Side branches were frequently removed from the scaffold limbs to keep the top thinned out. This was a close approximation to the system once desig- nated "long pruning." Control Plots (Unpruned). — Every fourth subplot was used as a con- trol from which yields and size of tree for intervening plots were com- puted as percentages, as will be explained later. The control trees grew rapidly and produced large crops of fruit; gradually the tops became very dense (fig. 9) and the fruit laterals in the lower interior of the tree died. As a result the fruit was mainly produced in the top of the trees on long, slender branches. Main limbs sometimes broke, even though artificially supported. Their removal constituted a form of pruning. Low-hanging branches were removed to permit the use of implements of cultivation. The varying amounts of wood removed are shown in table 3. RESULTS OP THE EXPERIMENTS THE EFFECT OF SYSTEMATICALLY HEADING BACK THE NEW GROWTH The apricot tree produces its fruit on short laterals arising from older shoots. The production of these laterals and the perpetuation of their vigor becomes, therefore, one of the chief concerns of the horticulturist. By far the best fruit was produced on short pendent laterals borne on the larger branches (figs. 10 and 11) . Year after year these small shoots yielded fruit of good quality. When they were pruned off, after having lost vitality, the cut was not made so close to the main limb that the ac- Bul. 574] Apricot Pruning 17 cessory buds near the base of the lateral were destroyed, and a new fruit- ing lateral frequently grew after an older shoot had been removed. The annual heading back in the dormant season increased the vigor of the fruiting laterals and prolonged their productive periods. When lightly pruned, as in plot 10 or in the control plots, opposite results were ob- tained, i.e., the few fruiting laterals that appeared were short-lived. Fig. 10. — Interior of central-shaft type. Tree A-2-1, January, 1924. Fruiting laterals had been well developed. The year-old, upright shoots in the upper part of the trees in plots 1 to 9 inclusive, were frequently headed back so severely that not more than 20 per cent of the mature shoot remained. Many commercial apri- cot orchards in southern California are pruned as severely with no det- rimental effects upon the trees. THE MECHANICAL FRAMEWORK OF THE TREE The mechanical framework of the tree is largely a question of orchard technic, but it has a rightful place in a discussion of pruning methods. The ten types of pruning included four types of framework, viz., cen- tral-shaft, vase, tall- vase (plot 10), and a dense top containing many slender branches (controls), The only types which gave good results were the first two, and between them there is little choice. Both form good spreading frameworks and produce satisfactory quantities of fruit. As a precaution, wire ties were generally installed in all the trees by 18 University of California — Experiment Station means of which one limb tended to counterbalance the pull of a limb on the opposite side of the tree. A strong screw eye was turned into the upper (inner) side of a limb approximately 8 feet above the ground. Fig. 11.— Interior of central-shaft type. Tree A-2-1, July, 1922. This illus- trates the production of fruit on interior laterals. The yield for the tree was 319 pounds. The average number of fruits per pound was 13.3. The end of a piece of 14-gauge wire was put through the screw eye and twisted on itself to make a strong loop. The other end of the piece of wire was similarly looped and twisted through a screw eye in another limb on the opposite side of the tree, drawn tight, and twisted. In case there were several limbs in the tree needing support one end of each stay Bul. 574] Apricot Pruning 19 wire was tied into a central iron ring, thereby obtaining a little better distribution of the load. The supporting wires are more economical than props and offer no obstacles to cultural operations. The framework of long-prnned trees (plot 10) suffered breakage prin- cipally on account of the greater length of the scaffold limbs. The amount of labor required to care for trees in this plot was much greater than in the others on account of the production of fruit on the long, TABLE 4 Average Weight of Wood Removed per Tree at the Annual Winter Pruning Compared with Fruit Produced Plots 1-10 Plots 5-8 Year Wood removed Yield per tree Wood removed Yield per tree 1922 1923 pounds 67 46 66 81 65 52 66 28 44 pounds 172 39 86 124 67 167 223 263 250 133 pounds 76 53 75 89 82 70 96 44 65 pounds 163 47 1924 86 1925 126 1926 59 1927 168 1928 1929.. 215 246 1930 257 1931 110 curved branches. Whether the fruit was removed at an early stage with a padded pole or was picked at maturity, the work involved added ex- pense. The maximum losses from defects in the mechanical support of the tree were in the control trees which never had any branches removed from the tops unless broken (fig. 9). RELATIONS BETWEEN THE AMOUNT OF WOOD REMOVED AND THE YIELD OF FRUIT For the initial discussion, the types of pruning are disregarded and only the severity of pruning considered — judging it from the weight of wood removed. The reliability of this criterion is due to the fact that all, or practically all, of the wood removed consisted of shoots which grew during the previous season. It would be more accurate to express this quantity as a percentage of the weight of the entire tree, but that method is obviously impossible. Reference to the second column of table 4 shows that there were four years in which the average weight of wood removed was substantially the same. Since the trees were larger in 1929 than in 1922, the removal of 67 pounds of wood in 1929 was relatively less se- vere than the removal of an equal weight in 1922. The same will apply 20 University of California — Experiment Station r^ _ SO OS ■»*< CO CO O -H -H -H -H © o o us CM OS O OS O 00 OS ■-! "" ~ ~ N**0 » O ■* M -H -H -H -H CO o o o !B O) t» X CM CM CM CM OS r-H .-C OS i-H CM CM CM -H -H -H -H o o o o N N M » O -h 00 lO CM CM CM CM -H -H -H -H M O O 1(5 (O N (M T(l CQ ^ CO CO CM CM -h CM OS N « lO co oo r^ oo -H -H -H -4H 00 00 Tf CM m a » o i« 113 * H i-H *H i-l CM o * t> ® CO 00 CO CM -H -H ^ -H io m ifl in © CM © OS r- co io ■* -H CM © CO M IB * * -H 41 41 -H lO "5 X id t- us r~- «*i CO US oo © M l<5 K5 * -H -H -H -H © US -H l« © £■» t~- © ^ CM © CO CM CM CO CO US -H -H -H ^ US CO CM CM N K5 N X ■<*■>*< Tt< •* OS US ^ CM f X N CO -H -H -H -H US US us © OS CO OS CD CO CO CM -h ^H H 1-H CM u- CC r^ oc 5 H Bul. 574] Apricot Pruning 21 to 1924 and 1926. The yields of fruit for these four particular years varied from 67 to 263 pounds. It would be obviously unsafe on this basis to make any assertions about the relation of pruning to fruit yield. It is evident that the severity of pruning (measured by the weight of wood removed per tree) had no general relation to the quantity of fruit produced in the following summer. TABLE 6 Plot Yields Expressed as Percentage of Controls Plot 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 Averages Trees pruned summer and winter 1 54 157 55 53 132 50 70 49 181 41 84 2 74 129 60 51 137 66 82 53 156 39 85 3 61 202 42 61 93 68 81 76 140 72 90 4 77 135 41 72 88 66 104 69 240 46 94 Trees pruned in winter 5 53 211 50 59 85 59 70 55 148 33 82 6 62 205 40 67 68 59 66 63 144 41 82 7 48 215 47 52 90 53 59 63 146 47 82 8 77 212 62 66 92 72 76 72 200 48 98 Trees girdled 64 168 57 61 52 89 63 77 Trees not headed back, moderately pruned 93 101 55 62 127 57 99 101 195 90 YIELDS FOR THE TEN-YEAR PERIOD It will be next in order to consider the annual yields of the various plots and to observe whether there are differences which may be related to the types of pruning employed. The records of yields presented in table 5 are averages of the 20 trees in each plot. They represent the total amount of ungraded fruit from each tree. " Inspection of the data shows that the control plots outyielded the pruned plots in nearly every year. One with a knowledge of southern California conditions might predict such a condition; as a matter of fact, trees of this variety need pruning primarily because they have such a tendency to produce excess amounts of fruit. The yields of the plots which were pruned are of primary interest to 22 University of California — Experiment Station fruit growers. Plots 4, 8, and 10 were the best in point of quantity of fruit produced, while plots 3, 5, 7, and were low in the quantity of fruit over the ten-year period. Before attempting to draw any conclusions from the yield records another question must be answered. "How do the yields of the various plots compare with those of the neighboring control plots V Table 6 shows plot yields as percentages of the nearest unpruned subplots. This method of computation gives a means for correcting for soil variation because each plot is judged by the performance of neighboring control plots. In general the actual yields were less than the theoretical. The yields were small in 1923 and 1926 and in them the yields of the various plots approximated or exceeded the theoretical yields. The yields w T ere large in 1922, 1927, 1928, 1929, and 1930. In four of these years the yields of the ten plots which received pruning were notably less than the theo- retical. The theoretical yields of 1930 constitute a notable exception. Plots 4 and 10 each held "first place" three times, plot 8 twice, and plots 2 and 7 each once. Plots 4, 8, and 10 were therefore superior plots. On the basis of chance each plot should have been superior only once in the ten-year period. Reviewing the data on yields it appears that there is a small, but con- sistent, difference in favor of plots 4, 8, and 10, although the quantity of ungraded fruit produced by the various plots shows no remarkably outstanding differences w T hen the 10 years are considered. The data show that the results of a short-term pruning experiment may be misleading. THE RELATION OF PRUNING TO QUALITY OF FRUIT Up to this point the discussion has dealt only with quantity of fruit produced by the plots, but that is, as everyone knows, only a part of the story. Unless the quality of the fruit is satisfactory there is a very re- stricted market for it and the price is generally low. The average number of fruits per pound was determined on a sample for each tree and the values for the 20 individual trees were averaged for the 10 plots. The results are presented in table 7. It is apparent that the fruits on the 9 plots which were headed back were larger than on plot 10 or on the control trees. In the last 5 years, when the crops were fairly large, the superiority of the winter heading-back systems was especially evident. There was no particular difficulty in obtaining fruit which had an average number of 10 or 12 per pound. The best sizes were obtained in 1926 when many trees yielded fruit which averaged 6 per pound. The number of fruits per pound on plot 10 exceeded 15 in 5 of the 10 years, and on the control trees in 6 of the 10 years. Bul. 574 Apricot Pruning 23 ^ CO r- CO o o o o +1 -H -H -H T»< t- CD o CO CO -»<■«*< -* "*l o o o o 4 -H 4 -H ffl t» lO N CO -*i ■* CO r|( IN O N o o o o -H -H -H -H O ^H ■<*< CO CM - 00 » OO N CM CM CM CM o o o o -H -H -fl -H lO CO Tt< l-H CM CO CO ■* N N TH rH o o o o HH 41 -H -H CO T 00 O © © © © CM CM CM CM o o o o -H 4! -H 4 CO CO © © » « OS OO t * » lO o o o o -H 4 41 -H CO t^ o o ifl N W 03 * CO CM CO CO CO CO CM CO O O O O 41 41 41 41 O t>- CO © M IN rt H ■* ^ lO CO o o o o 41 41 4! 41 as co o o CO CM CO CO CO CM CO CM O © O O l«^ O) N N M O © © © CM CM CM (M © © © © 41 41 4 41 00 CO © CO CM CM i— CO t*< lO CO «o © © © © 4 4 41 41 © CO ^h oo 00 X OO N —l CM